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AND 64 & 66, FIFTH AVENUE, NEW YORK 1912 ' ' ' v . . . EARTH SCIENCE* LIBRARY O^deot. CONTENTS CHAPTER PREFACE - PART I THE PROPERTIES OF MINERALS INTRODUCTION - i I. MORPHOLOGICAL CHARACTERS - 3 Habit of crystallized minerals, 4 ; symmetry of crystals, 5 ; law of symmetry and of the constant angle, 8 ; zones, 9 ; classification of crystals, 9 ; regular or cubic system, n ; hexagonal system, 15 ; tetragonal system, 17 ; orthorhombic or rhombic system, 19 ; law of rational intercepts, 20 ; monoclinic system, 22 ; triclinic or anorthic system, 23 ; symbols, 24 ; hemihedrism, 29 ; hemimorphism, 30 ; twinning, 31 II. PHYSICAL PROPERTIES - - 34 Cleavage, 35 ; tenacity, 38 ; hardness, 39 ; colour, 41 ; pellucidity and lustre, 42 ; refraction, 43 ; double refraction and polarization, 47 ; uniaxial crystals, 47 ; wave surface, 49 ; biaxial crystals, 51 ; absorption of light pleochroism, 54; thermal properties, 55 ; electrical properties, 56 ; magnetic properties, 57 ; taste and odour, 58 ; surface energy, 58 ; density, 60 III. CHEMICAL COMPOSITION - - 64 Polymorphism, 66 ; isomorphism, 67 ; pseudo- morphism, 67 ; classification by chemical com- position, 68 819493 vi MINERALOGY PART II DESCRIPTIVE MINERALOGY CHAPTER PAGE I. THE ROCK-FORMING MINERALS - 83 Quartz group, 84 ; felspar group, 86 ; felspathoid group, 95 ; scapolite group, 98 ; mica group, 98 ; amphibole and pyroxene group, 101 ; olivine group, 106 ; chlorite group, 109 ; talc, kaolinite, etc., in; zeolite group, 112; contact minerals, H3 II. THE ORES 118 Ores of platinum, 120 ; ores of gold, 122 ; ores of mercury, 127 ; ores of copper, 129 ; ores of silver, 142 ; ores of lead, 149 ; ores of zinc, 154 ; ores of nickel, 161 ; ores of cobalt, 165 ; ores of iron, 168 ; ores of manganese, 183 ; ores of bismuth, antimony, and arsenic, 188 ; ores of vanadium, 193 ; ores of tin, 193 ; ores of molybdenum, 199 ; ores of tungsten, 200 ; ores of uranium, 201 ; ores of other rare metals, 203 ; ores of aluminium, 204 ; appendix veinstones or gangue minerals, 206 III. THE SALTS AND USEFUL MINERALS OTHER THAN ORES - 208 Carbonates, 208; sulphates, 214; nitrates, 219; chlorides and fluorides, 220 ; phosphates, 224 ; berates, 225 ; appendix other useful minerals, 227 IV. GEMS - 229 Diamond, 230 ; corundum, 231 ; spinel, 232 ; beryl, 233 ; garnet, 234 ; topaz, 235 ; tourmaline, 236 ; zircon, 237 ; sphene, 238 ; turquoise, 238 ; chryso- beryl, 239 ; peridot, 240 ; opal, 240 ; chalcedony, 240 ; quartz, 240 ; felspar, 241 INDEX ........ 242 PREFACE TO THE FOURTH EDITION THE first edition of this introduction to the study of Mineralogy was published in 1892. Since then the book has been reprinted three times, but till now without revision. Starting with the idea of a simple revision, I have found it necessary to rewrite and to enlarge it ; but in doing this, I have been careful to retain the essential features of its original arrangement. As before, there are two parts, the first of which deals with the properties of minerals, and the second with the description of the more important species that either make up rocks, or occur as ores, as salts, or as gems. This subdivision, which is iden- tical with the one originally adopted, has been found convenient by students ; but, as in all attempts at the classification of natural products, it is not free from inconsistencies and overlaps. Thus, for example, calcite, which, it cannot be denied, is a rock- forming mineral, has been relegated to the Salts in company with the other carbonate of lime and those vii viii MINERALOGY of magnesia, strontia, and baryta namely, aragonite, dolomite, magnesite, strontianite, and witherite. Simi- larly apatite, which is an accessory constituent of many rocks, is placed in the phosphate division of the Salts ; and tourmaline, zircon, and garnet, which are also frequent accessory constituents of rocks, appear among the Gems. Again, pyrites, which is almost ubiquitous enough to be regarded as a rock-forming mineral, is placed with the Ores ; and even there it is difficult to find its best position, for, although it is chiefly mined for its sulphur content, its natural position is with the ores of iron. Such classificatory inconsistencies need not, however, occasion any real difficulty to the reader, since the description of any given mineral can be found by refer- ence to the index, where the number of the page, containing its descriptive paragraph, is distinguished by heavy type. In selecting mineral types for description, I have endeavoured to include only those that either play an important role in the economy of Nature, or are sought after by man for industrial purposes. Rare minerals, which have found no industrial application, although they may be of the greatest interest to the Crystal- lographer or to the Chemist, are, from the standpoint of the present work, mere lusus natures, and as such have been rigorously excluded. In the choice of a name, where there are several PREFACE ix synonyms for a mineral, I have been guided by the British Museum Index. For physical constants I have chiefly relied on Hintze's " Handbuch der Mineralogie," but have also consulted Rosenbusch and Wulfing's " Mikroskopische Physiographic," Weinschenk's " Ges- teinsbiidenden Mineralien," Brush and Penfield's " De- terminative Mineralogy," and Miers' " Mineralogy." The compilation of the paragraphs dealing with the distribution of the ores and other useful minerals has involved some research in the publications of the various mining institutions, in order to carry out the principle, adopted by me, of restricting the list to locali- ties of industrial importance. For kindly consenting to read the proofs I have to thank my friends W. Campbell Smith, of the British Museum, and R. H. Rastall, Fellow of Christ's College, Cambridge. F. H. H. SEDGWICK MUSEUM, CAMBRIDGE. 1911. MINERALOGY PART I THE PROPERTIES OF MINERALS INTRODUCTION THE geological study of the visible portion of the earth's solid crust has established the fact that it is chiefly made up of material which is either of igneous origin, having consolidated from a molten state, or con- sists of sediments that have accumulated at the bottom of former seas, and now lie piled up one above the other in strata many thousands of feet thick. These rock masses, whether igneous or sedimentary, are made up of the homogeneous inorganic substances known as Minerals. Besides those of which rocks are composed, there is a great variety of other minerals, filling chinks and fissures in the earth's crust, and comprising, inter alia, the valuable ore deposits which are the source of our metals. It is the business of the mineralogist to study the form, characters, and physical and chemical properties of these different kinds of mineral matter ; and the PROPERTIES OF MINERALS facts thus elicited afford a means by which the different species of minerals may be classified and distinguished. In accordance with this principle, Part I. of this book is divided into three chapters, of which the first deals with the morphological characters of minerals, the second with their physical properties, and the third with their chemical composition. CHAPTER I MORPHOLOGICAL CHARACTERS IN studying the properties of minerals, one of the first things that strikes the observer is their frequent occur- rence in geometrical forms bounded by plane surfaces. In these bodies, which are termed crystals, the com- ponent particles of matter are arranged in accordance with certain fixed laws of symmetry ; and that this is so, is evidenced by the fact that the physical properties of a crystal are found to bear a definite relation to its external geometrical boundaries. Mineral matter is said to be crystalline when it exists as part of a crystal or as an aggregation of crystals. In some minerals, however, the aggregation of the mole- cules is subservient to no law of symmetry; and in these there is consequently no interdependence between physical structure and external form. This is known as the amorphous state of matter. It must be borne in mind, however, that a crystalline mineral need not always present a definite geometrical form. The environment of the crystal during its forma- tion may have militated against the assumption of geometrical contours; but the crystalline or non-crys- 3 4 ' PROPERTIES OF MINERALS talline nature of a mineral will nevertheless be always indicated by its physical structure. The form or habit of crystallized minerals is deter- mined by the extent to which certain of the bounding planes (faces) of the crystals are developed at the expense of others. Such habits are the following : Tabular (or platy, etc.) : barytes. Prismatic (or columnar) : stibnite, epidote. A cicular (or needle-shaped) : cerussite. Capillary (or hair-like) : millerite. When the mineral is amorphous or minutely crystalline, it assumes an external shape which is bounded by other than plane surfaces. Such shapes are designated by the following terms : Nodular : blende, malachite. Globular : blende, calcite, marcasite. Botryoidal (like a bunch of grapes) : dolomite, chal- cedony. Mammillated : psilomelane. Reniform (or kidnev -shaped) : haematite. Stalactitic (pendent) : calcite, aragonite, limonite. Dendritic (branched) : copper. Wiry : silver. Mossy : copper. Leafy : gold. The internal structure of minerals having the external forms enumerated above is designated by the following terms : Granular. When composed of grains or small irregu- MORPHOLOGICAL 5 larly-shaped crystals in close juxtaposition : calcite (in marble), magnetite. Massive or Compact. When the outlines of the con- stituent grains are invisible : haematite. In addition, some minerals show an internal fibrous or concentric laminated arrangement; thus, a mammil- lated or globular external form may be associated with an internal fibrous structure, as in wavellite, pyrites, and haematite, or with a concentric laminated structure, as in malachite and haematite. The faces of crystals may be smooth, drusy, striated, or curved. When smooth they reflect clear images of distant objects, a property which is utilized in the measurement, by the reflecting goniometer, of the angle formed by two faces of a crystal.* Drusy faces are those which are roughened by the presence of minute projecting crystals. Striated faces are marked by a series of parallel lines which may be due to repeated (polysynthetic) twinning, as in albite (see p. 92), or to a rapid alternation (oscillation) of two sets of faces, as in pyrites. Curved faces are produced by rapid succes- sion of small, so-called vicinal faces, the angle between each pair being a little less than 180. Diamond, dolomite, and gypsum, are minerals which frequently have curved faces. Symmetry of Crystals. If a sufficiently large number of crystals be examined, it will be found that they * Note that the angles measured by means of the reflecting goniometer, and quoted in most books of mineralogy, are not the face-angles, but their supplements. 6 PROPERTIES OF MINERALS show differences in the symmetrical arrangement of their faces. It will also be found that they exhibit various degrees of symmetry. Thus, a crystal may possess what is termed centrosymmetry i.e., it has a centre of symmetry, through which every line will meet the crystal at similar points at its two ends.* It may also possess one or more planes of symmetry. A plane of symmetry is a plane dividing a body into two parts, each of which is the exact but inverse counter- part of the other ; that is to say, the two parts bear to one another the same relation that an image bears to its object, the mirror being equiva- lent to the plane of symmetry (see Fig. i). Every face of a crystal possessing centro- symmetry must have a corresponding face parallel to it on the opposite side of the FlG i._ A centre of symmetry, and there must also be PLANE OF f aces corresponding to these on the opposite oYMMETRY. sides of planes of symmetry, and making equal angles with them respectively. A group of faces which are thus mutually connected by symmetry is technically described as &form. Crystals possess other kinds of symmetry besides those depending on centrosymmetry and planes of symmetry. Thus, the faces of a crystal may be sym- metrically distributed about an axis of symmetry i.e., each of its faces can be brought into the position of another similar face by rotation about such an axis. * All crystals except certain hemihedral and hemimorphic forms see p. 29) possess centrosymmetry. MORPHOLOGICAL According as the angle through which the crystal is rotated, to produce this result, is J, J, J, or J, of the a, a b FIG. 2. FORM WITH NINE PLANES OF SYMMETRY. Three are shown in a, and six in b. total rotation to the original position, so the axis of symmetry is said to be binary, ternary, quaternary, or senary (or, according to some authors, digonal, trigonal, FIG. 3. HEXAGONAL FORM, WITH SEVEN PLANES OF SYMMETRY. VIEW (a) AND PLAN (&). Six are shown in &, and the seventh is the plane of projection. tetragonal, hexagonal). A plane of symmetry is always parallel to a possible face of the crystal ; an axis of symmetry is always parallel to a possible edge. 8 PROPERTIES OF MINERALS For convenience in mathematical treatment, crystal forms are referred to fixed lines which are chosen, when possible, from the intersections of their planes of symmetry, and are known as crystal axes. Axes of symmetry are not necessarily crystal axes, but an axis of symmetry either coincides with a crystal axis, or bisects the angle between two equal crystal axes, or is equally inclined to all the crystal axes. The Law of Symmetry and of the Constant Angle. The fundamental principle which underlies crystal- lography may be formulated as follows : Every crystal is enclosed by plane faces, and is subservient to the law of symmetry, which requires that similar parts of a crystal shall be similarly modified. For example, if one corner of a cube is replaced by a face, the law of symmetry requires that all eight corners shall be so replaced. The resulting eight faces of the octahedron comprise a simple form, as do also the original six faces of the cube ; the compound form in which both sets of faces are present is termed a combination. It is also to be observed that the angle between similar pairs of faces of a crystal of a given substance is con- stant. Thus, the angle between two adjacent faces of the octahedron measures always 109 28' 16" ; while the cor- responding angle between two adjacent rhombic pyra- mid faces (in : ill) of sulphur, for instance, measures always 106 26'.* This is the law of the constant angle. * See footnote on p. 5. The angle (in : 111) for sulphur, as quoted in the textbooks, is 73 34'. The face - angle is the supplement of this viz., 180 73 34' =106 26'. MORPHOLOGICAL 9 Zones. However complicated a crystal may appear to be, it will be found that the combination may generally be reduced to a few sets of faces, which, if produced, would intersect in edges parallel to the same straight line. A set of faces, therefore, which are parallel to the same line (the zone-axis) is termed a zone. The zone -axis is supposed to be drawn through the centre of the crystal at the intersection of the crystal axes. In the measurement of crystals by the reflecting goniometer, zones are of the greatest service. Once the crystal is set up so that a zone-axis is parallel with the axis of the instrument, the angles between all the faces belonging to that zone can be measured during a single rotation of the graduated circle. The reading of the scale is noted when the reflected image (the " signal ") from each face is suc- cessively in adjustment, and the angle between any two faces is the difference in their readings. Classification of Crystals. By considering crystals in the light of the three elements of symmetry described above, it is found that there are thirty-two possible cases, to which thirty-two classes of crystals correspond. They may, however, be conveniently grouped, by means of their planes of symmetry, into six systems. The greatest number of planes of symmetry possible in a crystal is nine; all forms possessing these belong to the REGULAR or CUBIC system.* Three of these planes are perpendicular to one another, and six bisect the angles between each pair of the first (see Fig. 2). * For the less symmetrical forms of the cubic system see p. 29. 10 PROPERTIES OF MINERALS The remaining systems are the following: The HEXAGONAL, with seven planes of symmetry, six of FIG. 4. TETRAGONAL FORM, WITH FIVE PLANES OF SYMMETRY. VIEW (a) AND PLAN (6). Four are shown in b, and the fifth is the plane of projection. which intersect in one straight line, and are inclined to one another at an angle of 30, the seventh being per- pendicular to the other six (see Fig. 3). FIG. 5 RHOMBIC FORMS, WITH THREE PLANES OF SYMMETRY. a and b are the view and plan respectively of the same crystal, and c is another rhombic combination showing three planes of symmetry. The TETRAGONAL, with five planes, four of which intersect in one straight line, and are inclined to one MORPHOLOGICAL 11 another at an angle of 45, the fifth being perpendicular to the other four (see Fig. 4). The RHOMBIC, with three planes of symmetry perpen- dicular to one another (see Fig. 5). The MONOCLINIC, with one plane of symmetry (see Fig. 6). The TRICLINIC, ANORTHIC, or ASYMMETRIC, with no planes of symmetry (see Fig. 7). The rhombohedral forms are considered by some to constitute a distinct system (the trigonal or rhombo- FIG. 6. MONOCLINIC FORM, FIG. 7. TRICLINIC CRYSTAL WITH NO WITH ONE PLANE OF SYMMETRY. PLANE OF SYMMETRY, BUT POS- SESSING CENTROSYMMETRY. hedral system), with three planes of symmetry, intersect- ing in one straight line, and inclined to one another at an angle of 60. They are often treated, however, as hemihedral derivatives of the hexagonal pyramids (see P. 30). The Regular or Cubic System of Crystals. This system comprises the following holohedral forms, which may occur alone or in combination : octahedron, cube, rhombic dodecahedron, icosi-tetrahedron, triakis-octa- hedron, tetrakis-hexahedron, and hexakis-octahedron. (For hemihedral forms see p. 29.) 12 PROPERTIES OF MINERALS For the purpose of comparison those forms are re- ferred to three equal crystal axes standing at right angles to one another (and at right angles to the three principal planes of symmetry). Each crystal axis is a quaternary axis of symmetry. There are further four a ternary axes of symmetry, j equally inclined to them, and ; six binary axes of symmetry, bisecting the angles between a them. I The octahedron is the simplest form of the regular system. It is composed of eight faces, each of which is an equilateral tri- angle, and cuts the three axes at unit distance. The face- angle between any two adjacent faces measured across an edge is 109 28' 16" (see Fig. 9). The cube is another simple and very common form of this system. It is contained by six square faces, each of which cuts one axis at right angles, and runs parallel to the other two, thus coinciding in direction with the principal planes of symmetry. The angle between c . FIG. 9. OCTAHEDRON. two faces measures 90 (see Fig. 10). The rhombic dodecahedron is contained by twelve equal rhombs (the diagonals of which bear to one another the ratio of i : V 2 )- Each face cuts two axes at unit distance, and is parallel to the third. The remaining six planes of symmetry of the regular system coincide MORPHOLOGICAL 18 in direction with the faces of this form. The angle be- tween two faces which meet in an edge measures 120. The icosi-tetrahedron is a form contained by twenty- four deltoids (a plane quadrilateral figure bounded by pairs of adjacent equal straight lines). Each face cuts FIG. io. CUBE. FIG. n. DODECAHEDRON. two axes at unit distance, the third at a distance measured by a rational quantity, m, less than unity. The series is limited on the one hand by the cube, on the other by the octahedron (see Fig. 12). The triakis-octahedron, or three-faced octahedron, is a FIG. 12. ICOSI-TETRAHEDRON. FIG. 13. TRIAKIS-OCTAHEDRON. form contained by twenty-four isosceles triangles. Each face cuts two axes at unit distance, the third at a distance equal to a rational quantity, m, greater than unity. The series is limited on the one hand by the octahedron, and on the other by the rhombic dodecahe- 14 PROPERTIES OF MINERALS dron. The form is also known as the pyramidal octahedron ; this designation has reference to the shape, which is that of an octahedron with a three-faced pyramid on each face (see Fig. 13). The tetrakis-hexahedron, or four-faced cube, is a form contained by twenty-four isosceles triangles. Each face cuts one axis at unit distance, one at a distance equal to a rational quantity, m, and is parallel to the third. The series is limited by the cube and the rhombic dodecahedron. The form is also termed the pyramidal cube, from the fact that it can be described as a cube FIG. 14. TETRAKIS-HEXAHEDRON. FIG. 15. HEXAKIS-OCTAHEDRON. each face of which is surmounted by a four-faced pyramid (see Fig. 14). The hexakis-octahedron, or six-faced octahedron, is a form contained by forty-eight scalene triangles. Each face cuts one axis at unit distance, another at a distance equal to a rational quantity, m, and the third at a distance equal to a rational quantity, n. By the varia- tion of the values for m and n* the hexakis-octahedron * These letters, m and n, are used to denote the value of the intercept, or distance cut off on the axis by the face, reckoning from the point of origin. They represent any rational number, such as i, 2, 3, or , , , etc. MORPHOLOGICAL 15 may be made to approximate successively to all the simpler forms of the system. It is therefore the most general of all the forms of the regular system (see FIG. 16. COMBINATIONS. a, Cube ; o, octahedron ; d, dodecahedron ; e, icosi-tetrahedron. Fig. 15). For examples of the various combinations of simple forms, see Fig. 16. The Hexagonal System.* The forms belonging to the system are referred to four axes, three of which (a) are equal and similar, and cross c one another at an angle of 60 ; while the fourth (the principal axis, c) is unequal and dissimilar to the other three, and stands at k Prism i i CO oo^P (no) Brachy-domes . . . CO I m m P oo (ok I) Macro-domes ... i oo m m P co (hoi) Brachy-pinacoid oo i 00 CO P 00 (oio) Macro-pinacoid ... I CO oo OO P CO (100) * In Miller's notation the three edges of a rhombohedron (p. 30) are taken as axes. Bravais' notation refers to four axes, three of which lie in one plane at 120 to each other, and have equal para- meters ( k Prism i i oo 00 P (no) Clino-domes ... oo i m m P oo (0*5 Ortho-domes i oo m m P oo (k o /) Clino-pinacoid oo I oo oo P oo (oio) Ortho-pinacoid I oo oo oo P oo (100) MODIFICATIONS IN THE SYMMETRY OF CRYSTALS. The symmetry of the forms of the different systems is subject to certain modifications, resulting in hemi- FIG. 38. OCTAHEDRON AND TETRA- HEDRON. The suppression of the shaded faces in the former gives rise to the latter form. FIG. 39. TETRA- HEDRON. With its corners trun- cated by a second tetrahedron. hedrism, hemimorphism, and twinning. In the first two cases the degree of symmetry is diminished ; in the last it is often increased. MORPHOLOGICAL 29 Hemihedrism. Hemihedral forms are those which possess only half the full number of faces required by the symmetry of the system to which they belong. Such forms can best be understood by reference to the FIG. 40. PENTAGONAL DODECA- HEDRON. Derived from the tetrakis-hexahedron by the suppression of the shaded faces. FIG. 41. PENTAGONAL DODECAHEDRON (Pd). In combination with the cube c. full-faced (holohedral) form. The suppression of alter- nate faces, or groups of faces, in the holohedral form gives rise to a hemihedral derivative of that form. Thus, the hemihedral derivative of the octahedron is the FIG. 42. Two KINDS OF RHOMBOHEDRA. Derived by the suppression of alternate faces of the hexagonal pyramid. tetrahedron, composed of four equilateral triangles, as exemplified in an important silver -copper ore (tetra- hedrite). Another example in the regular system is the pentagonal dodecahedron, which may be derived from 30 PROPERTIES OF MINERALS the corresponding tetrakis-hexahedron, or six-faced cube, by the suppression of alternate faces. Fig. 41 shows a combination of the pentagonal dodecahedron with the cube, often found in iron pyrites. The hexagonal pyramid is also subject to hemi- hedrism, giving rise to the rhombohedron, a six-faced form characteristic of the minerals calcite, dolomite, spathic iron ore (see Fig. 42). The dihexagonal pyramid gives rise similarly to the scalenohedron (see FIG. 43. SCALENOHEDRON. FIG. 44. HEMIMORPHITE. Fig. 43), and the tetragonal pyramid to the sphenoid. The rhombohedral and scalenohedral forms are charac- terized by the ternary axis of symmetry coincident with the principal crystal axis and three binary axes of symmetry. Hemimorphism. Hemimorphism is a property possessed by certain crystals of presenting different forms at the opposite ends of an axis of symmetry (generally the vertical axis), instead of being terminated similarly at both ends as in normal crystals. The phe- nomenon of hemimorphism is closely connected with MORPHOLOGICAL 31 that of pyro-electricity, since those minerals which are hemimorphic acquire positive and negative electricity at the ends or poles of the hemimorphic axis when warmed or cooled e.g., tourmaline and hemimorphite (silicate of zinc). Twinning". A twin is, as its name implies, a double crystal. The examination of a twin shows that it con- sists of two individual crystals, or two parts of one and the same individual, united on a common plane, or pene- trating one another symmetrically. To explain this phe- FIG. 45. TWINNED CRYSTAL. Produced by the rotation of one-half through an angle of 1 80 round the axis, 1 1'. nomenon, it is necessary to refer the two individuals to a plane the twinning plane with regard to which both are symmetrically disposed.* Now, it is found that the two twinned individuals can be brought into a position of complete parallelism if one of them be rotated through an angle of 180 on an axis (the twin axis t t' in Fig. 45) perpendicular to the twinning plane. The plane which unites the two individuals (the plane of * Note that the twinning plane can never be a plane of symmetry of the individual crystal if it is holohedral; on the other hand, it is always a possible face of the crystal. 32 PROPERTIES OF MINERALS composition] is often, but not necessarily, coincident with the plane of twinning. In some cases the two individuals completely penetrate one another, as in Figs. 46 and 47. These types can also be explained FIG. 46. CUBES TWINNED BY INTERPENETRATION . FIG. 47. Two MONOCLINIC CRYSTALS TWINNED BY INTERPENETRATION. by a hypothetical rotation of one individual about a twin axis. One of the most characteristic features of twinned crystals is the presence of re-entrant angles, whereas FIG. 48. GENICULATED TWIN OF TINSTONE. FIG. 49. CROSS-SHAPED TWINS OF STAUROLITE. the angles of simple crystals are always salient. Twin- ning frequently results in the production of knee- shaped (geniculated), arrow-headed, cross- and heart- shaped forms, as in the minerals tinstone, staurolite, MORPHOLOGICAL 33 and gypsum (Figs. 48, 49, and 50). Sometimes the same type of twinning is repeated in one and the same crystal, producing what is termed poly synthetic or lamellar twinning, the lamellae occasionally being so frequently repeated as to produce the effect of a mere striation, as FIG. 50. ARROW-HEADED TWIN OF GYPSUM. The dotted part shows the position of the rotated hah in the untwinned crystal. in plagioclase felspar. In some cases the compound crystal has a higher degree of symmetry than the untwinned crystals ; thus, rhombic and monoclinic minerals when twinned occasionally acquire pseudo- hexagonal symmetry e.g., aragonite and tridymite. CHAPTER II THE PHYSICAL PROPERTIES OF MINERALS ON page 2 it was stated that the physical properties of crystals bear a definite relation to their geometrical form. What is this relation ? Experiment has shown that the physical properties of crystalline bodies vary in a manner dependent on their direction in space, and that these directions are connected with the symmetry of the crystal, or, in other words, with the symmetrical arrangement of its molecules. Thus, while a sphere of glass when warmed remains a sphere, a sphere of sulphur becomes an ellipsoid, the axes of which are related to the axes of the crystal from which the sphere was cut. The optical properties of crystals are similarly related to their symmetry. These physical properties of a crystal are termed vector in contradistinction to the scalar properties, which are independent of direction. The vector properties are related to an internal structure which may be graphically represented by an ellipsoid. For crystals belonging to the hexagonal and tetragonal systems the ellipsoid is a spheroid i.e., it is one pro- duced by the rotation of an ellipse about one of its axes. In the regular or cubic system, the spheroid 34 PHYSICAL 35 is a sphere, the generating ellipse being in this case a circle. In crystals belonging to the rhombic, monoclinic, and triclinic systems, the ellipsoid has three unequal axes. Such an ellipsoid is produced by the rotation of an ellipse about one of its axes, while the other axis is being lengthened or shortened in such a manner that its ends describe ellipses instead of circles. The triaxial ellipsoid has three planes of sym- metry, which in the rhombic system are coincident with the three planes of crystal symmetry. In the monoclinic system only one of these planes is coinci- dent with a plane of crystal symmetry, while in the triclinic system, since there is no plane of crystal symmetry, the ellipsoid is independent of the crystal form. Amorphous bodies, on the other hand, only possess scalar properties. In the present chapter the physical properties of minerals are considered under the following heads : MOLECULAR COHESION, AS EXEMPLIFIED BY CLEAVAGE, FRACTURE, TENACITY AND HARDNESS ; PHENOMENA RELATING TO LlGHT ; CHARACTERS DEPENDING ON THE ACTION OF HEAT, ELECTRICITY, AND MAGNETISM ; TASTE AND ODOUR; SURFACE ENERGY; DENSITY. MOLECULAR COHESION. Cleavage. A piece of Iceland spar, when tapped with a light hammer, splits into a number of rhombo- hedral bodies of variable size but of similar form. A 36 PROPERTIES OF MINERALS piece of rock-salt, similarly treated, splits into cubes. Mica, on the other hand, splits into thin laminae. This property of crystals of separating along certain planes, related to the symmetry of the system to which the par- ticular crystal belongs, is termed cleavage. It is one of the most striking of the physical properties of crystals, and one that gives important evidence as to the mode of aggregation of the crystal particles : for the separation takes place in the direction of least cohesion i.e., per- pendicularly to that in which the particles exert their greatest attraction.* Planes of minimum cohesion are repeated over planes of symmetry and about axes of symmetry. Cleavage may be considered with regard to (i) quality or degree ; (2) direction. i. Quality or Degree. According to the character of the surface produced by splitting a crystallized mineral along its cleavage planes, we may distinguish between imperfect and perfect cleavage. A mineral possessing only an imperfect cleavage will, when broken, present a rough surface in which the planes of separation are frequently interrupted by cross-fractures and continued on a different level. The complete parallelism of the separated portions of the cleavage planes can be proved, however, by their being simultaneously illuminated when moved to and fro in a beam of light in such a * The cleavage of minerals is not to be confounded with that of rocks, which is a fissile structure due to the arrangement of the minerals constituting the rock, and in no sense a molecular structure. PHYSICAL 87 manner as to reflect the rays towards the eye of the observer. A mineral with perfect cleavage will split along faces which in most cases are more smooth and even than the original faces of the crystal themselves. 2. Direction. The planes of cleavage are always parallel to possible faces of the crystal ; and the number of intersecting planes varies with the number of faces which make up the form in question. Thus, a mineral with octahedral cleavage will split in four different directions ; with cubical, in three ; with prismatic, in two (except in the triclinic system) ; with pinacoidal, or basal, in one. Some minerals, however, possess cleavages parallel to the faces of more than one form, and these can usually be distinguished by their different quality, or degree of perfection. The following are examples of cleavage : Cubical: rock-salt, galena. Octahedral: fluorspar, diamond. Rhombic dodecahedral : zinc-blende. Pyramidal: sulphur. Prismatic : hornblende. Basal: mica, topaz. Pinacoidal: gypsum. Domatic : barytes. Rhombohedral : calcite. Two cleavages : felspar, barytes. In all these cases the cleavage may be regarded as perfect or fairly perfect. Examples of imperfect cleavage may be found in the minerals augite and olivine. 38 PROPERTIES OF MINERALS Planes of parting are to be distinguished from true cleavage, as they only occur at intervals in the crystal, and are probably due to secondary twinning or incipient alteration. Planes of parting are seen, for instance, in corundum, diallage, and magnetite. The radiating lines of easy separation known as percussion figures, which are obtained when a crystal face is struck with a sharp point, are closely allied to cleavage. They are particularly well exhibited by mica (see page 101). Fracture. The nature of the surface produced by breaking a mineral is also a valuable diagnostic character. Fracture, as it is termed, must not be con- founded with cleavage. The former can only be pro- duced in a direction in which cleavage is not developed ; consequently, if the tendency to cleave is strongly developed in a mineral, it will be difficult to produce a true fracture. The fracture of minerals may be described in the following terms : Even: chalcedony. Uneven : tourmaline. Conchoidal: calcite, flint. Subconchoidal : quartz. Splintery : jade. Hackly: copper. Tenacity. Tenacity is manifested by the behaviour of a mineral when submitted to pressure, percussion, tension, torsion, or division. According to the results PHYSICAL 39 obtained the mineral is said to be brittle, malleable, ductile, elastic, or sectile. Haidinger found that the ductility of the metals, as shown by the ease with which they could be drawn into wire, decreased in the following order : gold, silver, platinum, iron, copper, zinc, tin, lead ; and their malleability, as shown by the ease with which they could be flattened out under the hammer, as follows : gold, silver, copper, tin, platinum, lead, zinc, iron. A mineral is elastic when, after being bent, it returns to its original shape. Mica is elastic, chlorite and talc inelastic. When a mineral can be cut by a knife it is sectile ; horn-silver is a familiar example. Hardness. The term " hardness " signifies the resistance offered by a body to the separation of its particles. The relative hardness of minerals may be utilized as a means of identification. It is measured by the force required to scratch (i.e., to separate the super- ficial particles of) the mineral with a steel point, or the sharp-pointed fragment of some mineral harder than the one to be experimented upon. The results obtained from one and the same mineral are found to vary with the crystal face, and with the direction in one and the same face. This directional variation is a function of the sym- metry of the crystal, and if, on any face of a crystal, a curve of hardness be constructed by joining up the ends of lines drawn from a central point, whose lengths represent the hardness existing in their direction, it will be found to have the symmetry appropriate to 40 PROPERTIES OF MINERALS the crystal experimented upon. Thus, the curve of hardness on the face of the cube is intersected by the trace of four planes of symmetry (see Fig. 51), and that on a face of the octahedron by the trace of three planes of symmetry (Fig. 52). The relative average hardness of minerals may be FIG. 51. FIG. 52. expressed by reference to the following scale, devised by the Freiberg mineralogist Mohs : 6. Felspar (adularia). 7. Quartz. 8. Topaz. 9. Corundum (sapphire). 10. Diamond. 1. Talc. 2. Gypsum. 3. Calcite. 4. Fluorspar. 5. Apatite. Each of these minerals can be scratched by those that follow it, and will itself scratch those that precede it in the scale. Minerals having the hardness I 2 can be scratched by the finger-nail ; those lying between 3 and 5 by the point of the knife (with gradually increasing difficulty) ; 6 can scarcely be marked by the knife, but is touched by a steel file ; 7 scratches glass ; 8 scratches the steel file ; while 9 is only to be scratched by the diamond. PHYSICAL 41 These so-called " degrees of hardness" are only points arbitrarily fixed in a graduated scale. There is no con- stant ratio between the different numbers. Thus, the degrees of hardness of talc, gypsum, and calcite, lie within much narrower limits than those of topaz, corundum, and the diamond. Experiments made by Rosiwal, by grinding with a standard powder, showed that the relative hardness of the minerals used for the scale may be expressed by the following figures, corundum being taken at 1,000: diamond, 140,000; corundum, 1,000 ; topaz, 175 ; quartz, 120 ; felspar, 37 ; apatite, 6*5 ; fluorspar, 5 ; calcite, 4*5 ; gypsum, 1*25 ; talc, 0*33. PHENOMENA RELATING TO LIGHT. Colour. The colour of a mineral is either an intrinsic property as, for instance, that of gold, copper, cinna- bar, ruby-silver or it is accidental, and due to the inclu- sion or admixture of a small quantity of some colouring substance, as in rose-quartz, jasper, fluorspar, some varieties of felspar (e.g., the so-called amazon-stone), and most gem-stones. When powdered, most minerals show a different colour to that which characterizes them in the mass. Thus, haematite, which maybe dark brown or black in the mass, yields powder of a cherry-red colour. This property is utilized as a test namely, by filing off a little of the mineral, or by drawing it across the rough surface of a piece of unglazed porcelain ; the resulting mark is termed the streak of the mineral. 42 PROPERTIES OF MINERALS The streak of a mineral is therefore the colour of its powder. Pellucidity. According to the amount of light trans- mitted by minerals, we may distinguish between their transparency, translucency, and opacity. The pellucidity of a mineral is to a certain degree dependent on its colour ; for a dark colour diminishes transparency. It is also influenced by the thickness of the specimen, some minerals which are opaque in thick pieces being translucent in thin plates or splinters. The opacity of minerals is not always an inherent quality, since in some cases it is the result of decomposition, hydration, etc. (e.g., the kaolinization of felspar and the serpen- tinization of olivine). Lustre. The lustre of a mineral is due, partly to the degree in which light is reflected, and partly to the nature of the reflected light. According to the degree or intensity of lustre, a mineral is said to be splendent (as blende), shining (as calcite), glistening (as magnetite), glimmering (as galena), or dull (as serpentine). According to the nature of the reflected light, the lustre is metallic (as in the native metals, pyrites, etc.), submetallic (as in pitchblende), adamantine (as in diamond and blende), vitreous (as in quartz), greasy (as in el&olite], pearly (as in diallage and bronzite), or silky (as in tremolite). There are a few other phenomena, produced either by peculiarities in the reflecting surface, or by the fact that reflection takes place from surfaces in the interior of the crystal. Such, for example, are the following : PHYSICAL 43 opalescence a peculiar milky or cloudy appearance produced by the diffraction of light by minute fissures in the interior of a crystal (e.g., opal, moonstone) ; chatoyancy a changeable banded lustre like that of the eye of a cat. Chatoyant stones, like chrysoberyl (" cat's-eye "), when cut suitably, flash out bands of light which shift their position according to the way in which the stone is moved. This phenomenon is the result of a fibrous structure. When two or three systems of striations intersect, a star composed of four or six luminous rays is produced. This phenomenon is termed asterism, and is displayed by certain varieties of ruby, sapphire, garnet, and mica. Schiller is the name given to certain metallic and pearly lustres pro- duced by the reflection from the surfaces of minute enclosed plates, rods, or particles (as in diallage, bronzite, hypersthene, and avanturine). Change or play of colours is a phenomenon of diffraction produced at the surface of some minerals by a fine lineation (e.g., labradorite). Iridescence and irisation refer to the prismatic colours produced by the interference of light in the interior or at the surface of a mineral. In the former case the phenomenon is due to the presence of minute fissures, in the latter to the presence of a thin super- ficial film. Refraction. Isotropic media are those in which the velocity of transmission of light is independent of the direction in which the ray is transmitted. Anisotropic media are those in which the velocity of transmission changes with the direction. 44 PROPERTIES OF MINERALS Gaseous, fluid, and amorphous substances and crystals of the regular system are isotropic ; substances crystallizing in the tetragonal, hexagonal, rhombic, monoclinic, and triclinic systems are anisotropic. In one and the same homogeneous isotropic substance light is transmitted without change of direction and without change in the velocity of transmission ; but in passing from one isotropic medium to another the ve- locity of light is changed, and there is consequently a change in direction, except in the particular case when the incident ray is normal to the plane separating the two media. This deflection of a ray of light in passing from one medium to another is termed refraction. The law of refraction may be formulated thus : In passing from an optically rare to an optically dense medium a ray of light is refracted or bent towards the normal to the bounding surface, and the angle of refraction bears a constant relation to the angle of incidence Jor the same two media. This constant ratio is the index of refraction, and may be expressed thus : __ sine of angle of incidence sine of angle of refraction As generally used, it refers to the index of refraction of an isotropic substance as compared to air. Since the velocity of light varies inversely with the density of the medium, the refractive index is inversely proportional to the velocity. The amount of deflection suffered by the incident ray when refracted depends also on its wave-length. PHYSICAL 45 It is consequently different for rays of different colour. Refraction is therefore accompanied by dispersion, by which white light is resolved into the component colours of the spectrum. Now consider the case of light travelling from a denser to a rarer medium. Since in this case the re- fracted ray is deflected from the normal to the bounding surface, there must be a certain angle of incidence for which the deflected ray is parallel to the bounding N' itiiliti/l/fftn/f/Jia}i///JJfl//J/J////J/////M/f/ \ A, T IN' "C FIG. 53. DIAGRAM ILLUSTRATING THE PHENOMENON OF REFRACTION. PQ and ST, the parallel bounding surfaces of a plate of an isotropic substance in air; AB, the incident ray in air; BC, the re- fracted ray in a denser isotropic medium ; CE, the course of the ray in emerging from the denser medium again into air (CE is parallel to BA] ; NN', the normal to the bounding plane between the two media ; i, angle of incidence ; r, angle of refraction. surface. This angle, which varies in different sub- stances, but is constant for one and the same substance, is known as the critical angle. A ray of light, traversing a denser medium, and coming upon the bounding plane with a rarer medium at the critical angle, continues its course on emergence parallel to that plane (see Fig. 54). 46 PROPERTIES OF MINERALS If the ray in the denser medium strikes the bounding plane with a rarer medium at an angle greater than the critical angle, it undergoes total reflection (see Fig. 55). Since in the first case n = sin go sine of critical angle' and sin 90 = i, n = sine of critical angle" Consequently, the greater the index of refraction of a substance, the smaller its critical angle. 30 FIG. 54. DIAGRAM SHOWING THE PATH OF A RAY WHICH MEETS THE BOUNDING SURFACE WITH A RARER MEDIUM AT THE CRITICAL ANGLE (9). FIG. 55. DIAGRAM SHOWING THE PATH OF A RAY WHICH MEETS THE BOUNDING SURFACE WITH A RARER MEDIUM AT AN ANGLE OF INCIDENCE GREATER THAN THE CRITICAL ANGLE (0). The following are examples of this relation: Substance. Water Glass . . . Diamond Index of Refraction. I'33 i'53 2*42 Critical Angle. 48 35' 40 45 24 25' The small critical angle of the diamond explains why so much light is reflected from the interior facets of a brilliant; and the fact that this reflection is PHYSICAL 47 accompanied by a large amount of dispersion of the coloured rays, determined by its high refractive index, accounts for the " fire " of this precious stone. Double Refraction and Polarization. When a ray of light passes from an isotropic medium into an anisotropic medium, the incident ray is separated into two rays, which traverse the anisotropic medium in different directions and at different velocities. Further, each ray is polarized i.e., its vibrations, instead of being in all azimuths at right angles to the direction in which the light travels, are now confined to one plane ; and the planes of polarization or vibration of the two rays are at right angles to one another. This is the phenomenon of double refraction. The passage of the two rays through the anisotropic medium is regu- lated in accordance with the elasticity of the particular medium ; and the distribution of the elasticity or, in other words, the nature of the figure of elasticity is a function of the symmetry of the system in which the substance crystallizes. We have to distinguish between crystals with a principal axis (tetragonal and hexagonal), and those without a principal axis (rhombic, monoclinic, and triclinic). Uniaxial Crystals. As explained on page 34, the figure of elasticity for crystals of the tetragonal and hexagonal systems, which have a principal axis, is a spheroid ; in other words, sections at right angles to the principal axis are circles, all other sections being ellipses. Both the velocity and the direction of vibration of the two rays 48 PROPERTIES OF MINERALS produced by double refraction in this class of crystals are represented by the cross-section through the centre of the spheroid of elasticity at right angles to the incident ray. The vibrations of the rays take place respectively parallel to the greatest and least diameters of the section, and the lengths of these express the velocity of the rays. Since the section at right angles to the principal axis is a circle, and all diameters are equal, rays which enter the crystal parallel to this axis traverse it as through an isotropic medium i.e., without double refraction. This axis is distinguished as the optic axis, and crystals of the tetragonal and hexagonal systems are said to be uniaxial. Every other section is an ellipse, which is the more elongated the greater the angle made by the incident ray with the principal axis. In all the ellipses, how- ever, the one diameter remains equal to the diameter of the circular section. The refracted ray which vibrates parallel to this diameter travels with a constant velocity, and has a constant index of refraction (designated by o>). It is called the ordinary ray (O). Since the length of the second diameter* varies with the inclination of the incident ray to the principal axis, the ray vibrating parallel to it travels with a similarly varying velocity, and has, therefore, no constant index of refraction. The latter reaches its maximum or minimum value when the incidence is at right angles to the prin- cipal axis. The ray vibrating parallel to the second diameter is known as the extraordinary ray (E), and its * It will be either the major or the minor axis of the ellipse, according as the spheroid is prolate or oblate. PHYSICAL 49 maximum or minimum index of refraction is represented by e. As with n, both co and e have a different value for differently coloured light. If the principal axis is the axis of greatest elasticity (in the case of a prolate spheroid), the crystal is said to be optically negative, and the refractive index of the ordinary ray is greater than that of the extraordinary ray (oT>e). If, on the contrary, the principal axis is the axis of least elas- ticity (in the case of an oblate spheroid), the crystal is optically positive, and e is greater than a* Wave Surface. The relation between the ordinary and the extraordinary ray in a uniaxial crystal can be best understood by a consideration of the nature of the wave surfaces produced by the propagation of the two sets of rays. Suppose a luminiferous wave motion initiated within a uniaxial crystal. Two sets of rays (the ordinary and the extraordinary) will be propagated outward through the crystal in all directions. After a given interval of time, imagine all the points to which the light has travelled in that period of time united by a surface (wave surface], one for each set of rays. The wave surface for the ordinary rays will be the surface of a sphere, because these rays travel with equal velocity in all directions. That uniting the extraordinary rays will be the surface of a spheroid, the shape of which will be the inverse of the spheroid of elasticity for the crystal * In positive crystals e w has a positive value; in negative crystals it has a negative value. (e w) is a measure of the double refraction. 4 50 PROPERTIES OF MINERALS in question ;* that is to say, its major axis, which is the maximum distance travelled in the given period of time, will be at right angles to the major axis of the spheroid of elasticity, since the latter represents the direction in which the vibrations take place, and is proportional to the velocity of propagation. Since in negative uniaxial crystals the axis of greatest elasticity coincides with the principal axis, in these crystals the extraordinary ray will be propagated with the greatest velocity in a direction at right angles FIG. 56. DIAGRAM OF THE WAVE FIG. 57. DIAGRAM OF THE WAVE SURFACES OF THE ORDINARY SURFACES OF THE ORDINARY AND EXTRAORDINARY RAYS IN AND EXTRAORDINARY RAYS IN A NEGATIVE UNIAXIAL CRYSTAL. A POSITIVE UNIAXIAL CRYSTAL. to the principal axis, while it will travel with its minimum velocity in the direction of this axis, and this minimum velocity will also be the velocity of the ordinary ray. In other words, the wave surfaces of the two sets of rays will be tangential at a point on the principal axis, and the sphere will be enclosed by the spheroid. * In other words, the wave surface of the extraordinary rays is a prolate spheroid for crystals, in which the spheroid of elasticity is oblate, and vice versa. PHYSICAL 51 On the other hand, in positive uniaxial crystals, in which the axis of least elasticity coincides with the principal axis, the extraordinary ray has its greatest velocity in the direction of this axis, and this greatest velocity is equal to that of the ordinary ray. The two wave surfaces are therefore also tangential at a point on the principal axis, but the spheroid is in this case enclosed by the sphere. It is useful to note as a memoria technica that the shape of Fig. 56 recalls the negative sign, and that of Fig. 57 the positive sign. Biaxial Crystals. In crystals of the rhombic, mono- clinic, and triclinic systems, which have no principal axis, the elasticity is represented by an ellipsoid with three unequal axes (Fresnel's ellipsoid). The three axes of elasticity are at right angles to one another, and are known respectively as the axis of greatest elasticity (a), the axis of mean elasticity (6), and the axis of least elasticity (c). There are two directions in crystals belonging to these systems in which there is no double refraction ; from analogy with uniaxial crystals these directions are called optic axes, and the crystals which possess them biaxial crystals. In biaxial crystals the optic axes do not coincide with axes of elasticity, but they are in the plane of, and are symmetrical to, the axes of greatest and of least elasticity. The axis of mean elasticity is therefore the normal to the plane of the optic axes. When the axis of greatest elasticity is the acute bisectrix of the angle between the optic axes, the crystal is said to be negative; 52 PROPERTIES OF MINERALS when the axis of least elasticity is the acute bisectrix, it is positive. The smaller the optic axial angle, the nearer does the optic character of a biaxial crystal ap- proximate to that of a uniaxial crystal. The angle between the optic axes is designated by 2V.* It varies slightly with the wave-length, and consequently with the colour of the light. Thus, for diopside 2V =- 58 52' for red (Li) light, 5843' for yellow (Na) light, and 583o' for green (Tl) light. This phenomenon is known as the dispersion of the optic axes. Every ray which enters a biaxial crystal in any direction other than an optic axis is separated into two rays, which are polarized in planes at right angles to one another, and travel with different velocities, and in different directions. The direction of vibration and the velocity of transmission are represented by the axes of the elliptical section of the ellipsoid of elasticity taken through the centre, and at right angles to the incident ray. Each of the refracted rays has a velocity of trans- mission which varies with the angle of incidence ; there is therefore no constant index of refraction. Three principal indices of refraction are, however, distinguished namely : a, the index of refraction for rays vibrating parallel to the axis of greatest elasticity (a) ; /3, the index of refraction for rays vibrating parallel to the axis of mean elasticity (0) ; and 7, the index of refraction for rays * As observed in air, the optic axes appear to include a larger angle on account of the refraction of the light travelling along on emergence into air. This apparent axial angle in air is denoted by 2E. PHYSICAL 53 vibrating parallel to the axis of least elasticity (c) ; and since the velocity of transmission in these directions is represented by the lengths of the axes a, fc, and c, n = ~, 6 = -= and c = - ft is usually taken as the measure of refraction of a biaxial mineral,* but some authors give . 7 a is a measure of the double refraction. Crystals of the rhombic, monoclinic, and triclinic systems can be distinguished from one another by the orientation of the ellipsoid of elasticity with regard to the crystal axes. In the rhombic system each of the three axes of elasticity (a, fc, and c) coincides with a crystal axis, and the plane of the optic axes lies in one of the pinacoids. In the monoclinic system only one of the crystal axes namely, the ortho-axis (6), which is normal to the one plane of symmetry coincides with an axis of elasticity. The two remaining axes of elasticity lie in the clino-pinacoid (the plane of symmetry). If the ortho-axis (b] coincides with the axis of mean elasticity, * The index of refraction may be determined by measuring with a goniometer the angle of least deviation of light traversing a prism of the substance, or, for substances whose refractive index is not too high, by measuring the angle of total reflection when the sub- stance is immersed in a denser liquid or placed against a glass prism. It may also be determined by immersion of the substance in a liquid of known refractive index, one with a refractive index approximating to that of the substance under examination being selected by experiment. The behaviour of the light band at the contact of the two media, when tested by Becke's method, determines whether the substance has a higher or a lower refrac- tive index than the liquid in which it is immersed. 54 PROPERTIES OF MINERALS the optic axial plane lies in the clino-pinacoid. If the ortho-axis (6) coincides with the axis of greatest, or of least, elasticity, the plane of the optic axes lies normal to the clino-pinacoid. In the triclinic system none of the axes of elasticity coincides with a crystal axis, for the reason that the crystal axes are chosen arbitrarily, there being no plane or axis of symmetry to dictate a choice. Absorption of Light Pleochroism. It is found as the result of experiment that light in passing through a crystal becomes partly absorbed, and, further, that the absorption varies with the direction of vibration within the crystal, or, in other words, that light of a given colour is more absorbed when polarized in one plane than in another. Since the plane of polarization varies with the direction of transmission, crystals which possess the power of absorption in a marked degree will, if white light be used, transmit light of a different colour in dif- ferent directions. Thus, a crystal of cordierite will, when viewed in transmitted light, appear blue in one direction, and yellow in another. This phenomenon is known as Pleochroism. If light vibrating in one plane (i.e., polarized light) be transmitted through a pleochroic biaxial mineral, parallel to each axis of elasticity in turn, three distinct colours are obtained, and these are know r n as the axial colours. In the case of cordierite they are Yellow for rays vibrating parallel to a. Light blue (\ Dark blue ,, ,, ,, c. Uniaxial crystals are dichroic only. PHYSICAL 55 THERMAL, ELECTRIC, AND MAGNETIC PROPERTIES. Thermal Properties. The conductivity for heat of minerals is related to the symmetry of the system in which they crystallize. That it varies with the direction can be shown by the following experiment : if a cleavage surface of stibnite (a rhombic mineral with brachy- pinacoidal cleavage) be coated with a thin film of wax and touched with the point of a heated wire, the melted portion of the wax will be found to have the shape of an ellipse, the major axis of which coincides with the vertical axis of the crystal, and the minor axis with the brachy- diagonal a phenomenon consistent with the symmetry of a rhombic crystal. Specific heat, although a valuable constant of minerals, is not much used in determinative mineralogy, by reason of the extreme care required for its determination. It is defined as the ratio of the quantity of heat required to raise the temperature of the mineral one degree to fhe quantity of heat required to raise the temperature of an equal mass of water one degree. Of more service, because readily observed, is the degree of fusibility, or the ease with which substances fuse when submitted to the action of heat. A splinter of the mineral is held with the platinum forceps in the flame of a Bunsen burner or of a blowpipe. Its fusi- bility is determined by comparison with the following substances (Von Kobell's scale *) : i. Stibnite. A rather large fragment fuses easily in the gas flame. * As modified by Penfield. 56 PROPERTIES OF MINERALS 2. Chalcopyrite. A fragment of the standard size (diameter 1*5 millimetres) fuses rather slowly in the gas flame. 3. Almandine Garnet. A fragment of the standard size fuses readily to a globule before the blowpipe. 4. Actinolite. The edges of a fragment of the standard size are readily rounded before the blowpipe. 5. Orthoclase. The edges of a fragment of the standard size are rounded with difficulty before the blowpipe. 6. Bronzite. Only the finest points are rounded before the blowpipe. Electrical Properties. The electrical conductivity of minerals is a very variable factor. The best conductors are the metals, and, among compounds, those which possess metallic lustre, like pyrites, chalcopyrite, galena, haematite, etc. On the other hand, minerals like quartz, felspar, calcite, barytes, fluorspar, garnet, are poor con- ductors. A process for the separation of metallic minerals from their gangue is founded on these differences in con- ductivity. In the electrostatic separation process the dry mixture of metallic mineral particles and gangue mineral grains, such as quartz, while in a neutral electrical state, are brought into contact with a surface highly charged with electricity ; the metallic particles, because they conduct electricity readily, become charged to the same condition as the surface, and fly from it; the quartz particles, on the other hand, require a longer time to receive the charge, and therefore cling to the PHYSICAL 57 surface until they have acquired the same electrical condition. This difference in the behaviour of the two classes of material permits a separation to be made by a suitable arrangement of machinery. Some minerals, and especially those which are hemi- morphic, become electrified when heated or cooled. This phenomenon is known as pyro-electricity. It is manifested by the presence of statical changes at oppo- site ends of the crystal positive at one, and negative at the other. Magnetic Properties. The property of being attracted by a magnet is possessed in greater or less degree by all minerals which contain iron, and this property can be made use of in the separation of pow r dered mixtures of minerals. The process of magnetic separation by means of the electro-magnet has been successfully applied to ore-dressing opera- tions, and the method is advantageously used in laboratory investigations of sands and rocks to effect the isolation of minerals for chemical analysis. For laboratory work the sands are sifted to a uniform size, and the rock crushed to grains not exceeding 0*25 millimetre, the powder being removed by washing. Minerals having a high magnetic permeability, such as magnetite, pyrrhotite, and haematite, are first removed by a weak permanent magnet (an operation which is facilitated by covering the poles with a movable paper cap). The minerals of which iron is a constituent can 58 PROPERTIES OF MINERALS then be removed from the non-ferriferous minerals, and also separated from one another, by regulating the intensity of the field of an electro-magnet by the suitable adjustment of movable pole-pieces.* In this way, for example, the pyroxene, olivine, horn- blende, and biotite, can be separated from the plagio- clase felspar with which they are associated in an olivine gabbro, or monazite can be separated from zircon and quartz in a " monazite sand." TASTE AND ODOUR. Only those minerals which are to some extent soluble in water have a taste. Examples of such minerals are salt, soda, Epsom salt, alum, nitre. Odour is emitted from some minerals when they are rubbed, struck with a hammer, or heated. Sulphur compounds are characterized by the familiar foetid odour of sulphuretted hydrogen or by the choking smell of sulphurous acid. Arsenic compounds (like mispickel) have a smell of garlic. There are also characteristic clayey and bituminous odours that are emitted by argillaceous and bituminous minerals. THE SURFACE ENERGY OF MINERALS. The condition of molecular equilibrium established at the free surface of a substance is known as its surface energy. It is a function of internal cohesion, and, consequently, is generally greater in solids than in * For details of the process, see T. Crook, Science Progress, 1907, p. 18. PHYSICAL 59 liquids. In solids it is responsible for the phenomenon known as the surface condensation of gases ; in liquids it manifests itself as surface tension. The surface tension of liquids tends to make them occupy the least space possible in proportion to their mass, and to this is due the inclination of liquids towards the globular form, w T hich reaches its maximum development in mercury. The surface tension of mercury is, of course, exceptionally high among liquids. After mercury comes water; oils have a low surface tension relatively to water. Among solids, the surface energy is greatest in elements, such as the metals, graphite, etc. ; and it is manifested in these by the power of condensing gases at the surface, as exemplified by spongy platinum, finely divided carbon, etc. Next to the elements, the sul- phides of the heavy metals have the highest surface energy; while the compounds of the non-metals and of the light metals (such as quartz, felspar, calcite, etc.) are deficient in that respect. When a solid substance is in contact with a liquid, whether it will draw up or depress the liquid in its immediate neighbourhood depends on the ratio between the surface energies of the two substances. The angle of contact made by the liquid with the solid is there- fore a measure of the ratio between the surface energies of the solid and the liquid. Since the phenomenon of wetting is brought about by the strong surface energy of the solid overcoming the relatively weak surface tension of the liquid and its internal cohesion, the degree of wetting is also measured by the contact angle 60 PROPERTIES OF MINERALS (or " wetting angle "). When into water in which a small quantity of oil is present a mixture of solids is introduced, there is a selective wetting by the oil of the solid substances that have a high surface energy. On this principle is based the separation of metals and metallic sulphides from their gangue minerals by the so-called " oil-concentration processes." The separation is assisted by agitation, by means of which air or other gaseous bubbles are induced to form on the oily mineral particles, and they are thus enabled to float on the water in which the gangue minerals, free from oil, are sunk by gravity. The oil process can be most advantageously used for separating minerals whose difference in density is insufficient to permit of the application of the usual concentration by gravitation methods. Thus chalco- pyrite may be separated from magnetite, galena and blende from barytes, sulphides of copper from oxide of tin, etc. Similarly, when a diamantiferous " deposit " is washed over a grease-coated surface, the diamonds are retained, while the accompanying minerals are carried away by the water. DENSITY. Density is the mass of a unit volume. On the centi- metre-gramme-second system it is expressed as grammes per cubic centimetre. The specific gravity of a sub- stance is the ratio of its density to that of water at 4 C. The mass of a cubic centimetre of pure water PHYSICAL 61 at 4 C. and under normal pressure being 0-999973 gramme, " specific gravity " is for all practical pur- poses identical with "density." It is usually deter- mined by weighing the body first in air and then in water : the difference is the weight of a mass of water equal in bulk to the body; and this difference divided into the weight in air is the specific gravity. Another method, which is very serviceable in cases where the density is not above 3*3, is to make use of one of the so-called " heavy liquids." The con- centrated solution is diluted, by the gradual addition of water, until a fragment of the mineral under examina- tion remains just suspended. Its specific gravity is then determined either by weighing a measured volume or by the Westphal balance. By an adroit adjust- ment of the strength of such a solution, it may be used as a means of separating a mixture of minerals of different density, such as are found associated in rocks. The best known heavy liquids are the following : Sonstadt's aqueous solution of iodide of potassium and mercury (maximum density 3*18) ; Klein's aqueous solu- tion of borotungstate of cadmium (maximum density 3-28) ; Braun's solution methylene iodide (maximum density 3*32) ; Retger's solution thallium-mercuro- nitrate (liquid at 76 C. with a density of 5*0). A useful liquid for a preliminary removal of the light minerals is bromoform (maximum density 2*9). It has the advan- tage of being cheap, and it is cleaner and less sticky than some of the above-mentioned liquids. 62 PROPERTIES OF MINERALS If minerals be arranged according to their density, it will be found that the native metals rank as the heaviest. Gold is 19 (sinking to 15 in proportion as it is alloyed with copper and silver) ; platinum, 17 ; mercury, 13*6 ; lead, 11*37; silver, io'6 ; copper, 8*84. Next come the metallic ores e.g., cinnabar, 8*1 ; galena, 7-5 ; cassit- erite or tinstone, 6*9 ; mispickel, 6-05 ; cuprite, 6'o ; chalcocite, 575 ; bornite, 5-2 ; magnetite, 5*17 ; pyrites, 5*03 ; ilmenite, 4-84 ; chalcopyrite, 4*2 ; blende, 4*06 ; chalybite, 3*86 ; limonite, 3*8. Among minerals that are not compounds of the heavy metals, barytes takes the first place, with a density of 4*48; apatite is 3-2; fluorspar, 3-18; calcite, 272; gypsum, 2*32 ; rock-salt, 2*14 ; sulphur, 2*07 ; graph- ite, 2*16. The common rock-forming minerals have a density between 3*5 and 2*5 : thus the pyroxenes range from 3*5 to 3*27; olivine is 3*4; the amphiboles vary from 3*4 to 2*9 ; the micas from 3*2 to 27 ; the felspars from 276 to 2*56 ; while quartz is 2*65. The gems vary from 4*6 to 2*2 : thus zircon is 4*69 ; the garnets range from 4*3 to 3*15 ; corundum (sapphire and ruby) is 4*0 ; topaz, 3*53 ; diamond, 3*52 ; turquoise, 275 ; emerald, 27 ; opal, 2*2. Thus it will be seen that the metallic ores betray themselves, even to the casual observer, by their great relative weight, a fact which is naturally of inestimable value. Most of the ore-dressing processes are based on the relatively high density of the valuable metallic ores as PHYSICAL 63 compared with their associated gangue minerals. The principle of " gravity concentration " is made use of in a variety of ways in dollies, jigs, pulsators, percussion and shaking tables, and in the hydraulic sluicing of alluvial gold and tin-ore ; while the prospector applies it daily in the simple process of washing gold or other ores in the pan, the batea, or on the vanning shovel. Among precious stones density is also of great im- portance, since it enables the jeweller to determine with certainty whether a given stone is really what its colour and general appearance may indicate. CHAPTER III CHEMICAL COMPOSITION MINERALS are either simple elementary substances such as gold, sulphur, and diamond, or they consist of compounds in which the elements are in certain fixed proportions. The elements are divided by chemists into metals, of which the most important are gold, platinum, silver, tin, copper, lead, mercury, iron, nickel, cobalt, zinc, man- ganese, aluminium, barium, calcium, magnesium, potas- sium, and sodium ; and non-metals, such as oxygen, sulphur, nitrogen, phosphorus, chlorine, fluorine, carbon, silicon. Of the metals, only gold, platinum, silver, lead, copper, mercury, and iron (rare), occur native. With regard to the compound substances occurring in Nature, these are either compounds of the metals with simple non-metallic elements, such as oxygen, sulphur, arsenic, and the halogens (chlorine, fluorine, etc.), or they are oxy-salts (and sulpho-salts). The compounds with the simple non-metals are known as oxides, sul- phides (and sulphur salts), arsenides, chlorides, fluorides, etc. ; while the oxy-salts (and sulpho-salts) are sup- posed to be derived from the corresponding oxy-acids 64 CHEMICAL 66 (and sulph-acids) by the replacement of their hydrogen by metals. Thus Nitrates are formed from hydrogen nitrate or nitric acid HNO 3 . Carbonates are formed from hydrogen carbonate or car- bonic acid H 2 CO 3 . Sulphates are formed from hydrogen sulphate or sul- phuric acid H 2 SO 4 . Metaborates are formed from hydrogen metaborate or metaboric acid HBO 2 . Phosphates are formed from hydrogen phosphate or phosphoric acid H 3 PO 4 . Orthosilicates are formed from hydrogen orthosilicate or orthosilicic acid, H 4 SiO 4 . Metasilicates are formed from hydrogen metasilicate or metasilicic acid H 2 SiO 3 . Hydrogen, in combination with oxygen, also enters into the combination of a great number of minerals (hydrates) as " water of constitution " and " water of crystallization." The compound which plays by far the largest part in the constitution of the earth's crust is silicon dioxide or silica (i.e., a compound of silicon with oxygen, SiO 2 ), which either in the free state as colloidal silica or as quartz, or in combination with metallic bases as sili- cates, forms more than half the solid crust, and enters into the composition of nearly all its component rocks. Next in importance is alumina, which is an oxide of aluminium. In union with silica, it constitutes the 5 66 PROPERTIES OF MINERALS basis of an important series of silicates (felspar, mica, clay, etc.), which are the chief ingredients of a great number of rocks. Carbon dioxide often appears in combination with lime or magnesia, forming immense deposits of limestone and dolomite. Lime (oxide of calcium), besides being present as a carbonate in limestone, occurs, in combination with sulphuric acid, in anhydrite and gypsum. Other com- pounds which largely enter into the composition of the rock-forming minerals are magnesia and the alkalies potash and soda. Iron and manganese play a part of great importance in many rocks. The oxides and salts of iron may be said to constitute Nature's colour-box ; for the rich shades of yellow, red, and brown, which are so effective in coast scenery, and are seen wherever bare rock is exposed, are chiefly due to iron. The deep chocolate colour of some soils, and the varied colours of sands, are to be ascribed to the same cause. Manganese is widely distributed in marine sediments, as has been demonstrated by the Challenger researches of Murray and Renard; and like iron, its oxides are a common surface deposit. Polymorphism. There are several instances of minerals that differ in crystalline form and physical properties, but are identical in chemical composition. The property by which one and the same chemical substance has the power of appearing in two or more different states of molecular aggregation is termed poly- morphism. Calcite and aragonite (both carbonate of CHEMICAL 67 lime), and rutile, anatase, brookite (all titanic acid), are familiar instances. The former is a case of dimorphism, the latter of trimorphism. Isomorphism. Isomorphous minerals are those which, having a similar chemical composition, crystal- lize in identical or closely related forms, and are capable of forming homogeneous mixed crystals e.g., barytes (sulphate of barium), celestite (sulphate of strontium), and anglesite (sulphate of lead), all of which crystallize in allied rhombic forms. The phe- nomenon of isomorphism points to a close connection, between the atomic constitution of the molecule and the arrangement of the latter in the structure of the crystal. Pseudomorphism. In the mineral kingdom there are certain substances which exhibit the form of one mineral while possessing the chemical composition and molecular structure of another. These remark- able bodies are termed pseudomorphs. Although arising from various causes, they are all due to some secondary process which acts on the original mineral in such a way as to remove or decompose its substance, while retaining its crystal form. Pseudomorphs are classified according to their mode of origin. Quartz is sometimes found in crystal forms characteristic of the mineral calcite ; and tinstone, in the form of felspar: these are pseudomorphs produced by replace- ment, molecule by molecule. Galena, which is sul- phide of lead crystallizing in the regular system, 68 PROPERTIES OF MINERALS is occasionally found presenting the crystal form of pyromorphite (phosphate of lead crystallizing in the hexagonal system) : this is a case of pseudomorphism produced by chemical change. Other pseudomorphs are produced by incrustation i.e., the deposition of one mineral in regular layers on the faces of already exist- ing crystals of another (for instance, quartz on chaly- bite). These are termed epimorphs. In the special case, where a change in molecular structure takes place with- out a corresponding change in chemical composition, such as brookite (titanium dioxide) to rutile (also titanium dioxide), the pseudomorphs are termed para- morphs. Classification by Chemical Composition. In the following list the more important mineral species are classified according to their chemical composition. Those that occur most commonly are distinguished by being printed in heavy type. I. THE NATIVE ELEMENTS. A. NATIVE METALS AND ALLOYS. Gold, Au. Silver, Ag. Electrum gold-silver alloy, Au,Ag. Copper, Cu. Iron, Fe. Lead, Pb. Platinum, Pt. Iridosmine indium-osmium alloy, Ir,Os. Mercury, or Quicksilver, Hg. CHEMICAL Amalgam silver-mercury alloy, Ag,Hg. Arsenic, As. Antimony, Sb. Allemontite antimony-arsenic alloy, Sb,As, Bismuth, Bi. B. NON-METALS. Sulphur, S. Diamond, C (regular). Graphite, C (pseudo-hexagonal). II. THE COMPOUNDS OF METALS WITH ELEMENTS OF THE ARSENIC AND SULPHUR GROUPS. A. ARSENIDE GROUP. Niccolite arsenide of nickel, NiAs. Smaltite diarsenide of cobalt, CoAs 2 . Chloanthite diarsenide of nickel, NiAs 2 . Sperrylite diarsenide of platinum, PtAs 2 . B. MONOSULPHIDE GROUP. Argentite monosulphide of silver, Ag 2 S. Blende monosulphide of zinc, ZnS. Galena monosulphide of lead, PbS. Chalcocite, or Copper Glance monosulphide of copper, Cu 2 S. Stromeyerite monosulphide of copper and silver (Cu,Ag) 2 S. Cinnabar monosulphide of mercury, HgS (hexa- gonal). Metacinnabarite monosulphide of mercury, HgS (regular). 70 PROPERTIES OF MINERALS Millerite monosulphide of nickel, NiS. Alabandite monosulphide of manganese, MnS. Pyrrhotite, or Magnetic Pyrites monosulphide of iron, FeS. Realgar monosulphide of arsenic, AsS. C. BISULPHIDE GROUP. Hauerite disulphide of manganese, MnS 2 . Pyrites disulphide of iron, FeS 2 (cubic). Marcasite disulphide of iron, FeS 2 (rhombic). Molybdenite disulphide of molybdenum, MoS 2 . Sylvanite telluride of gold and silver (Au,Ag)Te 2 .* D. SESQUISULPHIDE GROUP. Stibnite, or Antimonite sesquisulphide of anti- mony, Sb 2 S 3 . Bismuthinite, or Bismuth Glance sesquisulphide of bismuth, Bi 2 S 3 . Orpiment sesquisulphide of arsenic, As 2 S 3 . E. ARSENOSULPHIDE GROUP. Cobaltite, or Cobalt Glance, CoAs 2 .CoS 2 . Gersdorffite, or Nickel Glance, NiAs. 2 .NiS 2 . Mispickel, or Arsenopyrite, FeAs 2 .FeS 2 . F. SULPHUR SALTS. Bornite, or Erubescite sulphoferrite of copper, 3Cu 2 S.Fe 2 S 3 . Chalcopyrite, or Copper Pyrites sulphoferrite of copper, Cu 2 S.Fe 2 S 3 . * Included here on account of the close relationship of tellurium to sulphur.^ CHEMICAL 71 Polybasite sulphantimonite of silver and copper, 9 (Ag,Cu) 2 S.Sb 2 S 3 . Pearceite sulpharsenite of silver and copper, 9(Ag,Cu) 2 S.As 2 S 3 " Stephanite sulphantimonite of silver, 5Ag 2 S.Sb 2 S 3 . Tetrahedrite, or Grey Copper sulphantimonite of copper, 3Cu 2 S.Sb 2 S 3 . Tennantite sulpharsenite of copper, 3Cu 2 S.As 2 S 3 . Bournonite sulphantimonite of lead and copper, 2PbS.Cu 2 S.Sb 2 S 3 . Pyrargyrite sulphantimonite of silver, 3Ag 2 S.Sb 2 S 3 . Proustite sulpharsenite of silver, 3Ag 2 S.As 2 S 3 . Enargite sulpharseniate of copper, 3Cu 2 S.As 2 S 5 . Freieslebenite sulphantimonite of lead and silver, 5 (Pb, Ag 2 )S.2Sb 2 S 3 . III. THE COMPOUNDS OF METALS WITH ELEMENTS OF THE CHLORINE GROUP. A. SIMPLE CHLORIDES, ETC. Rock-salt chloride of sodium, NaCl. Sylvite chloride of potassium, KC1. Sal Ammoniac chloride of ammonium, (NH 4 )C1. Kerargyrite, or Horn Silver chloride of silver, AgCl. Embolite bromochloride of silver, Ag(Br,Cl). Fluorspar, or Fluorite fluoride of calcium, CaF 2 . B. COMPOUND CHLORIDES, ETC. Cryolite double fluoride of aluminium and sodium, 3NaF.AlF 3 . Carnallite hydrated double chloride of potassium and magnesium, KCl.MgCl 2 + 6H.,O. 72 PROPERTIES OF MINERALS C. OXYCHLORIDES, ETC. Matlockite oxychloride of lead, PbCl 2 .PbO. Mendipite oxychloride of lead, PbCl 2 .2PbO. Atacamite hydrated oxychloride of copper, CuCl 2 .3Cu(OH) 2 . IV. THE COMPOUNDS OF METALS WITH OXYGEN. A. MONOXIDE GROUP. Cuprite, or Ruby Copper monoxide of copper, Cu 2 O. Zincite, or Spartalite monoxide of zinc, ZnO. Melaconite, or Tenorite monoxide of copper, CuO. Periclase monoxide of magnesium, MgO. Brucite magnesium hydrate, Mg(OH) 2 . B. EPITRITOXIDE GROUP. Spinel double oxide of magnesium and aluminium, MgO.Al 2 3 . Hercynite double oxide of iron and aluminium, FeO.Al 2 8 . Gahnite, or Zinc -spinel double oxide of zinc and aluminium, ZnO.Al 2 O 3 . Magnetite double oxide of iron, FeO.Fe 2 O 3 . Franklinite double oxide of iron, zinc, and man- ganese, (Fe,Zn,Mn)O.(Fe,Mn) 2 O 3 . Chromite double oxide of iron and chromium, FeO.Cr 2 O 3 . Hausmannite double oxide of manganese, MnO.Mn 2 O 3 . Chrysoberyl, or Alexandrite double oxide of beryllium and aluminium, BeO.Al 2 O a . CHEMICAL 73 C. SESQUIOXIDE GROUP. Braunite sesquioxide of manganese, Mn 2 O 3 . Corundum sesquioxide of aluminium, A1 2 O 3 . Haematite sesquioxide of iron, Fe 2 O 3 . Ilmenite sesquioxide of iron and^titanium, (Fe,Ti) 2 O 3 . Goethite hydra ted sesquioxide of iron, Fe 2 O 3 .H 2 O. Turgite hydrated sesquioxide of iron, 2Fe 2 O 3 .H 2 O. Limonite hydrated sesquioxide of iron, 2Fe 2 O 3 .3H 2 O. Manganite hydrated sesquioxide of manganese, Mn 2 O 3 .H 2 O. Diaspore hydrated sesquioxide of aluminium, A1 2 O 3 .H 2 O. Bauxite hydrated sesquioxide of aluminium, A1 2 O 3 .2H 2 O. Gibbsite hydrated sesquioxide of aluminium, A1 2 3 . 3 H 2 0. Psilomelane hydrated sesquioxide of manganese and barium, (Mn, Ba)O.MnO 2 .H 2 O. D. DIOXIDE GROUP. Pyrolusite dioxide of manganese, MnO 2 . Rutile dioxide of titanium, TiO 2 . Anatase dioxide of titanium, TiO 2 . Brookite dioxide of titanium, TiO 2 . Cassiterite, or Tinstone dioxide of tin, SnO 2 . Zircon double oxide of zirconium and silicon, ZrO 2 .SiO, Tridymite dioxide of silicon, SiO 2 (rhombic). Quartz dioxide of silicon, SiO 2 (hexagonal). Chalcedony dioxide of silicon, SiO 2 . Opal hydrated dioxide of silicon, SiO 2 + wH 2 O. 74 PROPERTIES OF MINERALS V. OXYGEN SALTS OF THE METALS. A. CARBONATES. Witherite carbonate of barium, BaCO 3 . Strontianite carbonate of strontium, SrCO 3 . Cerussite carbonate of lead, PbCO 3 . Aragonite carbonate of calcium, CaCO 3 . Calcite carbonate of calcium, CaCO 3 . Magnesite carbonate of magnesium, MgCO 3 . Dolomite double carbonate of magnesium and calcium, MgCa(C0 3 ) 2 . Ankerite carbonate of magnesium, calcium and iron, (Mg,Ca,Fe)C0 3 . Barytocalcite double carbonate of barium and calcium, BaCa(CO 3 ) 2 . Chalybite, or Siderite carbonate of iron, FeCO 3 . Rhodochrosite carbonate of manganese, MnCO 3 . Calamine carbonate of zinc, ZnCO 3 . B. HYDRATED CARBONATES. Chessylite, or Azurite hydrated basic carbonate of copper, 2CuCO 3 .Cu(OH) 2 . Malachite hydrated basic carbonate of copper, CuCO 3 .Cu(OH) 2 . Natron hydrated carbonate of sodium, Na 2 CO 3 .ioH 2 O. Trona hydrated carbonate of sodium, Na 2 C0 3 .HNaC0 3 .2H 2 0. C. CHROMATES AND SULPHATES. Crocoite chromate of lead, PbCrO 4 . Anhydrite sulphate of calcium, CaSO 4 . Celestite sulphate of strontium, SrSO 4 . CHEMICAL 75 Barytes sulphate of barium, BaSO 4 . Anglesite sulphate of lead, PbSO 4 . Thenardite sulphate of sodium, Na 2 SO 4 . D. HYDRATED SULPHATES. Gypsum, or Selenite hydrated sulphate of calcium, CaSO 4 .2H 2 O. Kieserite hydrated sulphate of magnesium, MgSO 4 .H 2 O. Epsomite, or Epsom Salt hydrated sulphate of mag- nesium, MgSO 4 -7H 2 O. Mirabilite, or Glauber's Salt hydrated sulphate of sodium, Na 2 SO 4 .ioH 2 O. Melanterite hydrated sulphate of iron, FeSO 4 .7H 2 O. Chalcanthite hydrated sulphate of copper, CuSO 4 -5H 2 O. Brochantite hydrated basic sulphate of copper, CuSO 4 .3Cu(OH) 2 . Kalinite, or Potash alum hydrated sulphate of potassium and aluminium, K 2 SO 4 . A1 2 (SO 4 ) 3 + 24H 2 O. Alunite, or Alumstone hydrated basic sulphate of potas- sium and aluminium, K 2 SO 4 A1 2 (SO 4 ) 3 .2A1 2 (OH) 6 . E. PHOSPHATES, ARSENATES, AND VANADATES. Monazite phosphate of cerium, lanthanum, and didy- mium, (Ce,La,Di)PO 4 . Xenotime phosphate of yttrium, YPO 4 . Vivianite hydrated phosphate of iron, Fe 3 (PO 4 ) 2 .8H 2 O. Erythrite, or Cobalt Bloom hydrated arsenate of cobalt, Co 3 (As0 4 ) 2 .8H 2 0. Annabergite, or Nickel Bloom hydrated arsenate of nickel, Ni 8 (AsO 4 ) 2 .8H 2 O. Libethenite hydrated basic phosphate of copper, Cu(CuOH)PO 4 . 76 PROPERTIES OF MINERALS Olivenite hydrated basic arsenate of copper, Cu(CuOH)AsO 4 . Clinoclase hydrated basic arsenate of copper, (CuOH) 3 As0 4 . Scorodite hydrated arsenate of iron, FeAsO 4 .2H 2 O. Torbernite hydrated phosphate of uranium and copper, Cu(U0 2 ) 2 (PO 4 ) ?i .8H 2 0. Autunite hydrated phosphate of uranium and calcium, Ca(U0 2 ) 2 (P0 4 ) 2 .8H 2 0. Wavellite hydrated phosphate of aluminium, 2(A10H) 3 (P0 4 ) 2 . 9 H 2 0. Lazulite hydrated phosphate of iron and magnesium, (MgFe)(A10H) 2 (P0 4 ) 2 . Turquoise hydrated phosphate of aluminium, with small quantity of phosphate of copper, 2A1 2 O 3 .P 2 O 5 .5H 2 O. Pyromorphite chloro-phosphate of lead, 3 Pb 3 (P0 4 ) 2 .PbCl 2 . Mimetite chloro-arsenate of lead, 3Pb 3 (AsO 4 ) 2 .PbCl 2 . Vanadinite chloro-vanadate of lead, 3Pb 3 (VO 4 ) 2 .PbCl 2 . Apatite (a) chlor-apatite chloro-phosphate of calcium, 3Ca 3 (PO 4 ) 2 .CaCl 2 ; (b) fluor-apatite fluoro-phosptiate of calcium, 3Ca 3 (PO 4 ) 2 .CaF 2 . F. NITRATES. Nitre, or Saltpetre nitrate of potassium, KNO 3 . Nitratine, or Soda nitre nitrate of sodium, NaNO. G. BORATES. Borax hydrated borate of sodium, Na 2 O.2B 2 O 3 .ioH 2 O. Boracite chloro-borate of magnesium, 6Mg0.8B 2 O s .MgCl 2 . CHEMICAL 77 H. TlJNGSTATES AND MOLYBDATES. Wolfram tungstate of iron and manganese, (Fe,Mn)WO 4 . Scheelite tungstate of lime, CaWO 4 . Stolzite tungstate of lead, PbWO 4 . Wulfenite molybdate of lead, PbMoO 4 . I. NlOBO-TANTALATE. Columbite niobo-tantalate of iron and manganese, (Fe,Mn)O.(Nb,Ta) 2 O 5 . J. TlTANATE AND TlTANO-SILICATE. Perovskite titanate of calcium, CaO.TiO 2 . Sphene, or Titanite titano-silicate of calcium, CaO.TiO 2 .SiO 2 . K. SILICATES. (a) Orthosilicates. Forsterite orthosilicate of magnesium, SiO 2 .2MgO. Fayalite orthosilicate of iron, SiO 2 .2FeO. Olivine orthosilicate of iron and magnesium, Si0 2 .2(Fe,Mg)O. Tephroite orthosilicate of manganese, SiO 2 .2MnO. Monticellite orthosilicate of calcium and magnesium, SiO 2 .CaO.MgO. Phenacite orthosilicate of beryllium, SiO 2 .2BeO. Willemite orthosilicate of zinc, SiO 2 .2ZnO. Andalusite orthosilicate of aluminium, SiO 2 .Al 2 O 3 . Kyanite orthosilicate of aluminium, SiO 2 .Al 2 O 3 . Topaz fluoro-orthosilicate of aluminium, Si0 2 .Al 2 (0,F 2 )0 2 . Nepheline--- orthosilicate of sodium, potassium, and aluminium, 2SiO 2 .Al 2 O 3 .(Na,K) 2 O. 78 PROPERTIES OF MINERALS Anorthite orthosilicate of calcium and aluminium, 2SiO 2 .Al ? O 3 .CaO. Meionite orthosilicate of calcium and aluminium, 6SiO 2 .3Al 2 O 3 .4CaO. Grossular-garnet orthosilicate of calcium and alu- minium, 3SiO 2 .Al 2 O 3 .3CaO. Pyrope orthosilicate of magnesium and aluminium, 3SiO 2 .Al 2 O 8 .3MgO. Almandine orthosilicate of iron and aluminium, 3SiO 2 .Al 2 O 3 .3FeO. Spessartite orthosilicate of manganese and aluminium, 3SiO 2 .Al 2 O 3 .3MnO. Uvarovite orthosilicate of calcium and chromium, 3SiO 2 .Cr 2 O 3 .3CaO. Melanite orthosilicate of calcium and iron, 3SiO 2 .Fe 2 O 3 .3CaO. Sodalite* chloro-orthosilicate of sodium and aluminium, 3SiO 2 .Al 2 O 3 .(AlCl)O.2Na 2 O. Haiiyne sulpho-orthosilicate of sodium, calcium, and aluminium, 3SiO 2 .Al 2 O 3 .(AlNaSO 4 )O.Na 2 O.CaO. Nosean sulpho-orthosilicate of sodium and aluminium, 3SiO 2 .Al 2 O 3 .(AlNaSO 4 )O.2Na 2 O. (b) Metasilicates. Rhodonite metasilicate of manganese, SiO 2 .MnO. Wollastonite metasilicate of calcium, SiO 2 .CaO. Enstatite metasilicate of magnesium, SiO 2 .MgO. Hypersthene metasilicate of iron and magnesium, Si0 2 .(Fe,Mg)0. Hedenbergite metasilicate of iron and calcium, 2SiO 2 .FeO.CaO. * For a discussion of the constitution of the sodalite group see Brogger and Backstrom, Zeits. fur Min., vol. xviii., p. 209. CHEMICAL 79 Diopside metasilicate of magnesium and calcium, 2SiO 2 .MgO.CaO. Tremolite metasilicate of magnesium and calcium, 4SiO 2 .3MgO.CaO. Anthophyllite metasilicate of magnesium and iron, 2SiO 2 .(Fe,Mg)O. Actinolite metasilicate of magnesium, iron, and cal- cium, 4SiO 2 .3(Mg,Fe)O.CaO. Acmite, or Aegirine metasilicate of sodium and iron, 4SiO 2 .Fe 2 O 3 .Na 2 O. Riebeckite metasilicate of sodium and iron, 4SiO 2 .Fe 2 O 3 .Na 2 O + 2(FeO.SiO 2 ). Augite metasilicate of calcium, iron, and aluminium, 2 Si0 2 .CaO.(Mg,Fe)0 + SiO 2 .(Al,Fe) 2 O 3 .(Mg,Fe)O. Hornblende metasilicate of calcium, magnesium, iron, and aluminium, 2Si0 2 .CaO.(Mg,Fe)0 + SiO 2 .(Al,Fe) 2 O,.(Mg,Fe)O. Spodumene metasilicate of lithium, sodium, and aluminium, 4SiO 2 .Al. 2 O 3 .(Li, Na) 2 O. Leucite metasilicate of potassium and aluminium, 4 Si0 2 .Al 2 3 .K 2 0. Beryl metasilicate of beryllium and aluminium, 6SiO 2 .Al 2 O 3 .3BeO. (c) Isomorphous Mixed Silicates. Felspars isomorphous mixtures of Orthoclase and Albite, or of Albite and Anorthite. Orthoclase, or Potash - felspar silicate of potassium and aluminium, 6SiO 2 .Al 2 O 3 .K 2 O. Albite, or Soda -felspar silicate of sodium and alu- minium, 6SiO 2 .Al 2 O 3 .Na 2 O. Anorthite, or Lime -felspar orthosilicate of calcium and aluminium, 2SiO 2 .Al 2 O 3 .CaO. 80 PROPERTIES OF MINERALS ScapolitCS isomorphous mixtures of Meionite and Marialite. Meionite silicate of calcium and aluminium, 6SiO 2 .3Al 2 O 3 .4CaO. Marialite chloro-silicate of sodium and aluminium, i8SiO 2 .3Al 2 O 3 . 3 Na 2 O.2NaCl. Chlorites isomorphous mixtures of Serpentine and Amesite. Serpentine hydrated silicate of magnesium, 2SiO 2 .3MgO.2H 2 O. Amesite hydrated silicate of magnesium and alu- minium, SiO 2 .Al 2 O 3 .2MgO.2H 2 O. (d) Hydrated Silicates, with " Water of Constitution" Hemimorphite hydrated orthosilicate of zinc, Si0 2 .2ZnO.H 2 0. Dioptase hydrated orthosilicate of copper, Si0 2 .CuO.H 2 0. Chrysocolla hydrated orthosilicate of copper, SiO 2 .CuO.2H 2 O. Prehnite hydrated silicate of calcium and aluminium, 3SiO 2 .(Al,Fe) 2 O 3 .2CaO.H 2 O. Zoisite hydrated orthosilicate of calcium and alu- minium, 6SiO 2 .3Al 2 O 3 .4CaO.H 2 O. Epidote hydrated orthosilicate of calcium, aluminium, and iron, 6SiO r 3(Al,Fc) t O r 4CaO.H 1 O. Piedmontite hydrated orthosilicate of calcium, alu- minium, and manganese, 6SiO 2 .3(Al,Mn) 2 O 3 4CaO.H 2 O. Orthite hydrated orthosilicate of calcium, iron, alu- minium, and cerium, 6SiO 2 .3(Al,Ce) 2 O 3 .4(Ca,Fe)O.H 2 O. Staurolite hydrated silicate of iron and aluminium, 5SiO 2 .6Al 2 O 3 .2FeO.H 2 O. CHEMICAL 81 Vesuvianite, or Idocrase hydrated silicate of calcium, magnesium, aluminium, and iron, Si 2 7 (Al,Fe)(OH,F)(Ca,Mg) 2 . Cordierite hydrated metasilicate of magnesium, iron, and aluminium, ioSiO 2 4(Mg, Fe)O.4Al 2 O 3 .H 2 O. Muscovite, or Potash -mica hydrated orthosilicate of potassium and aluminium, 6SiO 2 .3Al 2 O 3 .K 2 O.2H 2 O. Paragonite, or Soda - mica hydrated orthosilicate of sodium and aluminium, 6SiO 2 .3Al 2 O 3 .Na 2 O.2H 2 O. Phlogopite, or Magnesian - mica hydrated silicate of magnesium, potassium, and aluminium, 6SiO 2 .2Al 2 O 3 .4(Mg, Fe)O.K 2 O.H 2 O. Biotite, or Magnesian-iron-mica hydrated orthosilicate of magnesium, potassium, iron, and aluminium, 6SiO 2 .2(Al, Fe) 2 O 3 .4(Mg, Fe)O.K 2 O.H 2 O. Margarite hydrated silicate of calcium and aluminium, 2SiO 2 .2Al 2 O 3 .CaO.H 2 O. Chloritoid, or Ottrelite hydrated silicate of iron and aluminium, SiO 2 .Al 2 O 3 .FeO.H 2 O. Talc hydrated silicate of magnesium, 4SiO 2 .3MgO.H 2 O. Kaolinite hydrated silicate of aluminium, 2SiO 2 .Al 2 O 3 .2H 2 O. (e) Hydrated Silicates, with " Water of Crystallization " (Zeolite Group). Natrolite hydrated silicate of sodium and aluminium, 3SiO 2 .Al 2 O 3 .Na 2 O.2H 2 O. Analcite hydrated silicate of sodium and aluminium, 4SiO 2 .Al 2 O 3 .Na 2 O.2H 2 O. Chabazite hydrated silicate of sodium, calcium, and aluminium, 4SiO 2 .Al 2 O 3 .(Ca, Na 2 )O.6H 2 O. 6 82 PROPERTIES OF MINERALS Stilbite hydrated silicate of sodium, calcium, and alu- minium, 6SiO 2 .Al 2 O 3 .(Ca, Na 2 )O.6H 2 O. Heulandite hydrated silicate of sodium, calcium, and aluminium, 6SiO 2 .Al 2 O 8 .(Ca, Na 2 )O.3H 2 O. (/) Bovosilicates. Datolite hydrated boro-silicate of calcium, 2SiO 2 .B 2 O 3 .2CaO.H 2 O. Axinite hydrated boro-silicate of calcium, iron, man- ganese, and aluminium, 8SiO 2 .B 2 O 3 .2Al 2 O 3 .6(Ca, Fe, Mn)O.H 2 O. Tourmaline hydrated boro-silicate of sodium, mag- nesium, and aluminium. According to Clarke,* the tourmaline series consists of salts of an acid : Al 5 (Si0 4 ) 6 (B0 2 ) 2 .B0 3 H 2 .H 12 . * See Amer. Journ. of Set., vol. viii., 1899, p. in. PART II DESCRIPTIVE MINERALOGY CHAPTER I THE ROCK-FORMING MINERALS ALTHOUGH no doubt the greater number of the rocks that constitute the visible portion of the earth's crust are of sedimentary origin, it is to the crystalline rocks that we must look for variety in mineral composition ; and even those minerals that are found in the sedimen- tary strata are derived in great measure from rocks of igneous or metamorphic origin. The common con- stituents of the crystalline rocks are quartz, the felspars, the micas, the amphiboles, the pyroxenes, and the olivines; while chlorite, serpentine, talc, kaolinite, the carbonates of lime and magnesia (calcite, dolomite, etc.), hydrated silicates of alumina (clay), the zeolites, etc., are produced by their decomposition, and are termed secondary minerals. The relative proportion in which the minerals composing the superficial crust of the earth occur has been roughly calculated to be as follows : 83 84 DESCRIPTIVE MINERALOGY Felspar ... ... ... ... 48 per cent. Quartz ..; 35 Mica, chlorite, talc, etc. ... ... 13 ,, Hornblende, augite, olivine, and serpentine ... ... ... i Carbonates of lime and magnesia i Clay (hydrated silicate of alumina) i ,, Other substances (including ores, salts, etc.) i These numbers serve to give a rough idea of the composition of the external layers of the earth. Lower zones must have a different composition ; for there the basic silicates, together with the heavy metals and their compounds, doubtless play a more important role. QUARTZ GROUP. Quartz. Pure silica. SiO 2 (silicon, 467). Crystal- lizes in the hexagonal (rhombohedral) system. Usual habit, prismatic with rhombohedral terminations. The prism faces have a characteristic horizontal striation ; occasionally they are absent, the crystals then consist- ing of the double pyramid of twelve faces. Other and rarer forms also occur; indeed, so numerous are they, and so varied their association, that no two quartz crystals can be said to be absolutely identical. The crystals are frequently distorted, the distortion being the result of the undue development of some of the faces at the expense of their fellows, but in every case the angle between any given pair of faces remains ROCK-FORMING MINERALS 85 constant. One kind of distortion gives rise to what is known as sceptre-quartz. When pure, quartz is colourless. Streak, white. Lustre, vitreous. Transparent. Index of refraction, 1*551. Double refraction, weak (e &> = 0*009), positive. Rhombohedral cleavage, imperfect. Brittle. Fracture, conchoidal. Hardness, 7. Density, 2*65. Insoluble in acids (excepting hydrofluoric), in potash (distinction from opaline silica). The mineral may be recognized by its hardness, Infusible. Insoluble FIG. 58. QUARTZ. Bi-pyramidal crystal a com- bination of the rhombo- hedral forms R (1011) and z (oili). FIG. 59. QUARTZ. A combination of the rhom- bohedral forms R (roll), z (ion), and the prism M (lolo). pellucidity, and vitreous lustre. Usually it is quite colourless and transparent, but coloured and trans- lucent or even opaque varieties also occur e.g., brown or yellow (smoky quartz, cairngorm, jasper), pink (rose- quartz), purple (amethyst), green (chry sop rase), white and translucent (milky quartz). When pure and crystallized, it is known as rock-crystal; when crypto-crystalline, as chalcedony. The chief rocks in which quartz occurs are granite, * quartz-porphyry, felsite, rhyolite, gneiss, mica-schist, 86 DESCRIPTIVE MINERALOGY quartzite, sand, sandstone, clay, and shale. It is also a common companion of the ores, being a frequent vein-stone, or gangue material. THE FELSPAR GROUP. Under the name felspar is included a series of im- portant rock-forming minerals, which, while varying in chemical composition, are similar in physical character and crystalline form. They are silicates of alumina, with one or more of the bases potash, soda, and lime and can be regarded as isomorphous mixtures of three primary minerals namely, potash -felspar K 2 O. Al 2 O 3 .6SiO 2 ; soda-felspar, Na 2 O.Al 2 O 3 .6SiO 2 ; and lime-felspar, CaO.Al 2 O 3 .2SiO 2 . These primary felspars exist in nature: the potash-felspar as orthoclase (pseudo- monoclinic) and microcline (triclinic); the soda-felspar, as albite (triclinic) ; and the lime-felspar, as anorthite (triclinic). There are two principal isomorphous series namely, a lime-soda series (the plagioclases) and a potash-soda series (anorthoclase, etc.), the former being the most important. If the symbol Ab be used to represent the albite molecule J(Na 2 O.Al 2 O 3 .6SiO 2 ), and the symbol An for the anorthite molecule CaO.Al 2 O n .2SiO 2 , then, as Tschermak has shown, the lime-soda or plagioclase series may be represented by the general formula m Ab + n An. The nature of the variation in chemical composition is illustrated by the following selected points in the series : ROCK-FORMING MINERALS 87 Percentage Composition. SiO 2 A1 2 O 3 NaaO CaO Ab 687 19-5 u-8 o Ab 3 Anj 62-0 24-0 87 5 '3 Ab 2 An x 6o-2 25-2 7'9 67 Abj An x 55-6 28-3 57 io'4 Abj An 2 5 J 7 30-9 4-0 13-4 Ab, An 5 47-4 33-8 2'0 16-8 An 43*2 367 O 20' I For convenience, the following names are given to the intermediate members of the series : Oligoclase = Ab to Ab 3 An t Andesine = Ab 3 Ar^ to Ab x An x Labradorite = Ab,. Anj to Abj An 3 Bytownite = Ab x A 3 to An The physical properties also show a progressive varia- tion from one end of the series to the other. Thus, the density and the index of refraction increase from the albite to the anorthite end, as will be seen in the following table : Formula. Density. Mean Index of Refraction. Ab - - 2-605 i'535 Ab 3 An x - 2-649 J'545 Ab 2 Ar^ 2'66o i'549 Ab-L Arij 2-679 i-558 Abj An 2 2-710 1-567 Abj An 5 ^ 2733 !'577 An - 2-765 1-585 88 DESCRIPTIVE MINERALOGY The potash-soda series may be represented by the general formula (K,Na) 2 O.Al 2 O3.6SiO2. It is known as the anorthoclase or microcline-albite series. The first known member of this series was described by Forstner as soda-orthoclase from Pantelleria, near Sicily. Its chemical composition can be represented as 2(K 2 O.Al 2 O 3 -6SiO 2 ) +3(Na 2 O.Al 2 O 3 -6SiO 2 ). Orthoclase. K 2 O.Al 2 O 3 .6SiO 2 (potash, 17 ; alumina, 18; silica, 65 per cent). Although now generally assumed to be triclinic, with pseudo-monoclinic sym- metry, orthoclase is still usually treated as a monoclinic mineral. The common forms are : Basal plane, p (ooi). Prism, / (no). Clinopinacoid, M (oio). Orthopinacoid, k (100). There Orthodome, x (101). Orthodome, y (201). Clinodome, n (021). Hemipyramid, o (in). are three chief crystal habits viz. : (i) elongated along the edge PM, as in the Baveno crystals ; (2) tabular along M, as in the variety known as sanidine from the Drachenfels ; (3) elongated vertically, with dominant prism faces, as in the variety known as adularia, from St. Gothard. Crystals of the sanidine and adularia habits generally have a roof-like termination produced by the combination of the basal plane (P) with one or other of the orthodomes x and y. ROCK-FORMING MINERALS Crystals of orthoclase are frequently twinned, most often on what is known as the Carlsbad type. In this the two individuals are usually united on the clino- pinacoid (M) ; and the basal planes (P) of the crystals a c FIG. 61. CRYSTALS OF ORTHOCLASE. a, Baveno habit ; b, sanidine habit ; c, adularia habit. are inclined in opposite directions, each being brought into close juxtaposition with the orthodome of the other individual. The relative position of two individuals twinned on this type can be explained by an imaginary FIG. 62. ORTHOCLASE CRYSTALS TWINNED ON THE CARLSBAD TYPE. rotation of one of them through 180 about the vertical axis. Another type of twinning is the Baveno, in which the crystals are twinned about the clinodome n (021), and united by that plane. Since the angle made by the 90 DESCRIPTIVE MINERALOGY basal plane (P), and by the clinopinacoid (M), with the clinodome n is, in each case, very nearly 45, crystals of the Baveno habit, twinned on the Baveno type, differ very little in shape from the untwinned individuals ; but that they are twins is shown by the position of the cleavages. A third type of twinning is the Manebach, in which the crystals are twinned about the basal plane P (ooi), and united by that plane. The faces M of the two individuals fall into the same plane. FIG. 63. ORTHOCLASE TWINNED ON THE BAVENO TYPE. FIG. 64. ORTHOCLASE TWINNED ON THE MANEBACH TYPE. When pure and free from inclusions, orthoclase is colourless and transparent (adularia, sanidine) ; but more frequently it is opaque, and either white, flesh- coloured, or red. Although this opacity and coloration may in some instances be due to extraneous matter included in the crystal during its formation, it is more frequently the result of alteration produced by weather- ing. During this process the felspar substance under- goes chemical change, kaolin (a hydrated silicate of alumina) being formed, while silica is set free. In ROCK-FORMING MINERALS 91 certain cases muscovite and talc are also products of its decomposition. Streak, colourless. Lustre, vitreous. Index of re- fraction, 1*524. Double refraction, weak (7 - a = 0*006). Fracture, conchoidal. Brittle. Hardness, 6. Density, 2*5. Fusibility, 5. Insoluble in acids. There are two perfect cleavages namely, parallel to the basal plane, P, and to the clinopinacoid, M ; and it is noteworthy that the cleavage planes intersect at right angles. There is also an imperfect prismatic cleavage. Orthoclase is an essential constituent of the more^ acid igneous rocks, such as granite, syenite, and por-T) phyry, also of the foliated granitic rocks or gneisses. The clear fissured sanidine variety is common in the volcanic rocks rhyolites and trachytes. It is found in those sedimentary rocks which are derived directly from the waste of granitic rocks, such as felspathic grits and sandstones. Orthoclase is also common as a vein-stone, being a chief constituent of the pegmatites. Microcline. Triclinic potash felspar. Identical in chemical composition with orthoclase. The angle made by P (ooi) with M (oio) is about 89 30'. A characteristic feature of microcline is its polysynthetic twinning on both the albite and pericline types. The resulting twin-lamellation is in two directions at right angles, producing a rectangular cross-hatching, which is especially visible in suitably oriented thin sections when examined between the crossed nicols of a polarizing microscope. In other physical properties microcline does not differ from orthoclase ; and it is to be noted 92 DESCRIPTIVE MINERALOGY that if we imagine microcline crystals to be polysyn- thetically twinned on an ultra-microscopic scale, such crystals would be indistinguishable from orthoclase : on this reasoning some authors regard orthoclase as triclinic, with pseudo-monoclinic symmetry acquired by twinning. Albite. Na 2 O.Al 2 O 3 .6SiO 2 (soda, ir8; alumina, 19-5; silica, 68*7 per cent.). Crystallizes in the triclinic system, the angle between the basal plane (P) and the brachypinacoid (M) being 86 24'. Common forms are : Basal plane, P (ooi). Prisms, T (110) and / (no). Brachypinacoid, M (oio). Macropinacoid, k (100). Macrodome, x (ioi). Macrodome, y (201). Brachydome, n (021). Pyramid, o (ill). Twinning on the Carlsbad, Baveno, and Manebach types, as in orthoclase; but the charac- teristic feature of albite, and, indeed, of all plagioclase felspars, is the Albite type of twinning, in which the crystals are twinned about the brachypinacoid (M), and FIG 65 united on that plane. In this type there is usually a repeated or polysynthetic arrangement of individuals, the crystal being divided into a number of parallel lamellae, each of which is in the twinning position with regard to its immediate neighbours ; alternate lamellae, however, are similarly situated. The structure of the twinned crystal can be explained by an imaginary rotation of alternate ROCK-FORMING MINERALS lamellae about an axis normal to the brachypinacoid (M). The basal planes of all the lamellae lie on the same side of the crystal, but slope alternately towards and away from one another so as to produce a series of parallel ridges and depressions, the general effect of which is a parallel striation parallel to M, which is best seen on the surfaces produced by the basal cleavage. In the Pericline type the crystals are twinned about a plane normal to the brachydiagonal, and the relative position of the two individuals may be imagined as FlG. 66. POLYSYNTHETIC TWIN OF PLAGIOCLASE. ("Albite-type.") FIG. 67. DUAL TWIN OF ALBITE. P, Basal plane of first individual ; P!, basal plane of second indi- vidual. produced by rotation through 180 of one of them about the brachydiagonal. In this type the M faces of the two individuals do not fall into the same plane. Albite is usually colourless and transparent. Streak, colourless. Lustre, vitreous. Index of refraction, 1*533. Double refraction, positive and moderate (7 - a = 0*008). Basal cleavage, perfect. The brachypinacoidal and prismatic cleavages, imperfect. Fracture, uneven. Brittle. Hardness, 6. Density, 2*63. Fusibility, 4. Insoluble in acids. 94 DESCRIPTIVE MINERALOGY Albite and orthoclase occur in intimate intergrowth, the albite being in narrow lamellae intercalated in the orthoclase along planes parallel to the orthopinacoid. In sections parallel to the basal plane or to the clino- pinacoid, the included albite appears as strips and patches, which are distinguishable from the orthoclase by their twin striation. Such intergrowths are known as perthite and microperthite. Anorthite. CaO.Al 2 O 3 .2SiO 2 (lime, 2O'i; alumina, 367 ; silica, 43*2 per cent.) Crystallizes in the triclinic system, the angle between the basal plane (P) and the brachypinacoid (M) being 85 50'. The habit of the crystals is varied. Twinning occurs on the Albite, Pericline, Manebach, and Carlsbad types. Colourless. Streak, colourless. Lustre, vitreous. Transparent. Index of refraction, 1*585. Double refraction, negative and moderate (7 a = 0*013). Basal cleavage, perfect. Brachypinacoidal cleavage, less perfect. Fracture, conchoidal. Brittle. Hardness, 6*5. Density, 275. Fusibility, 5. Decomposed by hydrochloric acid with gelatinization. The isomorphous series of plagioclase felspars formed by albite and anorthite has already been dealt with (p. 86). The plagioclase felspars occur in many igneous rocks e.g., in the intrusive diorites, gabbros, dolerites, and in the volcanic andesites, porphyrites, and basalts. The variety oligoclase often accompanies orthoclase in granite and trachyte. Oligoclase and albite are also ROCK-FORMING MINERALS 95 frequent constituents of the schists, occurring in them in a clear granular form in association with quartz (secondary or granulitic felspar). Like orthoclase, plagioclase is prone to decom- position, giving rise to epidote, zoisite (in the so-called saussurite), calcite, and kaolin. THE FELSPATHOID GROUP. Nepheline. Orthosilicate of soda, potash, and alumina: (NaK) 2 O.Al 2 O 3 .2SiO 2 . Pure sodium-nepheline which has been prepared artificially has the composition: SiO 2 = 42*3, Al 2 O 3 = 35*g, Na 2 O = 2i*8; but in nepheline, as it occurs naturally, the proportion of Na 2 O to K 2 O is usually about 5 : i. Crystallizes in the hexagonal system in small six-sided prisms (combination of hex- agonal prism and basal plane). Colourless to white or grey. Transparent to translucent. Lustre, vitreous to resinous. Streak, white. Index of refraction, 1*54. Double refraction, negative and weak (co e = '005). Basal and prismatic cleavages, imperfect. Hardness, 5*5-6. Density, 2*6. Fusibility, 4. Soluble in hydro- chloric acid with separation of gelatinous silica, the solution giving cubes of common salt, when evaporated. Nepheline is found in the cavities of volcanic ejected blocks (Monte Somma, Laacher See) ; as an essential constituent of certain lavas (phonolite, nepheline- basalt, tephrite, etc.) ; and as a constituent of the soda-series of the plutonic rocks (syenites and alkali- gabbros). 96 DESCRIPTIVE MINERALOGY Leucite. Metasilicate of alumina and potash : K 2 O. Al 2 O 3 .4SiO 2 (silica, 55; alumina, 23-5; potash, 21-5 per cent.) Pseudo-regular. This mineral occurs crystallized in icositetrahedra, and no doubt at the temperature at which it was formed crystallized in the regular system, but on cooling broke up into rhombic or monoclinic sectors. In consequence the crystals exhibit weak double refraction ; but when heated to 500 C. they become optically isotropic. Index of refraction, 1*508. Colour, dirty white or grey. Lustre, vitreous. Transparent to opaque. Streak, white. Prismatic cleavage very imperfect. Brittle. Fracture, uneven to conchoidal. Hardness, 5 - 6, Density, 2*45 -2*50. Infusible. Slowly decomposed by hydrochloric acid with separation of silica. Leucite occurs as a constituent of the more recent volcanic rocks leucitophyre, leucite-tephrite, and leuci- tite; also in members of the alkali series of the plutonic rocks. Sodalite. Chloro-orthosilicate of aluminium and sodium: 3SiO 2 .Al 2 O 3 .(AlCl)O.2Na 2 O. Regular, with cubic habit ; also massive. Colourless to yellowish ; greenish white; or pale blue. Index of refraction, 1*484. Lustre, vitreous. Transparent to opaque. Streak, white. Cubic cleavage, fair. Fracture, conchoidal to uneven. Hardness, 5'5 - 6. Density, 2*2- 2*4. Fusi- bility, 3*5 - 4. Gelatinizes easily with hydrochloric acid. Occurs in blue, greenish, or colourless grains in syenites and in volcanic ejectamenta. ROCK-FORMING MINERALS 97 Haiiyne and Nosean. Isomorphous sulpho-ortho- silicates of alumina, lime, and soda. The soda end of the isomorphous series with little or no lime is nosean = 3SiO 2 .Al 2 O 3 .(AlNaSO 4 )O.2Na 2 O, with SiO 2 = 317, SO 3 = 14-1, A1 2 O 3 = 26-9, and Na 2 O = 27-3. When Na 2 : Ca = 3 : 2, the composition of haiiyne is SiO 2 = 32*0, SO 3 = 14*2, A1 2 O 3 = 27*2, CaO = 10*0, Na 2 O = 16*6. These minerals crystallize in the regular system. Habit, dodecahedral. Colour, blue (haiiyne) or grey (nosean). Lustre, vitreous. Transparent to opaque. Streak, white. Index of refraction, 1*496. Dodeca- hedral cleavage, fair. Fracture, sub - conchoidal to uneven. Hardness, 5-6. Density, 2'25-2*5. Gelatinize easily with hydrochloric acid ; on evaporation, needles of gypsum are formed in the case of haiiyne, none in the case of nosean. Haiiyne and nosean are essentially volcanic minerals occurring in volcanic ejectamenta and in phonolites, andesites, and basalts. Melilite. Silicate of alumina, iron, lime, magnesia, and soda: i2(Ca,Mg)O.2(Al,Fe) 2 O 3 .9SiO 2 . Tetragonal; occurring in small square tables and prisms, also in irregular grains. Colour, white to yellow. Lustre, vitreous. Translucent. Index of refraction, 1*629. Double refraction, weak. Hardness, 5-5*5. Density, 2'g-3'i. Fracture, conchoidal to uneven. Brittle. Fusibility, 4. Gelatinizes easily with hydrochloric acid. Occurs as a constituent of certain basalts (melilite- basalt) and of nepheline and leucite rocks. 98 DESCRIPTIVE MINERALOGY THE SCAPOLITE GROUP. Silicates of alumina, lime, and soda + sodium chloride. The scapolite group, like the lime -soda plagioclase group, may be regarded as isomorphous mixtures of two molecules viz., the meionite (Me) molecule (4CaO.3Al 2 O 3 .6SiO 2 ), with silica = 40*5, alumina = 34*4, and lime = 25*1 per cent, and the marialite (Ma) mole- cule (Na 4 Al 3 Si 9 O24Cl), with silica = 63-9, alumina = 18*1, soda = 147, and chlorine = 4*20 per cent, (oxygen for chlorine to be deducted). Wernerite includes scapolites with Me : Ma ranging from 3 : i to i : i ; Mizzonite those with Me : Ma ranging from i : 2 to i : 3. Couse- ranite and Dipyre are varieties of mizzonite. The scapolites crystallize in the tetragonal system, with prismatic habit. Colourless to white, also bluish, greenish, or reddish. Streak, white. Lustre, vitreous. Index of refraction, 1*55-1*59. Cleavage parallel to the prism of the second order (100), fair. Fracture, conchoidal to uneven. Brittle. Hardness, 5 - 6. Density, 2*57-2*74. Of infrequent occurrence in igneous rocks; oftener in gneisses and crystalline schists and in contact-altered limestones, calc-silicate rocks, etc. THE MICA GROUP. The micas are hydrated * silicates of alumina and the alkalies, potash, soda, or lithia, with which iron and * Most of the water is only given off at a high temperature, and must be regarded as water of constitution.] ROCK-FORMING MINERALS 99 magnesia are associated in some varieties. They crystallize in the monoclinic system, but possess pseudo- hexagonal symmetry. The crystals consist of six-sided tablets, of which the six sides are made up of the four faces of a prism, and two of the clino-pinacoid, the broad terminal faces being those of the basal plane. Mica has a very perfect cleavage parallel to the basal plane, permitting of its separation into laminae of extraordinary thinness ; it is characteristic for mica, as distinguished from other allied minerals (talc, chlorite), that these laminae are elastic, and cannot therefore be permanently bent. The density varies from 2*76 to 3*2. Hard- ness, 2-3. For practical purposes the micas FIG. 68. MICA- may be conveniently separated into CRYSTAL. i i i j ji i -i Basal plane- M two broad subdivisions: the white or prism- A clino- light-coloured. and the black or dark- pinacoid;'*,clino- dome. coloured varieties. Chief among the former is muscovite, or potash-mica, which is essenti- ally a hydrated silicate of alumina and potash. This variety of mica is not attacked by hydrochloric acid. It is generally pale-coloured to silvery white, with pearly lustre on the cleavage surfaces. It occurs in flakes, scales, and laminae, in many granites, gneisses, and phyllites. Fragments are often present in sand- stones and shales (derived, no doubt, originally from granitic rocks) ; and by their parallel arrangement impart to these rocks a fissile character. 100 DESCRIPTIVE MINERALOGY Less common than muscovite are the following light- coloured micas : Paragonite, or soda-mica. Lepidolite, or lithia-mica. Biotite. The most important dark mica ; essentially a hydrated ferro-magnesian and aluminous silicate. Unlike muscovite, this mica is attacked by hot hydro- chloric acid. It has a dark brown to black colour and sub-metallic lustre, and is transparent to opaque. It occurs in granites and mica-traps and in certain syenites, diorites, trachytes, and andesites. Loose crystals of a reddish-brown biotite (vubellan) are frequently found im- bedded in volcanic tuff. By decomposition mica readily passes into chlorite, assuming then a green colour. Those varieties in which there is much magnesia and little iron are distinguished as phlogopite. They are generally somewhat lighter in colour than biotite. Those which are rich in lithium and iron are known as zinnwaldite. The micas may also be conveniently classified by the relation of their percussion-figures to the position of the plane of the optic axes as shown by the inter- ference figure obtained by the examination of a cleavage flake under the microscope in convergent polarized light. The percussion-figure is a 6-rayed star obtained by driving a needle into a cleavage flake by means of a sharp blow with a light hammer. It will be found that one of the rays is either parallel to, or at right angles to, the plane of the optic axes. This, therefore, ROCK-FORMING MINERALS 101 gives the direction of the plane of symmetry. Micas in which the plane of the optic axes is perpendicular to the plane of symmetry are known as micas of the first class, and include all the alkali-micas; those in which the optic axial plane is parallel to the plane of symmetry are known as micas of the second class, and include biotite, phlogopite, and zinnwal- dite. Those anomalous ferro- magnesian micas which are found to belong to the first class are named anomite by Tscher- mak, to whom this classification owes its origin. FIG. 69. MICA, SHOWING THE PERCUSSION FIGURE (THE 6- RAYED STAR, OF WHICH THE LONGEST RAY is PARALLEL TO THE PLANE OF SYMMETRY, b), AND THE PRESSURE FIGURE (THE 6-RAYED STAR, SHOWN BY PECKED LINES). THE AMPHIBOLE AND PYROXENE GROUPS.* These minerals are silicates, mainly of magnesia and lime; but some varieties contain iron and alumina or manganese, soda or lithia in addition. All the varieties resist the action of acids excepting hydrofluoric. Their density varies from 2*90 to 3-55 ; their hardness from 5 to 6. The commoner species of both groups crystal- lize in the monoclinic system, but rhombic and triclinic varieties also occur. The main feature distinguishing the amphiboles from the pyroxenes is the angle between * " Amphibole " and " pyroxene " are used here as group names, while "hornblende" and " augite " are reserved for the specific rock-forming varieties which crystallize in the monoclinic system. 102 DESCRIPTIVE MINERALOGY the faces of the prism, which in the former measures 124, in the latter 87. The crystals are usually short-columnar, and consist of prisms and pinacoids, terminated by a pair of pyramidal faces. In the amphiboles the prism-faces usually predominate over the pinacoids, the ortho- pinacoidal faces being often even absent ; in the pyroxenes, on the other hand, they are about equally developed. The cross-section of an amphibole crystal FIG. 70. CROSS-SECTION OF AN AMPHIBOLE CRYSTAL, SHOWING THE LINES OF PRISMATIC CLEAVAGE INTERSECTING AT AN ANGLE OF 124. FIG. 71. CROSS-SECTION OF A PYROXENE CRYSTAL, SHOWING THE LINES OF PRISMATIC CLEAVAGE INTERSECTING AT AN ANGLE OF 87. is consequently lozenge-shaped ; that of a pyroxene octagonal. The crystals are occasionally twinned namely, on the orthopinacoid (see Fig. 71). A cleavage exists in both minerals parallel to the faces of the prism. In the amphiboles it approaches a high degree of perfection, the cleaved surfaces being smooth and lustrous ; but in the pyroxenes it is far less perfect, and the cleavage surfaces are consequently uneven. ROCK-FORMING MINERALS 103 The common variety of both groups is a black mineral ; but green, blue, and white varieties also occur. The chemical constitution of the more important pyroxenes and amphiboles is given in the following table : PYROXENES. Rhombic. Enstatite MgO.SiO 2 . . Hypersthene (Mg,Fe)O.Si0 2 . Monoclinic. Diopside MgO.CaO.2SiO 2 Augite CaO.MgO.2SiO 2 + MgO.(Al,Fe) 2 3 .Si0 2 . JEgmne (Acmite) Na 2 O.Fe 2 O 3 .4SiO 2 . Triclinic. Rhodonite MnO.SiO 9 . AMPHIBOLES. Rhombic. Anthophyllite (Mg,Fe)O.SiO 2 . Monoclinic. Tremolite CaO.3MgO.4SiO 2 . Actinolite Ca0.3(Mg,Fe)0.4Si0 2 . Hornblende CaO.3(MgFe)O.4SiO 2 + Ca0.2MgO.Al 2 O 3 .3SiO 2 . Arfvedsonite 4(Na 2 ,Ca,Fe)O.4SiO 2 + 2(Ca,Mg)O.2(Al J Fe) 2 O 3 .2SiO 2 . Triclinic. ^nigmatite (Cossyrite) 2Na 2 O.9FeO.(Al,Fe) 2 O 3 .i2SiO 2 . The following facts emerge from a study of the above table : (i) that these minerals are, in the main, metasilicates ; (2) that the pure metasilicate of mag- nesia is rhombic, that of lime and magnesia, monoclinic,* and that of manganese, triclinic; (3) that the mono- clinic varieties, known as augite and hornblende, are characterized by the presence of the sesquioxides of * The pure lime metasilicate, wollastonite, is monoclinic. By some authors it is considered to be a pyroxene, although it does not possess the characteristic prismatic cleavage. 104 DESCRIPTIVE MINERALOGY aluminium and iron ;* (4) that in the monoclinic class the amphibole molecule is double that of the pyroxenes, which explains why, at high temperatures, augite is more stable than hornblende. Diopside Augite. Crystallizes in the monoclinic system with short - columnar habit in the direction of the vertical axis. The common forms are : Orthopinacoid, a (100). Clinopinacoid, b (oio). Prism, m (no). Hemipyramid, s (in). M : m = 92 50'. Twinning on the orthopinacoid, a (100). Colour, various shades of green to colourless (diop- side) ; also brown to black (augite). Lustre, vitreous. FIG. 72. AUGITE. a, Orthopinacoid ; b, clinopinacoid ; s, hemipyramid ; m, prism. FIG. 73. TWINNED CRYSTAL OF AUGITE. Transparent to opaque. The plane of the optic axes coincides with clinopinacoid. Index of refraction, 1*7 ; double refraction, positive and strong (7 -a = 0*030). Streak, white to grey. Prismatic cleavage, fairly perfect. * Augite may be regarded as a combination of ;;/ molecules of diopside with n molecules of Tschermak's silicate (MgFe)0.(AlFe)-A.SiO 2 . ROCK-FORMING MINERALS 105 Parting, parallel to the orthopinacoid (diallage). Frac- ture, uneven to subconchoidal. Hardness, 5-6. Density, 3-2-3-6. Fusibility, 4. Insoluble in acids. Common augite occurs in dolerite, basalt, and in certain trachytes and andesites. The green and white varieties (diopside) are found in peridotites, and as an accessory constituent in some metamorphic limestones. Diallage is an essential constituent of gabbro; while the rhombic pyroxenes occur in some varieties of diorite, gabbro, dolerite, andesite, and peridotite. Actinolite Hornblende. Crystallizes in the mono- clinic system, with long or short columnar habit, due FIG. 74. HORNBLENDE. m, Prism ; b, clinopinacoid ; r, hemipyramid. FIG. 75. TWINNED CRYSTAL OF HORNBLENDE. to the predominance of the prism, m (no), frequently without the orthopinacoid, a (100), but seldom without the clinopinacoid, b (oio) (see Fig. 72). Twinning about the orthopinacoidal plane is frequent. Colour, various shades of green to almost colourless (actinolite) ; also dark brown to black (hornblende). Streak, white to grey. Lustre, vitreous. Transparent to opaque. Index of refraction, i '64. Double refraction 106 DESCRIPTIVE MINERALOGY strong, negative. Optic axial plane, the clinopinacoid. Prismatic cleavage, perfect. Fracture, uneven to sub- conchoidal. Brittle. Hardness, 5-6. Density, 2*9-3-4. Fusibility, 3-4. Insoluble in acids. Common hornblende occurs in certain varieties of granite and syenite, also in diorite, trachyte, and ande- site. Actinolite is found in blades, needles, and fibres in schists, amphibolites, and epidiorites. Nephrite, a variety of actinolite, forms a closely knit plexus of minute fibres and blades in the hard and tough sub- stance which is so much prized under the name of jade. Tremolite occurs in metamorphic limestones. THE OLIVINE GROUP. The olivines are ortho-silicates of lime, magnesia, iron, and manganese. They crystallize in the rhombic system, and form an isomorphous series, of which the following are the chief members : Forsterite: 2MgO.SiO 2 . Monticellite : CaO.MgO.SiO 2 . Fayalite: 2FeO.SiO 2 . Tephroite: 2MnO.SiO 2 . Common Olivine : 2(Mg,Fe)O.SiO 2 . Common olivine occurs in tabular or prismatic com- binations of pinacoids and domes ; also in irregular grains. Twinning on the brachydome (on). Colour, black, olive-green, or yellow. Lustre, vitreous. Trans- parent to translucent. Index of refraction, 1-678. ROCK-FORMING MINERALS 107 Double refraction, positive and strong (7 -a = '036). Optic axial plane (ooi). Brachy-pinacoidal cleavage, imperfect. Brittle. Fracture, conchoidal. Infusible. Decomposed by hydrochloric acid with separation of gelatinous silica. Certain yellowish-green and leek- green varieties of olivine are used as gem-stones, under the names of chryso- lite and peridote. FlG - 76.-OLivi NE . ,. . . c, Basal plane ; a, As a rock constituent, ollVine IS macro - pinacoid ; characteristic of the basic and ultra- basic rocks, occurring in basalts, dole- m, prism ; tf, macro- 1 i f dome. rites, and gabbros, and being the cruet constituent of the peridotites. Under the influence of the weather and percolating waters it is peculiarly liable to alteration, giving rise in some cases to serpentine, with separation of iron ore, in others to dolomite or calcite. SECONDARY MINERALS. The rock constituents described above have been produced by crystallization from a molten, or at least a plastic, condition ; but a considerable proportion of the rocks that make up the earth's crust are composed of minerals that have been accumulated and deposited by water. Such rocks, for instance, are the sandstones, clays, shales, and many limestones. The minerals com- posing these rocks are of earlier origin than the rocks themselves, being derived from pre-existing mineral aggregates by processes of disintegration and denuda- 108 DESCRIPTIVE MINERALOGY tion. These mineral aggregates, of course, may also have been of aqueous origin, but if we could trace back the history of such a series of changes sufficiently far, we should arrive finally at a primary crust, which must have had an igneous birth. The part played by water in the disintegration and decomposition of rocks, and in the distribution and rearrangement of the materials thus produced, is best exemplified by a specific case. Granite is a rock composed chiefly of quartz, felspar, with one or more micas or other ferro-magnesian con- stituent. Submitted to meteoric influences (rain, frost, percolating water, etc.), it decomposes into a loose, crumbling mass, which is ultimately washed away to form new combinations. Let us endeavour to trace the history of the three constituent minerals. First the quartz : this mineral, though chemically unaffected, becomes mechanically separated from its associates, and reduced by trituration to small partially rounded grains, in which condition it goes to form deposits of sand, and these when consolidated give rise to sand- stone. Mixed with the quartz, one would naturally expect to find occasional fragments of felspar and of light mica ; and such indeed is the case, as" may be seen in certain felspathic and micaceous varieties of sandstone. But most of the felspar is decomposed under the influence of acid surface waters : its alkalies are re- moved in solution as carbonates, and there remain behind certain hydrated silicates of alumina, which, although they constitute several distinct minerals, are ROCK-FORMING MINERALS 109 for convenience generally referred to as kaolin. These substances are also found in clays, which are in great part derived from the decomposition of felspathic rocks. Finally, the dark mica, or the ferro-magnesian con- stituent, is first converted into chlorite, which may in turn undergo decomposition, the products being re- moved in solution. With regard to the dissolved portions, the alkalies (potash and soda) and alkaline earths (lime and magnesia) unite with materials derived from other sources to form chemical precipitates of various salts e.g., calcite, dolomite, magnesite, gypsum, rock- salt, etc. Most of these minerals are included with the salts in Chapter III. (p. 208); but a few notes on chlorite, serpentine, talc, kaolinite, epidote, and the zeolites are appended here. Chlorite Group. The minerals included under this head are hydrated silicates of magnesia, iron and alumina, the water of which (about 12 per cent.) is only given off at a high temperature. Monoclinic, with pseudo-hexagonal symmetry. Generally in small scales or fibres, sometimes aggregated to spherular or spiral forms. The well-crystallized varieties are known as ortho- chlorites ; those that occur in scales and fibres, leptochlo- rites. The orthochlorites are regarded by Tschermak as isomorphous mixtures of a serpentine molecule (Sp) and an amesite molecule (At). 110 DESCRIPTIVE MINERALOGY The isomorphous series may be represented thus : Serpentine : Sp = 2H 2 O.3(Mg,Fe).O.2SiO 2 . Penninite : Sp. At. Prochlorite : Sp 3 , At l7 . Corundophilite : Sp. At 4 . Amesite : At = 2H 2 O.2(Mg,Fe)O.Al 2 O 3 .SiO 2 . The chlorites are dark green in colour. Streak, white. Index of refraction, r6. Double refraction, weak (y - a = 0*003). Basal cleavage, perfect. The cleavage flakes are pliable, not elastic, as with mica. Hardness, 2-3. Density, 2*6-3*0. Fusible with diffi- culty. Decomposed by sulphuric acid. In one form or another the chlorites are a very frequent alteration product, especially in the more basic igneous rocks. The green colour of many of the latter (greenstones) is due to the presence of the mineral. Serpentine. Hydrated silicate of magnesia and iron : 2H 2 O.3(Mg,Fe)O.2SiO 2 . Crystal system, uncertain. Occurs massive as an aggregate of blades, scales, and fibres. Colour, dull green, often stained red and yellow by iron oxides. Lustre, dull to resinous. Translucent to opaque. Streak, white to grey. Hardness, 3-4. Density, 2'5-2'7. Index of refraction, rtf. Double refraction, negative, moderate (y - a = o*oi). Fusi- bility, 6. Decomposed by hydrochloric acid. Occurs as an alteration product of olivine and other ferro-magnesian silicates. Veins of fibrous serpentine (chrysotile) are worked as a source of commercial asbestos. In these the direction of the fibres is at ROCK-FORMING MINERALS 111 right angles to the vein, whereas in fibrous hornblende, which is also known and used as asbestos, the long axis of the fibres is parallel to the seams in which they occur. Talc, steatite, or soapstone, is a hydrated silicate of magnesia (H 2 O.3MgO.4SiO 2 ) crystallizing in the monoclinic system, but with pseudo-hexagonal sym- metry. It is a pale green or colourless mineral, very soft, and with a greasy feel. Lustre, pearly. Trans- lucent. Index of refraction, 1*55. Double refraction, strong (y -a = 0*040). Hardness, i. Density, 27-2*8. Cleavage, basal; flakes, non-elastic. Fusibility, 6. In- soluble in acids. Talc occurs as an alteration product of magnesian minerals in igneous and metamorphic rocks. Kaolinite is a constituent of kaolin, and probably of all clays. It is a hydrated silicate of alumina (2H 2 O.Al 2 O 3 .2SiO 2 ), crystallizing in the monoclinic system, but with pseudo - hexagonal symmetry. It occurs in minute colourless six-sided plates and scales. Hardness, 1-2. Density, 2-34-2-57. Kaolinite is one of the minerals produced in the alteration of felspar. Under the influence of the change known as kaolinization, felspar loses its glassy appearance, becomes dull, and finally crumbles down into a white mealy powder (kaolin), which contains kaolinite. Kaolin mixed with quartz is, consequently, often found in the immediate neighbourhood of granite. Epidote. Silicate of lime, alumina, and iron : H 2 O.4CaO.3(Al,Fe) 2 O3,.6SiO 2 . Monoclinic with elon- 112 DESCRIPTIVE MINERALOGY gated habit parallel to the orthodiagonal axis. Colour, yellowish green. Lustre, vitreous. Translucent. Streak, grey. Index of refraction, 1*75. Double re- fraction, negative, strong (y a = 0-04), Perfect cleavage parallel to the basal plane. Hardness, 6*5. Density, 3*4. Fracture, uneven. Fusibility, 3*5. Partially de- composed by hydrochloric acid. Epidote occurs as a frequent alteration product of the ferro-magnesian minerals, in gabbros, diorites, epidiorites, and hornblendic and chloritic schists. It is also found veining these rocks, or associated with other secondary minerals in the amygdules of old lavas. The Zeolite Group. A group of hydrated silicates of various bases : alumina, potash, soda, lime, baryta, and strontia. They are secondary products, occurring in igneous rocks as the infillings of amygdaloidal cavities (especially of melaphyres and basalts), or as pseudo- morphs after decomposed minerals (e.g., nepheline). Generally speaking, the zeolites are colourless to white, and occur in fibrous and radiate aggregates. The index of refraction is low. All of them also exhibit low double refraction. They are easily decom- posed by hydrochloric acid, with separation of gelatinous silica. Some of the more commonly occurring varieties are : heulandite, natrolite, analcime, phillipsite, laumontite, scolecite, and apophyllite. The composition of these is given in the following tables : ROCK-FORMING MINERALS 113 Name. Crystallographic System. Formula. Heulandite ... Monoclinic H 4 CaAl 2 (SiO 3 ) 6 + 3H 2 O Natrolite Rhombic Na 2 Al 2 Si 3 O l0 + 2H 2 O Analcime Regular NaAl(SiO 3 ) 2 + H 2 O Phillipsite Laumontite ... Scolecite Monoclinic Monoclinic Monoclinic (K 2 ,Ca)Al 2 (Si0 3 ) 4 + 4^H 2 H 4 CaAl 2 Si 4 O 14 + 2H 2 O Ca(AlOH) 2 (SiO 3 ) 3 + 2H 2 O Apophyllite ... Tetragonal H 7 KCa 4 (Si0 3 ) 8 + 4irH 2 NT Percentage Composition. so* A1 2 3 . CaO. Na 2 0. K 2 0. H 2 0. Heulandite 59-2 16-8 9'2 _ _ 14-8 Natrolite 47*4 26-8 16-3 9'5 Analcime 54'5 23-2 14-1 8-2 Phillipsite 48-8 207 7'6 6-4 i6'S Laumontite 51-1 217 11-9 I5'3 Scolecite 45*9 26*0 i4'3 irs Apophyllite 537 ~ 25-0 5-2 16-1 MINERALS OF THE GRANITE CONTACT ZONE. Andalusite. Silicate of alumina : Al 2 O 3 .SiO 2 (silica, 36*8; alumina, 63*2 per cent.). Rhombic; occurring in square thick-set prisms, terminated by the basal plane. Crystals of this mineral usually appear dark-coloured (reddish-brown), owing to the presence of included carbonaceous matter. In thin section, however, the grains are either colourless or pink. Lustre, vitreous. Translucent. Streak, colourless. Index of refraction, 114 DESCRIPTIVE MINERALOGY 1*638. Double refraction, negative, moderate (y-a on). Cleavage prismatic. Fracture, uneven. Brittle. Hardness, 7-7*5. Density, yi-^'z. Infusible. In- soluble in acids. Occurs in slates and shales that have undergone metamorphism in contact with granite ; also in gneisses and crystalline schists, and as an accessory constituent of granite (e.g., at the Cheesewring in Cornwall). Chiastolite is a variety of andalusite, con- taining graphitic material arranged along the diagonals of the prism. It is found in small light-coloured prisms in chiastolite-slate in the neighbourhood of granite (e.g., Skiddaw). Sillimanite. Silicate of alumina : Al 2 O 3 .SiO 2 (silica, 36*8; alumina, 63*2 per cent.). Rhombic; with pris- matic habit, without definite terminal faces, often in long and slender crystals ; sometimes fibrous (fibroUte). Colour, brown to greyish-green ; in thin section, colour- less. Lustre, vitreous. Transparent. Streak, white. Index of refraction, fairly high ( 1*667) ; double refrac- tion, moderate, stronger than that of andalusite. Cleavage parallel to the brachypinacoid, perfect. Hard- ness, 6-7. Density, 3^23 - 3*24. Infusible. Insoluble in acids. Occurs in gneisses and crystalline schists, often in the aureoles of metamorphism around granite, in association with cordierite, corundum, andalusite, and kyanite. Kyanite. Silicate of alumina: Al 2 O 3 .SiO 2 (silica, 36*8 ; alumina, 63*2 per cent.), like sillimanite and andalusite. Triclinic ; usually in long prismatic crystals. ROCK-FORMING MINERALS 115 Colour, white to blue. Lustre, vitreous. In thin section, colourless to pale blue, with weak pleochroism. Index of refraction, high (172). Double refraction, negative, strong (y-a = o'oi6). Cleavage parallel to the macropinacoid, perfect, with partings parallel to the basal plane. Fracture, fibrous. Brittle. Hard- ness =4 -7. Density = 3*5-37. Kyanite occurs in gneisses and crystalline schists in association with garnet, staurolite, and sillimanite ; often in the aureoles of metamorphism round granite. It is also found in sands and clays in association with rutile, tourmaline, zircon, etc. According to Vernadsky, kyanite, when heated to 1300 C., is converted into sillimanite. Staurolite. Hydrated silicate of alumina, iron, and magnesia: 2H 2 O.6(Fe,Mg)O.i2Al 2 O 3 .nSiO 2 . Rhombic. In prismatic forms terminated by the basal plane; commonly twinned, forming symmetrical Maltese and St. Andrew's crosses. Colour, reddish-brown. Lustre, vitreous to resinous. Translucent to opaque. Index of refraction, high (174). Double refraction, moderate but slightly stronger than quartz. Brachypinacoidal cleavage, perfect. Fracture, conchoidal to uneven. Hardness, 7-75. Density, 3'3-3'8. Infusible. Un- attacked by acids. Occurs in the crystalline schists, and in rocks of the granite contact-zone. Cordierite. Hydrated silicate of alumina, iron, and magnesia : H 2 O.4(Mg,Fe)O.4Al 2 O 3 .ioSiO 2 . Rhombic ; with pseudo-hexagonal symmetry. Often twinned. Colour, dark blue ; in thin section usually colourless. 116 DESCRIPTIVE MINERALOGY Lustre, vitreous to resinous. Transparent to trans- lucent. Pleochroic. Refractive index, 1*536. Double refraction (y- = '007), slightly lower than that of quartz. Brachypinacoidal cleavage perfect. Parting parallel to basal plane. Fracture, subconchoidal. Hardness, 7-7*5. Density, 2*6. Cordierite occurs in granites and gneisses (cordierite- gneiss) ; more rarely in volcanic rocks (e.g., basalt, andesite). It alters easily into mica-like decomposition products (pinite, esmarkite, praseolite, gigantolite, etc). Idocrase or vesuvianite. Hydrated silicate of lime and alumina ; probably 2H 2 O.i2CaO.3Al 2 O 3 .ioSiO 2 , Small quantities of manganese, iron, magnesium, and alkalies, are often present. Tetragonal. In square prisms, with basal plane and pyramid. Colour, dark green, brown, red, or yellow. Lustre, vitreous. Index of refraction, high (172). Double refraction, weak (w - e = *ooi). Prismatic cleavage imperfect. Fracture, uneven. Hardness, 6-7. Density, 3.35-3.45. Fusi- bility, 3. Scarcely attacked by acids. Occurs in lime- stones that have undergone alteration by contact with igneous rocks. Axinite. A borosilicate of calcium and aluminium : 7CaO.2Al 2 O 3 .B 2 O 3 .8SiO 2 , in which lime maybe partially replaced by manganese, iron, and magnesium. Tri- clinic. In broad crystals with acute edges. Colour, honey-yellow to clove-brown. In thin section, colourless to pale yellow or violet. Lustre, vitreous. Transparent to translucent. Index of refraction, high (1*677). ROCK-FORMING MINERALS 117 Double refraction, moderate (y-a = -oo9). Brachy- pinacoidal cleavage, distinct. Fracture, conchoidal to uneven. Brittle. Hardness, 6-5-7-0. Density, 3*271- 3*29. Occurs in the contact-aureoles of granite. Topaz. A silicate and fluoride of aluminium : Al 2 O 2 (OH,F) 2 .SiO 2 . Rhombic. In short prismatic crystals with pyramidal and basal terminations. Colour- less, wine-yellow or tinted blue, red, or green. In thin section, colourless. Lustre, vitreous. Transparent to translucent. Index of refraction, 1*62. Double refrac- tion, moderate (y-a = *oog). Basal cleavage, perfect. Hardness, 8. Density, 3*4-3*6. Infusible, and un- attacked by acids. Occurs in some granites and pegmatites ; also in the contact-aureoles of granite. Datolite. A basic orthosilicate of calcium and boron : H 2 O.2CaO.B 2 O 3 -2SiO 2 with SiO 2 = 37-6, B 2 O 5 = 21-8, CaO = 35'0 and H 2 O = 5*6 per cent. Monoclinic. In stumpy, prismatic forms, or irregular grains. Colourless to white. Lustre, vitreous. Transparent to translucent. Index of refraction, moderate (1*65). Double refraction, very strong (y a= -0448). No defined cleavage. Frac- ture, subconchoidal. Hardness, 5. Density, 2*9-3*0. Fusibility, 2-2*5. Not attacked by acids. Occurs in association with zeolites and calcitein the amygdaloidal cavities of basalt ; also in the zones of contact-meta- morphism. CHAPTER II THE ORES THE ores are those minerals from which the metals can .be profitably extracted. They consist of the oxides, sulphides, chlorides, carbonates, sulphates, etc.. of the metals, and, in a few cases, of the native metals them- selves e.g., gold, platinum, silver, and copper. Although they exceed the rock-forming minerals in number, in bulk they constitute an insignificant fraction of the earth's crust, occurring as ore-bodies in its fissures and cavities, or as small particles and grains disseminated through its constituent rocks. Only exceptionally are masses of ore encountered that are large enough to bear comparison with the rocks themselves e.g., in the case of certain iron ores. It is outside the scope of this small book to discuss either the mode of occurrence or the genesis of the ore- deposits. For the present purpose it must suffice to distinguish between (i) ore -deposits formed in situ, whether occurring as lodes, veins, masses, or beds, and of whatever mode of origin whether formed by mag- matic differentiation (i.e., concentration in an igneous magma), pneumatolysis (deposition from vapours), 118 ORES 119 hydatogenesis (deposition from water), or metaso- masis (chemical replacement) ; and (2) detrital or placer deposits, in which the minerals have been derived by erosion from pre-existing formations, and have been accumulated in hill-talus, river-gravels, or sea-beaches e.g., alluvial gold, stream tin, and iron sand. The first-named is by far the larger of the two classes of ore-deposits, and is in general also the more important as a source of the metals. For, although a larger quantity of gold or tin may in any given district be temporarily won from superficial detrital deposits (' placers '), these become comparatively soon ex- hausted, or their working is prohibited by State legisla- tion on account of the injury done to the soil by the disposal of the debris from the gold-washings ;* and the miner must then perforce turn his attention to the more lasting veins and beds of ore that extend deep down into the crust of the earth. Here again a further dis- tinction is necessary between the primary ores, found in those parts of the lodes and beds that are below the permanent water-level, and the secondary, oxidized ores that characterize those superficial portions within the belt of weathering. Whereas the latter are remarkable for their variety and complexity, and comprise the bulk of the multifarious minerals classed as ores, the former are in general limited to a few simple sulphides. * Thus, Central California, which at one time had an output of gold from placer-mining, valued at three and a half millions sterling per annum, has, in consequence of the action of the State, entirely ceased to produce gold from this class of deposit. 120 DESCRIPTIVE MINERALOGY Thus, the great class of secondary copper ores are derived in the main from the primary sulphide of copper and iron known as chalcopyrite or copper pyrites. Similarly, both lead and silver ores mostly take their origin from primary sulphides of these metals, which often occur in isomorphous association in the same mineral (galena). Again, blende (sulphide of zinc) is the primary source of zinc ores, and also of rather rare cadmium minerals, the two metals being closely allied and frequently associated. Gold, also, although not chemically combined with, is closely associated with, and even mechanically in- cluded in, pyrites, chalcopyrite, and mispickel, and is only set free by the breaking down (oxidation) of these pyritic ores when brought within the zone of weathering by the natural lowering of the ground-water level during the ordinary process of denudation. In the following pages a brief description is given of the metals : platinum, gold, mercury, copper, silver, lead, zinc, nickel, cobalt, iron, manganese, bis- muth, antimony, arsenic, vanadium, tin, titanium, molyb- denum, tungsten, uranium, and aluminium. ORES OF PLATINUM. The native metal is the sole source of commercial platinum. A compound with arsenic (sperrylite) is known, but it has only been found in one or two places. On account of its infusibility and the difficulty with which it is attacked by acids, platinum constitutes a valuable material for chemical vessels crucibles, ORES 121 dishes, etc. It is also largely used in the electric light industry and in the dental and photographic trades, as well as by manufacturing jewellers. At 8 per ounce, it is almost twice as valuable as gold. Native Platinum occurs in small flattened grains and scales, but occasionally in larger nuggets. It crystal- lizes in the regular system ; but crystals are rare, only small cubes having been occasionally found. Its colour lies between a steel grey and a silver white. It takes a higher polish than silver. Hardness, 4*5-5 ; fracture, hackly ; density, 14-19, amounting in chemi- cally pure platinum to 21*5. Platinum is thus one of the heaviest metals known. Malleable and ductile. Infusible before the blowpipe, except in the very thinnest wire. Insoluble in acids, except aqua regia, in which it is easily dissolved to platinum chloride. It is also attacked by caustic alkalies. Platinum occurs as a primary constituent of the peri- dotites of the Urals (Nischne Tagilsk, Mount Solovief) ; but the chief source of supply are the placers in the valleys of the rivers (Issa, Wyja, Tura, and Njassma) draining the same districts. It is associated in the sands of these placers with chromite and magnetite. The ore consists of a mixture of platinum with osmium-iridium, and the metal is besides alloyed with palladium, rhodium, iridium and smaller quantities of osmium and iridium. Iron is invariably present, in quantities from 4 to 13 per cent. The output of the Russian deposits amounts to 200,000 ounces per annum. 122 DESCRIPTIVE MINERALOGY Outside Russia, the State of Colombia (districts of Choco and Barbacoas) is the largest producer (7,000 ounces per annum). It is also found in British Columbia (Tulameen River), Northern California, Brazil (Minas Geraes), Assam, Borneo, and New Zealand (River Tayaka"). Sperrylite. Arsenide of platinum : PtAs 2 (platinum 56*47 per cent). Crystallizes in the regular system, with pentagonal hemihedrism. Habit, cubic or octahedral. Colour, tin white. Opaque. Lustre, metallic. Streak, black. Fracture, conchoidal. Brittle. Hardness, 6-7. Density, 10*6. Infusible. Soluble in aqua regia. A rare mineral, and only interesting as the one ore of platinum known besides the native metal. Occurs in Ontario, Canada (Vermilion Mine), and North Carolina (Cowee Valley). ORES OF GOLD. The chief supply of the noble metal is native gold, other ores being comparatively rare. It is true the tellurides (sylvanite, krennerite, calaverite, petzite, etc.) are worked in a few places, but, in proportion to the production of the whole world, the supply from this source is extremely small. Gold amalgam (a compound of the noble metal with mercury) is of no importance as an ore. The original source of the bulk of native gold is in auriferous quartz veins, and in conglomerate beds (" banket ") ; but a small proportion is possibly a ORES 123 primary syngeftetic constituent of ingenous rocks. Within the belt of weathering the gold of these deposits occurs free, and the ore is " free-milling " i.e., it is amenable to amalgamation when, after suitable crushing, the ore mixed with water (the pulp) is passed over copper plates coated with quicksilver ; but below the permanent water-level it is closely associated with, and to a considerable extent mechanically included in, pyrites (auriferous pyrites), and in the treatment of such ores (" pyritic ores ") it is essential to reduce them to a sufficient state of fine subdivision to enable the gold to be extracted by cyanide solution. What the exact genetic relation is between the pyrites and the gold has not been satisfactorily settled, but it is clear that a community of origin is indicated. Besides being associated with pyrites, gold, being iso- morphous with silver, lead, and copper, is almost always present in the ores of these metals, and it is usually an important by - product both in the metallurgy of silver and of copper. Sometimes, how- ever, the gold is, from the economic standpoint, the dominant constituent, as in the gold-copper pyrrhotite veins of Rossland in British Columbia, and in the gold- silver telluride veins of Transylvania, of Cripple Creek in Colorado, of Tonapah in Nevada, of Mexico, and of Kalgoorlie in West Australia. A large quantity of gold is won from transported material. Such are the screes and talus debris of mountain slopes (drift gold), the sands of the rivers that drain them (alluvial gold), and the beach gravels that 124 DESCRIPTIVE MINERALOGY accumulate near the mouths of the rivers (beach placer gold). Transported or placer gold is derived from the disintegration of the gold-bearing quartz veins, so common in the old crystalline rocks of mountain dis- tricts. The metal is also found in mudstones, sand- stones and conglomerates, in which it may have either accumulated at the time of the formation of the deposit or been introduced subsequently by deposition from solution. The most important gold-fields are situated in Australasia (Western Australia, Victoria, New South Wales, Queensland, New Zealand), the United States (California, Nevada, Arizona, Montana, Colorado, and Alaska), Canada (British Columbia and the Yukon), Mexico, Colombia, Venezuela, Guiana, Chili, Peru, Brazil, South Africa (Witwatersrand, Barberton, and Lyden- burg, in the Transvaal, and Rhodesia), West Coast of Africa, East Indies (Sumatra, Java, Borneo), British India (Kolar gold-field in Mysore), Russia (Siberia and the eastern slopes of the Ural Mountains). Native Gold crystallizes in the regular system ; but crystal forms Cas a rule, octahedral or dodecahedral) are rare, indistinct, and distorted. More usually it occurs in branched or wiry aggregates, in leaf-like ex- pansions, or in a finely divided condition as mustard gold, paint gold, and sponge gold ; also as minute grains, generally in intimate association with pyrites dissemi- nated through quartz. It is also found in gravels and sands as dust, and in loose grains and nuggets. Large nuggets are occasionally found : thus, one from Upper ORES 125 California weighed 161 pounds ; and others have been found in Australia, one of which yielded 2,268 ounces of gold. Gold is the most malleable and the most ductile of all metals. It is soft, having a hardness of only 2*5-3. Its fracture is hackly. Its colour and streak vary from a reddish to a brassy yellow, the variation being caused by the presence of small quantities of silver and copper. The amount of silver present in native gold varies from i to 40 per cent. ; alloys with over 20 per cent, are termed electrum. The density of native gold ranges from 15*6 to 19*4, being less with increasing percentage of silver. The density of pure gold is 19*37. The metal is insoluble in single acids, but dissolves readily in a mixture of nitric and hydrochloric acids (aqua regia). Fusible with ease in the flame of the blowpipe (2*5-3 on Von KobelPs scale). Auriferous Pyrites. In the deeper-seated portions of quartz veins, the gold is often found in intimate but mechanical association with iron, copper, arsenical, or magnetic pyrites ; and no doubt a considerable pro- portion of the so-called " free gold " occurring in the oxidized (weathered) zone of gold-bearing deposits has been liberated by the decomposition of these minerals. Gold Amalgam. An alloy of gold with mercury. In soft, yellowish-white grains and balls. Density, 15*5. Occasionally accompanies native gold in California and Colombia. 126 DESCRIPTIVE MINERALOGY Calaverite. Telluride of gold: AuTe 2 (Au 44*03, Te 55*97, per cent.). As a rule silver is also present. Occasionally crystallized in striated crystals of prismatic habit, but more commonly massive. Colour, bronze yellow, with metallic lustre. Fracture, uneven to semi- conchoidal. Brittle. Hardness, 2-3. Density, 9. Fuses easily (T on Von Kobell's scale), yielding on charcoal before the blowpipe a globule of gold. Soluble in aqua regia. Occurs in California (Calveras County), Colorado (Cripple Creek) and Western Australia (Kalgoorlie). Sylvanite. Telluride of gold and silver: AuAgTe 4 (Au 24-45, Ag 13*39, Te 62-16, per cent.). Crystal- lizes in the monoclinic system, with tabular habit after the clinopinacoid (oio), or with predominant orthodome (101) and basal plane (ooi), or pseudo- rhombic with dominant pinacoids (oio) and (100). Often twinned and reticulated (graphic tellurium), and in scaly to granular aggregates. Colour and streak, silver white to steel grey, with metallic lustre. Cleavage, perfect parallel to the clinopinacoid. Frac- ture, uneven. Sectile. Hardness, 1-2. Density, 7*9-8*3. Easily fused (i on Von Kobell's scale). Imperfectly soluble in nitric acid (with separation of gold) ; soluble in aqua regia, with ' separation of silver chloride. Occurs in Transylvania in Hungary, Colorado (Cripple Creek), Western Australia (Kalgoorlie). Krennerite. Telluride of gold and silver : (Au, Ag)Te 2 (the percentage of gold varies from 24-45 to 44'O3). Crystallizes in the rhombic system, with prismatic ORES 127 habit and basal termination. Vertically striated. Colour, silver white to light brassy yellow, with metallic lustre. Cleavage, perfect parallel to the basal plane. Fracture, uneven to semi-conchoidal. Hardness, 2-3. Density, 8*35. Easily fusible (i on Von Kobell's scale). Occurs in Transylvania (Nagyag), Colorado (Cripple Creek), and West Australia (Kalgoorlie). Petzite. Telluride of silver and gold: (Ag,Au) 2 Te. When free from gold, petzite contains theoretically 63*27 per cent, of silver ; it may contain, however, as much as 25 per cent, of gold. With Ag : Au=3 : I, the percentages are silver, 42 ; gold, 25*5. Crystal- lizes in the regular system, in cubes or distorted forms ; also massive or granular. Colour, leaden grey to steel grey, with metallic lustre. Hardness, 2-3. Density, 8'3-g'o. Fracture, uneven to semi-conchoidal. Sectile to brittle. Easily fusible (1*5). Occurs in the Altai, Transylvania, California, Colorado (Cripple Creek), and other places. ORES OF MERCURY. The only important source of mercury is the sulphide, cinnabar ; but the metal also occurs, though rarely, in the native state, globules of quicksilver being found as an alteration product in the oxidation zone of quick- silver deposits. Metacinnabarite, the black sulphide, is also an alteration product of cinnabar, and other rare associates are the chloride, the telluride, and the selenide of mercury. A gold amalgam (Au,Hg) and 128 DESCRIPTIVE MINERALOGY a silver amalgam (Ag,Hg) are also known, but on account of their rarity they are of no importance as ores. Mercury finds a variety of uses in the arts : it is greatly used in the extraction of gold and silver from their ores (in the so-called "amalgamation" processes), in dentistry, and in the manufacture of scientific in- struments (e.g., the barometer and thermometer) ; its salts are also employed in medicine. Cinnabar. Sulphide of mercury: HgS (mercury 86'2 per cent.). Crystallizes in the hexagonal system, with rhombohedral hemihedrism. Occurs in thick tabular crystals, composed of the basal plane in com- bination with a series of rhombohedral faces. More frequently, however, it is found massive or as an earthy incrustation. Colour, a bright crimson or cochineal red. Streak, scarlet. Transparent to translucent. Lustre, adamantine. Prismatic cleavage, perfect. Fracture, conchoidal to uneven. Sectile. Hardness, 2 - 2*5. Density, 8-8'2. Volatile before the blowpipe. Heated carefully in the open tube, yields metallic mercury and fumes of sulphur. Decomposed by aqua regia, with separation of sulphur. Cinnabar is important as the sole source of the mercury of commerce. It occurs in veins, irregular masses, or disseminated in grains through sandstone, and is often accompanied by pyrites, marcasite, chalco- pyrite, stibnite, realgar, and mispickel. The chief mines are in Southern Spain (Almaden), Austria (Idria in Carniola), Italy, Russia, California ORES 129 (New Almaden, New Idria, Sulphur Bank, Clear Lake), Mexico (Guad alcazar, Huitzuco), Peru (Huancavelica), China (Kweichou). ORES OF COPPER. Copper, one of the earliest metals known to man, was much prized by the ancients on account of its toughness. It was used by them, for instance, in an alloy with one-tenth of its weight of tin, for the manu- facture of weapons and tools. The alloy with zinc (brass) was also in great use for ornamental work. There are a great number of minerals containing copper, but comparatively few are of commercial importance as a source of the metal. In the upper weathered portion of the lodes are found the oxidized ores, which, besides the oxides, cuprite and melaconite, include the carbonates, malachite and chessylite ; the silicate, chrysocolla ; the sulphate, chalcanthite ; the sulphide, covellite ; and native copper. In the zone of secondary enrichment which occurs immediately below the belt of weathering are found the rich sulphides, chalcocite and bornite, and the sulpharseniate, enargite ; while in the deepest parts of the deposits the copper is confined to the primary ore, chalcopyrite, either alone or in association with iron pyrites (cupriferous pyrites). Besides these simple compounds, copper is also obtained in considerable quantity from certain complex ores, such as the sulphantimonite and sulpharsenite of copper, silver, iron, and zinc (e.g., fahl-ore and ten- nantite). 9 130 DESCRIPTIVE MINERALOGY Copper is one of the most valuable metals employed in the arts, and perhaps its greatest application is as a conductor of electricity. A great quantity also goes into consumption in the form of sheet copper or as castings, and the metal is largely used in the manu- facture of brass, bronze, and other alloys, in elec- trolysis, and as a chemical agent in the form of blue vitriol (copper sulphate) and other salts. The production of copper is about 847,000 tons (of 2,240 pounds) per annum (1910), distributed as follows : Tons. United States ... ... ... 483,000 Mexico ... ... ... 59,000 Spain and Portugal ... ... 48,000 Australasia ... ... ... 42,000 Chili ... ... ... ... 41,000 Japan ... ... ... ... 41,000 Germany ... ... ... 24,000 Russia ... ... ... 23,000 Canada ... ... ... 22,000 Peru ... ... ... ... 20,000 Africa ... ... ... ... 16,000 Other countries ... ... ... 28,000 847,000 Native Copper. Regular. When crystallized, this metal occurs in small and large crystals, having the forms of the octahedron, the cube, and the rhombic dodecahedron, all of which are usually much distorted, and aggregated to irregular branching masses. Most ORES 131 frequently, however, no crystalline form is visible, the metal occurring in irregular lumpy masses, in wiry and mossy coils, or as foil and in plates. Its colour is the familiar copper-red, but the surface is often tarnished, and is then of a dirty yellow or brown colour. Lustre, metallic. The metal is malleable, ductile, and tenacious. No cleavage. Fracture, hackly. Its hardness is 2*5-3 ; its density, 8*5-8*9 ; and its fusibility, 3 on Von Kobell's scale. Native copper is usually chemically pure, but it sometimes contains iron and silver. Soluble in nitric and hydrochloric acids. Copper occurs, chiefly in association with secondary ores,"* in Cornwall and many places in Europe, Siberia, Brazil, Chili, Bolivia (Corocoro), Australia and Tas- mania, the United States (e.g., in the copper-mines of Lake Superior). A great mass, weighing 400 tons, was discovered in one of the mines at the last-named locality, and forty men were employed for twelve months in its extraction. Cuprite, or red copper ore. Copper monoxide or cuprous oxide : Cu 2 O (copper 88*8 per cent.). Crys- tallizes in the regular system, in well -formed octa- hedra, either alone or in combination with the cube and the rhombic dodecahedron. It is also found massive and granular, or as a brick- red earth (tile ore). It has a brilliant cochineal-red colour, which is best seen in transparent, or at least translucent, crystals, * Metallic copper is easily produced from cuprite by the action of sulphuric acid : Cu a O + H 2 SO4 = Cu + CuSO4 + H 2 O. 132 DESCRIPTIVE MINERALOGY or on reducing opaque specimens to powder. Lustre, metallic to adamantine, and streak, brownish - red. Its octahedral cleavage is fairly perfect. Fracture, conchoidal to uneven. Brittle. Hard- ness, 3*5-4. Density, 5*7-6-2. Fusi- bility (Von KobelPs scale), 2*5 - 3. Before the blowpipe on charcoal it yields a metallic globule of copper. The ease with which it can be reduced : 10.77- E - ma k es ft one of the best ores for the Octahedron. ... . . extraction of the metal, but it is only found as a decomposition product in the upper or oxidized portions of copper sulphide lodes. Of widespread occurrence, if limited in quantity for instance, in Cornwall (Liskeard, Redruth), France (Chessy), Nassau, Harz, Saxony, Silesia, Bohemia, Hungary, Italy, Spain, Siberia, Australia (Wallaroo, Moonta, Burra Burra, and Cobar), Tas- mania (Mount Lyell), Namaqualand in South Africa, Arizona (Clifton, Morenci, Bisbee, and Globe), Lake Superior, Alaska (Mount Wrangell), Mexico (Boleo), Peru, Chili, Bolivia (Corocoro), etc. Melaconite, or tenorite. Black oxide of copper, or cupric oxide : CuO (copper 79-86 per cent.). Crystal- lizes in the triclinic system, with pseudo-monoclinic symmetry and tabular habit (100). Twinning parallel to the macropinakoid (100) and the brachydome (on). Also massive, powdery, earthy, scaly, and cellular. Colour, black to grey. Lustre, metallic to dull. ORES 188 Opaque. Streak, dirty green. Cleavage, basal. Frac- ture, conchoidal to uneven. Thin flakes are elastic. Hardness, 3-4. Density, 5'8-6'3. Fusibility, 3. Soluble in hydrochloric and nitric acids. Occurs in the oxidized portion of copper lodes as a decomposition product of chalcopyrite, bornite, etc., but not in sufficient quantity to be of great importance as a source of copper. It is found on Vesuvius in lava as a sublimation product. Other localities are Corn- wall, Spain (Huelva), Siberia (Bogoslowsk and Nischne Tagilsk), Japan, Australia, Bolivia, Mexico (Boleo), Chili, Peru, United States (Tennessee, Arkansas, and Michigan). Chalcocite, copper glance, or redruthite. Sulphide of copper: Cu 2 S* (copper 79*83 per cent.). Crys- tallizes in the rhombic system. The crystals have a pseudo-hexagonal symmetry, occurring often in flat six-sided tablets, composed of pyramids and brachydomes, or of prisms and brachypinacoids, ter- minated in both cases by the FlG " 78CoppER-GLANCE. T i i ^, f A Basal plane ; z, pyra- basal plane. They are frequently m id ; e , brachydome. twinned, the twinning plane being a face of the prism. More frequently, however, the ore is massive, platy, or nodular. Colour, dull black. Lustre, metallic. Superficially often iridescent or tarnished by incipient alteration to covellite or bornite. Streak, * The impure sulphide, CuS (with copper 66*9 per cent.), Covellite, is a blue microcrystalline mineral (hexagonal), and rather rare. 134 DESCRIPTIVE MINERALOGY blackish-grey. Opaque. Prismatic cleavage, imperfect. Fracture, conchoidal. Slightly sectile. Hardness, 2-5-3. Density, 5'5-5'8. Fusibility, 2-2-5. After care- ful washing and rinsing with carbonate of soda, yields, before the blowpipe, a globule of copper. Chalcocite is a rich ore of copper, of frequent occur- rence in copper lodes, especially in the zone of secondary enrichment. Notable examples are to be found in Cornwall, Saxony, Hungary (Kapnik, Rezbanya, Ora- vicza), Italy (Monte Catini), Caucasus (Kiadebek), Eastern Russia (Bogoslowsk, Nischne Tagilsk, Spassky), South-West Africa (Namaqualand), Transvaal (Mes- sina), Australia (Wallaroo, Moonta, Burra Burra), Japan (Ashio, Besshi), Alaska (Mount Wrangell), Cali- fornia (Shasta County), Montana (Anaconda, near Butte), Arizona (Bisbee, Jerome, Clifton, etc.), Bolivia (Corocoro). Chalcopyrite, or copper pyrites. Sulphide of copper and iron, or sulphoferrite of copper: Cu 2 S.Fe 2 S 3 (copper 34*56, iron 30*52 per cent.). Tetragonal, the commonest form being the hemihedral sphenoid ; but the crystals are usually small and distorted, and conse- quently difficult to determine. Twinning on various types; most frequently with (in) as twinning plane (see Fig. 77). Mostly, however, it occurs massive, in large nodular, kidney-shaped, or botryoidal masses, and in scattered grains and specks. Its colour is brassy to golden yellow, being a stronger yellow than that of iron pyrites. The surface of the mineral is often iridescent in blue or red tints, which are the result of tarnish ORES 135 (peacock ore). Streak, black. Cleavage, parallel to (201), imperfect. Fracture, conchoidal to imperfect. Hardness, 3*5-4. Density, 4'i-4*3. Fusibility, 2. Before the blowpipe, on charcoal, yields black mag- netic globule ; when mixed with carbonate of soda yields a ferruginous copper globule. Dissolves in nitric acid, with separation of sulphur. It may be distinguished from iron pyrites by its less hardness, and from gold by its brittleness, since it crumbles under the point of the knife. Chalcopyrite is the most widely distributed of all FIG. 79. TWINNED CRYSTALS OF CHALCOPYRITE. copper ores, and is responsible for the bulk of the world's output of copper. It occurs as a mineral of sedimentary origin, as a product of magmatic con- centration in igneous rocks, and as a true vein deposit. A well-known example of the sedimentary ores is the Kupferschiefer, a thin bed of bituminous shale in the Zechstein (Permian) formation of the Southern Harz, in which chalcopyrite occurs as a fine dust in association with bornite, chalcocite, iron pyrites, and galena. These sulphides have been deposited from a solution of the metallic sulphates by the reducing 136 DESCRIPTIVE MINERALOGY action of decomposing animal matter. Chalcopyrite is invariably found in copper lodes when depths below the zones of weathering and of secondary enrichment are reached. It is the primary copper mineral, from which the other sulphides (chalcocite and bornite), and the oxides and carbonates that characterize the upper portions of the lodes, are derived. It is impossible to enumerate the localities for chalcopyrite, since it occurs wherever copper is mined ; the mention of a few of the copper-mining districts where this mineral bulks largely must suffice: Germany (Mansf eld, Rammelsberg), Saxony (Annaberg), Spain and Portugal (Huelva District, Rio Tinto, Tharsis, etc.), Ural Mountains (Bogoslowsk), Caucasus (Kiadabek), Altai (Tschudak and Songatof), Finland (Pitkaranta), Norway (Sulitelma, R0ros, Vigs- nas, Foldal, Ytter0), Sweden (Falun), Italy (Monte Catini, Massa Marittima, Mossetana, Boccheggiano), Austro- Hungary (Kitzbiihel in the Tyrol, Schmollnitz in Hungary, and Rezbanya in the Banat), South Africa (Ookiep, Spectakel, Nababeep, and Kopeberg, in Little Namaqualand), Australia (Great Cobar in New South Wales ; Wallaroo, Moonta, and Burra Burra, in South Australia; Mount Morgan in Queensland), Tasmania (Mount Lyell), Japan (Ashio, Ani Osarisawa, Besshi, and Kosaka), China (Yunnan and Kweichou Provinces), Canada (Rossland, Sudbury, Capeltown), Montana (Anaconda and other mines at Butte), Arizona (United Verde, Copper Queen, and other mines in the Jerome, Bisbee, Clifton, Morenci, and Globe Districts), Cali- fornia (Mountain Copper in Shasta County), Alaska ORES 137 CHRISTIANA MAP SHOWING COPPER ORE DEPOSITS IN NORWAY. 138 DESCRIPTIVE MINERALOGY (Copper Mountain, Mount Wrangell), many places in the Appalachian States, Mexico (Cananea, Boleo, and Montezuma), Northern Chili, Bolivia (Corocoro), Peru (Cerro de Pasco). Bornite, erubescite, purple ore, or horseflesh ore. Sulphide of copper and iron, or sulphoferrite of copper : 3Cu 2 S.Fe 2 S 3 (copper 55*57, iron 16*36 per cent.)- Crys- tallizes in the regular system. Habit, cubic. Usually occurs massive or in disseminated grains. Colour, reddish-brown to copper-coloured. Often iridescent. Opaque. Lustre, metallic. Streak, greyish-black. Octahedral cleavage, very imperfect. Fracture, sub- conchoidal to uneven. Not very brittle. Hardness, 3. Density, 4*9-5-4. Fusibility, 2*5. With carbonate of soda on charcoal yields a globule of copper. Soluble in nitric or concentrated hydrochloric acid, with separa- tion of sulphur. Bornite is a valuable ore of copper, occurring in the oxidized or in the secondarily enriched portion of the lodes for example, in Cornwall, Norway, Harz, Saxony, Siberia (Spassky), South Africa (Namaqua- land), Chili, Peru, Bolivia, Mexico, United States (Shasta County in California and Butte in Montana), and Canada. Enargite. Sulpharseniate of copper: As 2 S r 3Cu 2 S (copper 48*36 per cent.). Crystallizes in the rhombic system. Habit, columnar in the direction of the ver- tical, with vertical striation, or tabular parallel to the basal plane. Also massive, granular, or columnar. ORES 139 Colour, grey to iron black. Lustre, metallic. Streak, greyish - black. Cleavage, perfect, parallel to the prisms (no). Fracture, uneven. Brittle. Hardness, 3. Density, 4'4-4'5. Fusibility, i. On charcoal with car- bonate of soda yields a globule of copper. Heated in the open tube, gives off arsenical and sulphurous fumes. Soluble in aqua regia. Enargite occurs, as an ore of copper in the United States (Anaconda mine at Butte in Montana, Tintic mines in Utah, etc.), Argentine (Sierra Famatina), Philippines (Luzon). Tetrahedrite, fahl - ore, or grey copper ore. Sulphantimonite of copper, with a variable amount of copper replaced by silver, iron, and zinc: 3(Cu 2 ,Ag 2 , Fe,Zn)S.(Sb,As) 2 S 3 . The corresponding sulpharsenite of copper, 3Cu 2 S.As 2 S 3 , is tennantite. The pure sulphantimonite of copper contains theoretically 46*84 per cent, of copper; the sulpharsenite of copper, 52*64 per cent. Crystallizes in the regular system, with tetrahedral habit. Twinned parallel to a face of the octahedron. Colour, steel grey to iron black. Opaque. Lustre, metallic. Streak, black to reddish. No cleavage. Fracture, conchoidal to uneven. Very brittle. Hard- ness, 3-4. Density, 4*4-5'i. Fusibility, 1*5. A valuable ore of copper and of silver (of which it may contain up to 30 per cent.). Tetrahedrite occurs in Cornwall, Harz (Andreas- berg), Saxony (Freiberg), Hungary (Kremnitz), Silesia, Bohemia (Przibram), Nassau (Dillenburg), Spain, Russia (Bogoslowsk), Chili, Bolivia (Huanchaca, where it is 140 DESCRIPTIVE MINERALOGY worked as a silver ore), Peru, Mexico, and United States (Arkansas, Utah, Nevada, California), Australia (Broken Hill). Malachite. The green hydrated basic carbonate of copper : CuCO 3 .Cu(OH) 2 , or 2CuO.CO 2 .H 2 O (copper 59*3 per cent.). Crystallizes in the monoclinic system, but usually occurs massive or with a smooth mamillary surface and concentric fibrous internal structure. Colour, bright green. Streak, pale green. Lustre, silky to dull. Hardness, 3*5-4- Density, 4. Fusibility, 3. Reduced on charcoal before the blowpipe to globule of copper ; colours the flame green. Gives off water when heated in the closed tube. Dissolves in acids with effervescence. Malachite is of universal occurrence in the upper part of copper lodes, together with the other oxidized ores of copper, and is a valuable ore, when in sufficient quantity for profitable extraction. Well-known occur- rences are the following : Cornwall, Chessy in France, Spain, Siberia (Nischne Tagilsk, especially at the mine Mednoroudiansk), South Australia (Burra Burra), Ka- tanga in Central Africa, United States (especially Arizona), Mexico (Boleo), Chili, etc. Chessylite, or azurite. The blue hydrated basic carbonate of copper: 2CuCO 3 .Cu(OH) 2 (copper 55*3 per cent.). Crystallizes in the monoclinic system, but also occurs in massive or in earthy forms. Colour, deep blue. Streak, pale blue. Lustre, vitreous. Fracture, conchoidal. Hardness, 3*5-4. Density, 3*5-3*8. Fusi- ORES 141 bility, 3. Behaviour before the blowpipe and with acids same as for malachite. It accompanies malachite in the oxidized form of copper lodes, but is of less frequent occurrence. For localities, see the list given under malachite. Chrysocolla. Hydrated silicate of copper: CuO.SiO 2 . 2H 2 O (copper 36 per cent.). Amorphous. Massive and compact. Opaline to earthy. Vitreous to greasy lustre, or dull. Translucent to opaque. Colour, green to blue. Streak, bluish-white. Fracture, conchoidal. Brittle. Hardness, 2-4. Density, 2-2*2. Infusible. Mixed with carbonate of soda on charcoal before the blowpipe, yields metallic copper. Decomposed by acids, with separation of silica. Heated in the open tube, yields water. Occurs as a decomposition product of copper ores in the belt of weathering of lodes. Russia (Bogoslowsk, Nischne Tagilsk), South Africa (Nama- qualand), United States (Michigan, Arizona, California), Mexico (Boleo), Chili. Dioptase. Hydrated silicate of copper: CuO.SiO 2 . H 2 O (copper 40*2 per cent.). Crystallizes in the hexagonal-rhombohedral system, with short columnar habit, with rhombohedral terminations. Colour, emerald green. Streak, green. Lustre, vitreous. Translucent to transparent. Rhombohedral cleavage perfect. Frac- ture, conchoidal to uneven. Brittle. Hardness, 5. Density, 3*3. Double refraction, strong, positive. Before the blowpipe turns black, but does not melt. Colours the flame green. With carbonate of sodium 142 DESCRIPTIVE MINERALOGY on charcoal yields a globule of copper. Decomposed by hydrochloric acid with separation of silica. Dioptase is of rather rare occurrence. A well-known locality is the Kirghese Steppes, where it occurs in lime- stone. It has also been found in Chili, Peru, Arizona, and Central Africa (Congo). Chalcanthite. Hydrated sulphate of copper : CuSO 4 . 5H 2 O (copper 25*4). Crystallizes in the triclinic system. Also occurs massive or as incrustations. Colour, blue. Streak, white. Lustre, vitreous. Brittle. Hard- ness, 2*5. Density, 2'2. Fracture, conchoidal. Fusi- bility, 3. Soluble in water. Yields water when heated in the open tube. Occurs in small quantities only, as a decomposition product of chalcopyrite. Atacamite. Hydrated oxychloride of copper : CuCl 2 -f 3Cu(OH) 2 (copper 59*4). Crystallizes in the rhombic system. Also occurs massive. Colour, dark green. Lustre, vitreous. Streak, light green. Hard- ness, 3-3-5. Density, 37-3*8. Fusibility, 3-4. Yields a globule of copper on charcoal before the blowpipe. Gives off water when heated in the open tube. Soluble in acids. Occurs chiefly in the dry desert regions of Chili and Peru (Atacama), where it is worked as an ore of copper ; also in Mexico (Boleo). ORES OF SILVER. Silver occurs in the native state, but this is not an important source of supply. The largest amount of the metal is obtained from argentiferous galena, (Pb,Ag 2 )S, ORES 143 which may be regarded as an isomorphous mixture of argentite (Ag 2 S) and galena (PbS), both crystallizing in the regular system. A rhombic sulphide of silver acanthite also exists, and has been found, for example, in the silver-mines of Freiberg. If this mineral be regarded as isomorphous with the rhombic sulphide of copper chalcocite an explanation is afforded of the constitution of argentiferous chalcocite or stromeyerite, (Cu,Ag) 2 S, which also occurs at Freiberg and in the Altai. Silver also replaces copper in tetrahedrite, some varieties of which contain up to 30 per cent, of silver, and are then regarded as silver ores. Other important ores of silver are the sulphantimonites and sulph- arsenites of silver, lead and copper. Their chemical relationship is shown in the following table : Name. Chemical Formula. Cystallographic System. Argentite Ag 2 S Regular Argentiferous galena (Pb,Ag 2 )S Regular Acanthite Ag 2 S Rhombic Stromeyerite (Cu,Ag) 2 S Rhombic Polybasite 9(Ag,Cu) 2 S.Sb 2 S 3 Monoclinic Pearceite 9(Ag,Cu) 2 S.As 2 S 3 Monoclinic Stephanite 5Ag 2 S.Sb 2 S 3 Rhombic Pyrargyrite ... 3Ag 2 S.Sb 2 S 3 Hexag-rhombohedral Proustite 3Ag 2 S.As 2 S 3 Hexag-rhombohedral Tetrahedrite 3 (Cu,Ag) 2 S.Sb 2 S 3 Regular Freieslebenite 5(Pb,Ag 2 )S.2Sb 2 S 3 Monoclinic Of the haloid compounds of silver, the chloride, kerargyrite, is important ; the chloro-bromide, embolite, less so ; while the iodide, iodyrite, is quite rare. 144 DESCRIPTIVE MINERALOGY Silver ores occur in veins belonging to two main groups : (i) Those associated with volcanic rocks of late Mesozoic or Tertiary age ; and (2) those of much earlier age. The first group is well illustrated by occurrences in the Carpathians of Transylvania (Nagyag-Verespatak) and Hungary (Schemnitz, Kremnitz and Nagybanya- Kapnik) ; in the Andes of Bolivia (Potosi, Huanchaca, Oruro), Peru (Cerro de Pasco), and Colombia (Tolima) ; in the Sierras of Mexico (Durango, Fresnillo, Zacatecas, Guanajuato, Puchuca) and of Arizona ; in the Sierra Nevadas of California (San Bernardino) and Nevada (Comstock, Esmeralda, etc.) ; in the Wahsatch Range of Utah (Hornsilver, etc., in Beaver County) ; in the Rockies of Colorado (Boulder, San Juan, Silver Cliff, Rosita, etc.) ; in the Coromandel peninsula (Hauraki) of New Zealand ; and finally in Japan (Akita and the island of Sado). The second group is illustrated by occurrences in the silver-lead deposits of Saxony (Freiberg, Annaberg, and Schneeberg), of the Harz (Clausthal and Andreas- berg), Bohemia (Przibram), and Norway (Kongsberg). A large amount of silver is now obtained from Cobalt in Northern Ontario, Canada, where silver ores (native silver and argentite) are associated with ores of nickel and cobalt (smaltite, niccolite, chloanthite, and cobalt - ite), as well as bismuth and mispickel. The world's output of silver amounts to close on 220,000,000 ounces per annum, having a value of, roughly, 23,500,000. ORES 145 Native Silver. Regular. Native silver, when crys- tallized, presents cubical or octahedral forms, but the crystals are usually distorted or united to divergent branching masses. Most frequently, however, the metal occurs in strings and wiry coils, occasionally even assuming a moss-like character ; it is also found as plate or foil, or in massive lumps. One such mass from Kongsberg in Norway, which is preserved in the Copenhagen Museum, weighs about 5 hundred- weight. Silver is very malleable and ductile, ranking next to gold in these qualities. In hardness it lies between gold and copper, its position in Mohs' scale ranging from 2*5 to 3. The density of native silver varies from 10* i to ii ; that of pure silver is 10*5. Although naturally of a white colour, it is often tarnished super- ficially to a red, brown, or blackish colour. Lustre, metallic; fracture, hackly. The native ore contains traces of copper, arsenic, antimony, and iron. It is soluble in nitric acid, and gives a precipitate with hydrochloric acid. Fusibility, 2 (Von Kobell's scale). Native silver occurs in veins, associated with the other ores of silver, or intermingled with native copper as at Lake Superior. Other occurrences in the United States are in Arizona, Nevada (Comstock Lode), Colo- rado, North Carolina, etc. It is also found in Ontario (Cobalt), and in Australia. Large deposits occur in the mines of Peru and Mexico. In Europe it is found in Norway (Kongsberg), the Harz, Saxony, Silesia, Hun- gary, Spain (Sierra Morena), and the Dauphine. 10 146 DESCRIPTIVE MINERALOGY Argentite. Sulphide of silver : Ag 2 S (silver 87 per cent.). Crystallizes in the regular system in cubes, octahedra, and rhombic dodecahedra, but also occurs massive. Colour and streak, a dull black or lead grey. Lustre, metallic. Opaque. Cubic cleavage imperfect. Fracture, conchoidal. Sectile. Hardness, 2-2*5. Den- sity, 7-7*4. Fusibility, 1*5. Yields a globule of silver on charcoal. An important ore of silver in many silver-mines, as, for example, those of Saxony, Bohemia, Hungary, and Norway, in Europe; those of Peru and Chili, in South America; those of Mexico, Arizona, Idaho, Colorado, Nevada (Comstock Lode), Utah, and Ontario (Cobalt), in North America ; and those of Australia and Tasmania. Stephanite, or brittle silver ore. A sulphide oi silver and antimony, or sulphantimonite of silver Represented by the formula 5Ag 2 S.Sb 2 S 3 , which gives a silver percentage of 68*36. Il occurs massive, or crystallized ir thick six-sided tablets or in shorl FIG. 80. STEPHANITE. prisms of the rhombic system c, Basal plane ; />, pyra- This mineral has an iron-blacl mid ; d, brachydome. . colour, metallic lustre, is soft (hard ness, 2-2*5), an d nas a density of 6-2-6*3. It cleaves parallel to the brachypinacoid (oio), has an uneven tc semi-conchoidal fracture, and is brittle. Before the reducing flame of the blowpipe on charcoal it yield: a button of silver. Stephanite occurs with other silver ores in Saxon] ORES 147 (Freiberg), Bohemia, Hungary (Schemnitz, Kremnitz, and Hodritsch), Chili, Peru, Mexico, and Nevada (Comstock Lode). Pyrargyrite, or dark ruby silver ore. A sulphide of silver and antimony, or sulphantimonite of silver : 3Ag 2 S.Sb 2 S 3 (silver 59*97, antimony 22*21, sulphur 17*82, per cent.). Crystallizes in the hexagonal-rhom- bohedral system (hemimorphic). Habit, short columnar with manifold rhombohedral and scalenohedral termina- tions. Often twinned. Also occurs massive. Cleavage, rhombohedral (1011). Fracture, conchoidal to uneven. Brittle. Hardness, 2-3. Density, 577-5*86. Lustre, metallic-adamantine. Translucent in thin splinters. Colour in reflected light, black or grey-black ; in trans- mitted light, deep cochineal red. Streak, red. Fusi- bility, i (Von Kobell). Before the blowpipe gives off dense antimonial fumes; yields a globule of silver when fused with carbonate of soda on charcoal. Occurs with other silver ores, and frequently with galena, the gangue being often calcite. Pyrargyrite occurs in Saxony, Bohemia, Hungary (Schemnitz, Kremnitz), Transylvania, the Harz (Andreas- berg), Norway, Spain, Montana, Nevada (Comstock Lode), Mexico, Chili, Peru, Bolivia, Canada (Cobalt). Proustite, or light ruby silver ore. A sulphide of silver and arsenic, or sulpharsenite of silver : 3Ag 2 S.As 2 S 8 (silver 65*4, arsenic 15*17, sulphur 19*43, per cent.). Crystallizes in the hexagonal-rhombo- hedral system (hemimorphic), in similar forms to pyrar- 148 DESCRIPTIVE MINERALOGY FIG. 81. PYRARGYRITE. P, Rhombohedron ( + R) ; z, a more obtuse rhom- bohedron (-% R) ; h, a scalenohedron ; n, prism. gyrite, with which it is isomorphous. Also massive. Cleavage, rhombohedral (1011). Fracture, conchoidal to uneven. Brittle. Hardness, 2. Density, 5 '55-5*64. Lustre, adamantine. Transparent to translucent. In reflected light, black or grey -black; in trans- mitted light, proustite has a brighter colour than pyrargyrite, inclining to scarlet-red. Streak, red. Fusibility, i (on Von Kobell's scale). Heated on char- coal before the blowpipe, gives off arsenical fumes (smelling of garlic), and yields a globule of silver with carbonate of soda. Occurrence, same as pyrargyrite. Chailarcillo, a mine in Chili, is a noted locality. Polybasite. A sulphide of silver, copper, and anti- mony, or sulphantimonite of silver and copper : 9(Ag,Cu) 2 S.Sb 2 S 3 (with 62 to 75 per cent, silver and from o to 10 per cent, copper). The corresponding arsenical compound is also known (pearceite). Mono- clinic, in thin six-sided tables with pseudo-rhombo- hedral symmetry. Also occurs in scaly aggregates. Metallic lustre. Colour, iron black; in thin fragments by transmitted light cherry red. Basal cleavage, perfect. Fracture, uneven. Hardness, 2-3. Density, 6-6'2. Fusibility, I (Von Kobell's scale). Before the blow- pipe gives off antimonial fumes ; with carbonate of soda on charcoal yields globule of cupriferous silver. ORES 149 Occurrence with other silver sulphides, in Saxony (Freiberg), Harz (Andreasberg), Bohemia (Joachims- thai, Przibram) Hungary (Schemnitz, Kremnitz, Hodritsch), Colorado, Nevada (Comstock Lode), Mon- tana, Arizona, Mexico, Peru, Chili. Kerargyrite, chlorargyrite, or hornsilver. Chloride of silver : AgCl (silver 7.5-3 per cent.)- Crystallizes in the regular system, with cubic habit, but usually occurs massive or in scales and plates. Colour, whitish-grey. Lustre, resinous to adamantine. Translucent. Mal- leable. Sectile. Hardness, 1-2. Density, 5*58-5-6. Easily fused (fusibility, i), yielding a globule of silver on charcoal. A valuable ore of silver, but not of very common occurrence. The largest deposits are in Mexico (Oajaca), Chili, and Peru. It is also found in Nevada (Comstock Lode), California (Pine Hill), and New South Wales (Broken Hill). Embolite. Chlorobromide of silver: Ag(Cl,Br) (silver 65 per cent.). Crystallizes in the regular system, but occurs in small disseminated particles. Colour, greenish. Translucent. Sectile. Hardness, 2-3. Den- sity, 579-5*80. Fusibility, i. Yields a globule of silver on charcoal. Of rare occurrence. Mexico (Oajaca), Chili (Chanarcillo). ORES OF LEAD. Lead occurs native, but it is a rare mineral. The bulk of the ore mined is galena. This, the simple monosulphide of lead, is the primary lead mineral, and 150 DESCRIPTIVE MINERALOGY is invariably encountered in lead ore deposits when the mines are carried below the belt of weathering. In the upper oxidized portions of the deposits it has been re- placed, although rarely completely, by the carbonate (cerussite) and the sulphate (anglesite). The red oxide, minium (Pb 3 O 4 ), also occurs, but is unimportant as an ore. Lead ores occur as vein deposits of hydatogenetic origin and as metasomatic replacements. In the vein deposits the dominant gangue material may be quartz, dolomite, or barytes. In the quartz veins the galena is accompanied by chalcopyrite, and by silver ores in the dolomite and barytes veins; while zinc-blende is present in all three types. The bulk of the metasomatic deposits were originally formed by the replacement of limestone by galena, the sulphide of lead being carried in solution by alkaline sulphides. In most cases a secondary concentration has accumulated the ores in fissures, cavities, joints and bedding planes, whence they have spread out into the adjoining country rock. Such deposits occur in limestones of all ages Archaean, Ordovician, Devonian, Carboniferous, Triassic, Cretaceous, and Tertiary. Lead has a multifarious application in the arts : it is used in the form of sheets, pipe, shot, glazier's lead, wire; for type metal and other alloys; and in the manufacture of the pigments white lead (basic carbo- nates) and red lead (oxide, Pb 3 O 4 ). Litharge (PbO) and lead acetate (sugar of lead) are also important articles of commerce. ORES 151 The world's production of pig lead amounts to a little over one million tons per annum. Galena. Sulphide of lead : PbS (lead 86'6 per cent.). Crystallizes in the regular system, generally as a com- bination of the cube and octahedron ; the cubical habit is usually dominant, with frequent twinning parallel to the octahedron (m). It also occurs massive in granular aggregates. In argentiferous galena enough silver is present to make the mineral a valuable source of that metal. FIG. 82. GALENA. a, Cube ; o, octahedron ; d, rhombic dodecahedron ; e, icosi-tetrahedron. A small amount of gold is usually associated with the silver. Colour, lead grey. Streak, greyish-black. Opaque. Lustre, metallic. When massive, dulL Tarnishes on exposure to air. Cubical cleavage, perfect. Fracture, even. Hardness, 2-2*5. Density, 7'4-7'6. Fusibility, 2. On charcoal with sodium carbonate, yields a globule of lead. Soluble in concentrated nitric acid, with separa- tion of sulphur. Galena is the most widely distributed, the most abundant, and the most important, ore of lead. The following are localities of some of the more im- 152 DESCRIPTIVE MINERALOGY portant lead -mining districts in which galena occurs below the oxidation zone, generally in association with blende : Cardigan and Montgomery (Welsh Potosi, Van, Cwm Ystwith), Flintshire (Halkyn), Denbigh- shire (Minera), Isle of Man (Laxey and Foxdale), Anglesey (Parys, Mona), Cornwall, Northumberland, Cumberland (Threlkeld), Durham (Weardale), York- shire (Swaledale), and Derbyshire ; Saxony (Freiberg, Schneeberg, Annaberg, Altenberg), Silesia (Katzbach), Nassau (Ems, Holzappel), Harz, Bohemia (Przibram, Pilsen, Kuttenberg), Styria (Graz), Carinthia (Raibl and Bleiberg), Carniola, Bosnia, Tuscany, Sicily, Sardinia, Spain (Murcia, Linares, Ciudad Real, Sierra Morena), Sweden (Sala), North Africa (Tunis, Constantine), Colorado (Leadville), Idaho (Coeur d'Alene), Utah (Frisco, Wisconsin, Oquirrh, Bingham), Illinois, Dakota, Arkansas, Nevada (Eureka), Missouri (Joplin), Mexico (Sierra Mojada), Canada (Ontario), New South Wales (Broken Hill), Tasmania (Zeehan, Mount Read), South Africa (Transvaal). Cerussite. Carbonate of lead : PbCO 3 (lead 77*5 per cent.). Crystallizes in the rhombic system, being iso- morphous with aragonite. The crystals are partly of a pyramidal habit, partly tabular, and are often twinned on a face of the prism (no). They are usually colour- less and pellucid ; but less transparent and white, or slightly tinted, varieties occur, occasionally forming delicate silky and fibrous aggregates. The crystals are characterized by a brilliant adamantine lustre. Pris- ORES 153 matic cleavage, imperfect. Fracture, conchoidal. Brittle. Hardness, 3-3*5. Density, 6-4-6*6. Fusibility, 1*5. Yields a globule of lead when fused on charcoal with carbonate of soda. Soluble in dilute nitric acid, with effervescence. Cerussite is an important ore occurring in the oxidized zone of lead deposits. Well - crystallized specimens, represented in most collections, are usually from Cornwall (Pentire Glaze), Nassau (Friedrichsegen mine, near Ems), Bohemia (Mies), Siberia (Nert- FIG. 83. SIMPLE AND TWINNED CRYSTALS OF CERUSSITE. t, Pyramid ; m, prism ; e, brachy prism ; 6, brachypinacoid. schinsk), New South Wales (Broken Hill), Colorado (Leadville), Rhodesia (Broken Hill). Anglesite. Sulphate of lead : PbSO 4 (lead 68*3 per cent.). Crystallizes in the rhombic system, being iso- morphous with barytes and celestite. Habit, tabular. Also occurs massive. Colourless. Streak, white. Lustre, adamantine to resinous. Transparent. Cleavage, pris- matic and basal, fair. Brittle. Fracture, conchoidal. Hardness, 3. Density, 6-3. Fusibility, 2. Yields globule of lead with carbonate of soda on charcoal. Soluble with difficulty in nitric acid. Anglesite occurs 154 DESCRIPTIVE MINERALOGY in the oxidation zone of lead ores as an alteration pro- duct of galena, and often constitutes an important ore of lead. It derives its name from Anglesey, where it is found at the Parys copper mine. Other well- known localities are Sardinia (Monte Poni), Westphalia (Siegen), Harz, Pennsylvania (Phcenixville), and many lead-mines in the United States. Pyromorphite. Chlorophosphate of lead : 3Pb 3 (PO 4 ) 2 .PbCl 2 (lead 76*4 per cent). Crystallizes in the hexagonal system. Habit, prismatic. Also occurs in uniform and botryoidal aggregates. Colour, green, yellow, or brown. Streak, white. Lustre, resinous. Translucent. Prismatic cleavage, imperfect. Fracture, subconchoidal. Brittle. Hardness, 3*5-4. Density, 6*5-7'i. Fusibility, 2. On charcoal, with carbonate of soda, yields a globule of lead. Soluble in nitric acid. Occurs in association with other ores of lead. ORES OF ZINC. Blende, the monosulphide of zinc, is the source of the bulk of the zinc of commerce (spelter). It is the primary zinc ore, and is invariably found in the deeper parts of the deposits ; while within the belt of weather- ing it is replaced by the carbonate (calamine), the silicates (smithsonite and willemite), and the oxide (zincite). As in the case of lead, zinc ores occur both as veins and as metasomatic replacements. ORES 155 The same three types of vein deposit hold for zinc ores as for lead ores, blende and galena being frequently associated in the same veins. The metasomatic ores have been formed by the replacement of limestone, sulphide of zinc being de- posited as blende from solution in alkaline sulphides. The subsequent distribution of the ore along fissures and other openings is regulated by solution, redeposi- tion, and concentration. Where oxidation is possible, secondary zinc ores are also formed. The chief uses of zinc are in the manufacture of galvanized iron (sheet iron coated with zinc), in electro- zincing, and for alloys (especially brass, which is an alloy of zinc with copper, and German silver, in which zinc is alloyed with copper and nickel). Its compounds are also used as pigments (zinc oxide in zinc white, and zinc sulphide in admixture with barium sulphate in lithopone), and for many other purposes. The world's output of spelter amounts to close on 800,000 tons per annum. NOTE. The metal cadmium is also obtained from zinc ores. Zinc and cadmium are closely allied metals, and their compounds occur in isomorphous inter- mixture. Thus, blende often contains from 0*3 to 3 per cent, of cadmium sulphide (CdS), as at Ouro Preto in Brazil, and in Kentucky, Illinois, and the Joplin District of Missouri, in the United States. Cadmium is used in fusible alloys for soft solders, electric fuses, and (in alloy with mercury) in dental amalgam. It may also take the place of bismuth in cliche metal. 156 DESCRIPTIVE MINERALOGY Zinc-Blende, sphalerite, or "Black Jack." Sul- phide of zinc: ZnS (zinc 67*06 per cent.). Iron is a common constituent of blende, especially of the darker- coloured varieties,* and cadmium is frequently present. Crystallizes in the regular system, with tetrahedral sym- metry. Habit, usually dodecahedral. Twinned crys- tals frequent, the twinning axis being the normal to a face of the octahedron (see Fig. 82). Also occurs massive, or in granular, nodular, and botryoidal aggre- gates. Colour, usually brown to black (" Black Jack"), but also yellow to colourless. Transparent to trans- FIG. 84. TWINNED CRYSTALS OF BLENDE. o, Octahedron ; a, cube ; d, rhombic dodecahedron. lucent. Lustre, resinous to adamantine. Streak, yellow or brown. Dodecahedral cleavage, perfect. Conchoidal fracture. Brittle. Hardness, 3-4. Den- sity, 3'g-4'i. Fusibility, 5. Gives off sulphur fumes when heated. Soluble in nitric acid, with separation of sulphur. Blende is the most widely distributed and most abundant ore of zinc, being found, in association with galena, below the oxidation zone of all zinc-lead ore * The iron is probably present as sulphide of iron (FeS) in isomorphous admixture with blende (ZnS). ORES 157 deposits. For localities, see those given for galena. A few may be specially mentioned : North of England (Alston Moor), Belgium, Sardinia, the Alps, Westphalia (Iserlohn), Upper Silesia, Hungary (Schemnitz), Carin- MAP OF THE MISSOURI-KANSAS ZINC AND LEAD FIELDS. thia (Raibl), Greece (Laurium), Northern Spain (As- turias), Algeria, New South Wales (Broken Hill), Missouri-Kansas (Webb City, Joplin, Galena, Duenweg, Oronogo, Granby, Alba Neck, Badger, Miami, and Carthage). 158 DESCRIPTIVE MINERALOGY Blende also occurs in association with chalcopyrite, as in the complex ores of Huanchaca in Bolivia. Calamine, or zinc spar (smithsonite of Dana). Carbonate of zinc : ZnCO 3 (zinc 52 per cent.). Crys- stallizes in the hexagonal system, with rhombohedral symmetry, being isomorphous with calcite. When crystallized, it occurs in small crystals with curved faces. Usually, however, it is found in kidney-shaped and botryoidal aggregates, or massive. Though colour- less when pure, it is often tinted grey, yellow, brown, or green. Lustre, vitreous to pearly. Streak, white. Hardness, 5. Density, 4-3-4-35. Infusible. Soluble in warm dilute hydrochloric acid, with effervescence. An important ore of zinc in those deposits, or por- tions of deposits, which lie within the belt of weather- ing. It is largely worked in the zinc-mines of Siberia (Nertschinsk), Aix-la-Chapelle (Altenberg), Northern Spain (Santander), Greece (Laurium), Virginia (Austin Mines), Illinois (Jo Davies County), Arkansas, Iowa, Kansas, Kentucky, Rhodesia (Broken Hill). Willemite. Silicate of zinc : SiO 2 .2ZnO (zinc 58*7 per cent.). Crystallizes in the hexagonal system, with rhombohedral symmetry, occurring in small crys- tals. More usually it is found massive, granular, or in kidney-shaped aggregates. Colour, white, yellow, or brown. Transparent to translucent. Lustre, vitreous. Cleavage, basal. Fracture, subconchoidal. Hardness, 5-6. Density, 4-02-4*18. Infusible. Gelatinizes with hydrocloric acid. ORES 159 In Europe it occurs at Aix-la-Chapelle (Altenberg) and a few other localities ; but it is only found in workable quantities in New Jersey in the United States (Franklin Furnace and Sterling Hill), where it occurs in association with zincite and franklinite, and constitutes a valuable ore of zinc. Hemimorphite (calamine of Dana). Hydrated sili- cate of zinc : SiO 2 .2ZnO.H 2 O (zinc 41 per cent.). Crystallizes in the rhombic system, with hemimorphic development i.e., the crystals present different com- binations at the two ends of the vertical axis (hence 9 9 FIG. 85. CRYSTALS OF HEMIMORPHITE. c, Basal plane ; b, brachypinacoid ; a, macropinacoid ; g, prism ; o, and^, macrodomes ; m, brachydorae. its name). The crystals are usually small tablets (parallel to oio), which are combined to fan-shaped, spherical, and kidney-shaped aggregates. Occurs also in fibrous and granular masses. Twinning parallel to the basal plane. Although often colourless or white, it is also grey, yellow 7 , brown, or even of a green or blue tint. The crystals are usually transparent, and have a glassy lustre. Streak, white. Prismatic cleavage, perfect. Fracture, uneven. Brittle. Hardness, 4-5. Density, 3*4-3*5. Strongly pyro- electric. Infusible. Loses water at a red heat only. 160 DESCRIPTIVE MINERALOGY An ore of zinc, often associated with the car- bonate (calamine), as at Aix-la-Chapelle (Altenberg), Westphalia (Iserlohn and Siegen), Carinthia (Bleiberg and Raibl), Spain (Santander), Siberia (Nertschinsk), Virginia (Austin Mines), Missouri (Granby, Duenweg, Joplin, Spring City, Aurora, Sarconie). Zincite. Oxide of zinc: ZnO (zinc 8031 per cent.)- Crystallizes in the hexagonal system, with hemimorphic development. Habit, columnar, with prism (1010), pyramid (1011), and basal plane (oooi). Occurs usually massive or in granular aggregates. Colour, dark red. Transparent to translucent. Lustre, ada- mantine to metallic. Streak, orange to yellow. Basal cleavage, perfect. Fracture, subconchoidal. Brittle. Hardness, 4-5. Density, 5*4 "5*7- Infusible. Soluble in acids. Occurs as an ore of zinc in association with willemite and franklinite in the zinc-mines of New Jersey (Franklin Furnace and Sterling Hill). Franklinite. An epitritoxide of zinc, manganese, and iron: (Zn,Fe,Mn).O(Fe,Mn) 2 O 3 (percentage of zinc variable). A member of the spinel group. Crystal- lizes in the regular system, with octahedral habit. Usually occurs massive or granular. Colour, iron black or dark brown. Streak, brown. Opaque. Lustre, metallic. Fracture, uneven. Brittle. Hardness, 6. Density, 5*15. Slightly magnetic. Infusible. Soluble in hydrochloric acid. Occurs as an ore of zinc in association with zincite ORES 161 and willemite in the zinc-mines of New Jersey (Franklin Furnace and Sterling Hill), where the zinc is first extracted, and the residue then treated as an iron ore. ORES OF NICKEL. The chief source of nickel is nickeliferous pyrrhotite (see under pyrrhotite), which occurs in the marginal portions of large intrusions of norite near Sudbury* in Ontario, Canada. In this ore the nickel appears to be present in the form of pentlandite (Fe,Ni)S. Another source is the indefinite hydrated silicate of nickel known as garnierite, which is mined at Noumea in New Cale- donia. Linnceite, the sulphide of cobalt and nickel, is also a source of nickel. The remaining nickel ores viz., niccolite, chloanthite, and gersdorffite are responsible for the production of a comparatively small amount of the metal ;t while the nickel minerals millerite, anna- bergite, and zaratite only occur as decomposition products in the weathered portions of the lodes. Nickel is used principally in the manufacture of certain white alloys e.g., German silver, which is an alloy of nickel with copper and zinc also for nickel-plating. Pentlandite. A sulphide of iron and nickel: (Fe,Ni)S (nickel 10 to 39 per cent.). Crystallizes in the regular system, but usually occurs massive. Colour, yellowish- * Sudbury is responsible for more than half the world's annual production of nickel. t The nickel derived from this source of supply is chiefly con- tained in the silver ores shipped from the Cobalt district of Ontario, Canada. 11 162 DESCRIPTIVE MINERALOGY bronze. Streak, black. Lustre, metallic. Octahedral cleavage. Fracture, uneven. Hardness, 3*5-4. Den- sity, 4*95-5. Fusibility, 5. Soluble in nitric acid. This mineral occurs in association with pyrrholite and chalcopyrite in the so-called nickeliferous pyrrhotite of Sudbury, Ontario, which is the most important source of the metal nickel. Niccolite. Arsenide of nickel : NiAs (nickel 44*1 per cent.). Crystallizes in the hexagonal system, but occurs mostly massive or in granular and botryoidal aggregates. Colour, copper red. Opaque. Lustre, metallic. Streak, brownish-black. No cleavage. Frac- ture, conchoidal to uneven. Fairly brittle. Hardness, 5. Density, 7*3-7*7. Fusibility, 2. On charcoal, yields a white, brittle, metallic globule, and gives off arsenical fumes. Soluble in aqua regia and in nitric acid, with separation of sulphur. Occurs as an ore of nickel in association with silver and cobalt ores. Cornwall, Harz (Andreasberg), Saxony (Schneeberg, Annaberg), Bohemia (Joachimsthal), Nor- way, Sweden, Chili (Chanarcillo, and Huasco), Ontario (Cobalt). Chloanthite. Arsenide of nickel: NiAs 2 (nickel 28*1 per cent.). Crystallizes in the regular system, with pentagonal hemihedrism. Usual form, the cube. Also massive or granular. Colour, tin white to steel grey. Opaque. Streak, greyish-black. Lustre, metallic. Octahedral cleavage, imperfect. Fracture, uneven. Brittle. Hardness, 5-6. Density, 6*3-7. Fusibility, 2, ORES 163 yielding magnetic globule on charcoal, with production of arsenical fumes. Decomposed by nitric acid, yield- ing a green solution. Occurs as an ore of nickel in the Erzgebirge, Harz, Saxony, Chili, Peru, United States, Canada (Cobalt), etc. Gersdorffite. Arsenosulphide of nickel: NiAsS (nickel 35*31 per cent.). Crystallizes in the regular system, with pentagonal hemihedrism. Usual form, the octahedron, often in combination with the cube. Also granular and massive. Colour, silver white to steel grey. Streak, greyish-black. Opaque. Lustre, metal- lic. Cubic cleavage, fair. Fracture, uneven. Brittle. Hardness, 5-5*5. Density, 5'6-6'2. Fusibility, 2. Gives off sulphurous and arsenical fumes on heating. Decom- posed by nitric acid. Occurs as an ore of nickel in Westphalia, Rhine Province, Nassau, Harz, Saxony, Hungary, Canada (Ontario). Millerite. Sulphide of nickel : NiS (nickel 64/69 per cent). Crystallizes in the hexagonal system, in acicular or capillary prisms or in fibrous aggregates. Colour, brassy yellow. Lustre, metallic. Opaque. Streak, greenish-black. Rhombohedral cleavage, per- fect. Fracture, uneven. Brittle. Hardness, 3-4. Den- sity, 5*3-5*6. Fusibility, 1*5-2. Yields a magnetic globule. Soluble in nitric acid, with separation of sulphur. Millerite is not itself of importance as an ore of nickel, 164 DESCRIPTIVE MINERALOGY but occurs with other nickel ores, as at the Gap Mine, Lancaster County, Pennsylvania," and at Cobalt, Ontario. Garnierite, genthite, noumeite. Rather indefinite hydrated silicates of nickel, magnesium, and iron : 2(Mg,Ni)O.3SiO 2 + wH 2 O (nickel 15-30 per cent.). No crystal form ; occurs massive, often as an incrustation. Colour, apple green. Translucent to opaque. Lustre, resinous. Streak, greenish-white. Fracture, uneven. Hardness, 3-4. Density, 2*2-2 '8. Infusible. Decomposed by hydrochloric acid, with separation of silica. Usually occurs in serpentine, sometimes in association with chromite. It is worked as an ore of nickel at Noumea in New Caledonia, where it is found in a decomposed serpentine rock ; also, but to a smaller extent, near Frankenstein in Silesia. Annabergite, or nickel-bloom. Hydrated arseni- ate of nickel: Ni 3 (AsO 4 ) 2 .8H 2 O or 3NiO.As 2 O 5 .8H 2 O (nickel 29*5 per cent.). Not crystallized. Occurs as earthy deposit, resulting from the decomposition of nickel ores. Colour, apple green. Streak, greenish- white. Hardness, i'5-2'5. Fusibility, 4. A decomposition product of nickel ores. Itself of no importance as an ore. Original locality, Annaberg in Saxony. Zaratite, or emerald-nickel. Hydrated nickel car- bonate : NiCO 3 .2Ni(OH) 2 .4H 2 O or CO 2 .3NiO.6H 2 O (nickel 26 per cent.). Not crystallized. Massive. ORES 165 Usually as an incrustation. Colour, emerald green. Lustre, vitreous. Translucent. Streak, pale green. Hardness, 3-3*2. Density, 2'6-2'j. Brittle. Infusible. Dissolves with effervescence in hydrochloric acid. Of no importance as an ore. A decomposition product of other nickel ores. Locality, Texas in Pennsylvania. ORES OF COBALT. The chief ores of cobalt are linnceite (the sulphide), smaltite (the arsenide), and cobaltite (the arsenosulphide). Glaucodot (the arsenosulphide of cobalt and iron) is of less importance; while erythrite or cobalt-bloom (a hydrated arseniate of cobalt) only occurs as a decomposi- tion product in the superficial portions of the lodes. Most nickel ores contain some cobalt, and the metal also occurs in mispickel (q.v.). Both cobalt and nickel are found in some copper ores. The main supply of cobalt is from the mines of Ontario and of New Caledonia. It is largely obtained as a by-product in the smelting of the silver ores from Cobalt in Ontario. The principal use of cobalt is as a pigment, especially in its application to glass- making and pottery. Z offer, a roasted cobalt ore, and the oxide, arsenate and phosphate of cobalt, are all used for imparting a blue colour to glass, or in glazing and painting on porcelain and glass. Linnaeite, a sulphide of cobalt and nickel: (Co,Ni) 3 S 4 (cobalt and nickel 57*88 per cent.). Crystallizes in the regular system, with octahedral habit. Also occurs 166 DESCRIPTIVE MINERALOGY massive or granular. Colour, light steel grey. Streak, blackish-grey. Opaque. Lustre, metallic. Cubical cleavage, imperfect. Fracture, uneven. Brittle. Hard- ness, 5-6. Density, 4*8-5. Fusibility, 2. Soluble in nitric acid. An important ore of cobalt and nickel, found in association with galena, chalcopyrite, and pyrites, as in Westphalia, Sweden, and Missouri in the United States. Smaltite. Arsenide of cobalt : CoAs 2 (cobalt 28*2 per cent.). Crystallizes in the regular system, with pentagonal hemihedrism. Usual form, the cube. Also massive or granular. Colour, tin white to steel grey. Opaque. Streak, grey-black. Lustre, metallic. Octa- hedral cleavage, imperfect. Fracture, uneven. Brittle. Hardness, 5-6. Density, 6-3-7. Fusibility, 2-3. Yields a magnetic globule on charcoal, and gives off arsenical fumes. Decomposed by nitric acid, yielding a pink solution. Occurs as an ore of cobalt in the Erzgebirge, Harz, Saxony, Chili, Peru, Ontario (Cobalt), United States, Transvaal. Cobaltite. Arsenosulphide of cobalt : CoAsS (cobalt 35*5 P er cent.). Crystallizes in the regular system. Usual form, the pentagonal dodecahedron, modified by the cube. Also granular or massive. Colour, silver white, with a reddish tint. Opaque. Streak, greyish- black. Lustre, metallic. Cubical cleavage, fair. Fracture, uneven. Brittle. Hardness, 5-6. Density, ORES 167 6-6*4. Fusibility, 2-3. Yields a magnetic globule on charcoal. Decomposed by nitric acid. Occurs in crystalline schists, together with chalco- pyrite. Also in ore veins, as in Westphalia (Siegen), Hungary, Caucasus, Norway (Skutterud and Snarum), Sweden (Tunaberg), Chili, Ontario (Cobalt). Glaucodot. Arsenosulphide of cobalt and iron : (Co,Fe)AsS (cobalt 4 to 25, iron 12 to 33 per cent.). Rhombic, with habit similar to that of mispickel. Colour, tin white to reddish silver white. Opaque. Metallic lustre. Streak, black. Basal cleavage, per- fect. Fracture, uneven. Brittle. Hardness, 5. Den- sity, 5*9-6. Fusibility, 2-3. Yields a magnetic globule on charcoal. Decomposed by nitric acid, with separa- tion of sulphur. Of little importance as an ore of cobalt. Occurs in Norway (Skutterud, Sulitelma), Sweden (Hakansboda), United States (New Hampshire). Erythrite, or cobalt -bloom. Hydrated arseniate of cobalt : Co 3 (AsO 4 ) 2 .8H 2 O or 3CoO.As 2 O 5 .8H 2 O (cobalt 29*5 per cent.). Crystallizes in the monoclinic system, in slender prismatic needles. Also massive, earthy, or as an incrustation. Clinopinacoidal cleavage, perfect. Colour, crimson to peach colour. Lustre, pearly to vitreous or dull. Streak, pale red. Trans- lucent. Sectile. Hardness, 1*5 -2*5. Density, 2*95. Fusibility, 2*5. Occurs as a decomposition product of cobalt ores. Of no importance as an ore. 168 DESCRIPTIVE MINERALOGY ORES OF IRON. The ores of iron are abundant and of widespread occurrence. Those in chief use for the extraction of the metal are the oxides, magnetite and hematite, the group of hydrated oxides embraced under the general term limonite, and the carbonate, chalybite. The sul- phides pyrites, marcasite,pyrrhotite, etc. are not mined for their iron, but for their sulphur content* ; but these minerals are the original source of much of the oxidized iron ore which is found within the belt of weathering of the earth's crust,f although the largest proportion of the oxides is derived, in the first instance, from the decomposition of the numerous iron-bearing minerals (ferromagnesian silicates, etc.) that occur in igneous rocks and are especially abundant in the more basic subdivisions. Chalybite is an intermediate product, and haematite and limonite ores often pass downwards into spathic ores (chalybite), from which they have been derived by oxidation and hydration ; while the chalybite itself has, in some cases, been traced to the decomposition of pyrites. Of the oxides, magnetite alone occurs in considerable quantity as a mineral of igneous origin, the deposits of this ore near the peri- phery of large intrusions of basic igneous rocks, having been formed by magmatic concentration, while the rock * The iron oxide residue, which remains after the iron sulphides have been roasted in the manufacture of sulphuric acid, is frequently used in blast-furnaces for the production of pig-iron. t For example, the limonite gossan deposits. ORES 169 was still in a state of igneous fusion. Chromite, and in a less degree haematite, also occur as minerals of igneous origin. Native iron occurs as a constituent of meteorites, and as small nodules in certain basalts (e.g., Disco Island in West Greenland). There seems to be some doubt whether it occurs in the latter as an original or as a secondary constituent. The existence of iron in the native state is only of scientific interest : native iron is not an ore of the metal. The world's annual output of iron ore amounts at the present time to about 133,000,000 tons, from which some 60,000,000 tons of pig-iron are produced. The dis- tribution of the ores mined is shown by the following table, which summarizes the output (for 1909) of the ten principal producers : United States ... ... 53,034,000 tons Germany ... ... 25,095,000 ,, United Kingdom ... ... 14,980,000 ,, France ... ... ... 12,254,000 Spain ... ... ... 9,056,000 ,, Sweden ... ... 3,823,000 Austria ... ... ... 2,450,000 ,, Canada ... ... ... 239,000 ,, Belgium ... ... 203,000 ,, An interesting estimate of the actual and potential ore reserves of the world has been published by the Eleventh International Geological Congress, which sat in Stockholm in 1910. It is quoted here, as it serves 170 DESCRIPTIVE MINERALOGY SCALE or ENGLISH MILES . , 1 1 9 :.. ; L A P L A N D O IRON ORE OCCURRENCES IN SCANDINAVIA." ORES 171 admirably to illustrate the distribution of the known iron ores of the world : Actual Potential Reserves. Reserves. Million Tons. Million Tons. Europe ... 12,032 41,029 America ... 9,855 81,822 Australia ... 136 69 Asia ... ... 260 457 Africa... ... 125 not estimated Total ... 22,408 123,377* Magnetite, or magnetic iron ore. The epitritoxide of iron : Fe 2 O 3 .FeO (iron 72*4 per cent.). Crystallizes in the regular system, occurring in very perfect octa- hedra or rhombic dodecahedra. Twinning on the spinel type : twin- ning plane (m). More frequently, however, it is found in granular and compact masses ; in minute particles, scattered through many igneous rocks and crystalline schists ; FlG - 86. MAGNETITE. or in loose rounded granules (" iron sand ") that have been washed out of igneous rocks. It possesses an iron-black colour, black streak, and metallic lustre. Its fracture is conchoidal to uneven. There is no cleavage, but planes of parting parallel to the octahedron are characteristic. Hardness, 5-5- 6-5. Density, 4^9 -5*2. Its magnetic properties are pro- nounced, and it was these qualities that caused it to an unknown amount. 172 DESCRIPTIVE MINERALOGY be esteemed by the ancients under the name of the lodestone. When powdered it is easily soluble in hydro- chloric acid. Before the blowpipe it fuses only with difficulty (fusibility, 5-5*5). Magnetite is often associated with plutonic igneous rocks of basic composition, and such association denotes a community in origin. Important deposits occur in Sweden (Kirunavaara, Luossavaara, Gellivare, and Taberg), Norway (Sydvaranger and Dunderland), Russia (Gora Blagodat and Gora Magnitnaja in the Urals and the Caucasus), Spain (Lugo), Transvaal (Bushveld). In England, the ore has been worked at Rosedale in Yorkshire. Haematite. Sesquioxide of iron : Fe 2 O 3 (iron 70 per cent.). Hexagonal-rhombohedral. This iron ore com- prises two varieties : the crystallized or specular iron ore, and the amorphous and earthy material known as red hczmatite. The crystals are partly of a rhombohedral and pyram- idal habit, partly tabular, according as rhombohedral and pyramidal faces or the basal plane predominates. The prismatic faces are always subordinate. The rhom- bohedral faces are often curved, passing gradually over into the basal plane. Specular iron ore also occurs in granular and scaly aggregates, this variety being known as micaceous hematite. The ordinary red iron ore is not crystallized, its structure being either crypto- crystalline or earthy. It occurs in nodular and botryoidal masses, having a smooth exterior and a radiating internal struc- ture (kidney ore], and it also forms stalactitic aggregates. ORES 178 The earthy variety is termed red ochre, and is used as a paint. Lustre, metallic to dull. Colour, iron black to dark steel grey. Streak, cherry red. The FIG. 87. SOME COMMON FORMS OF HEMATITE. r, Rhombohedron ; 5, a more obtuse rhombohedron ; e, a negative rhombohedron ; n, a deutero-pyramid. colour of the earthy varieties is red. No true cleavage. Fracture, conchoidal to uneven. Infusible before the blowpipe. Soluble in concentrated hydrochloric acid. Haematite is an important source of iron, and the ore FIG. 88. KIDNEY IRON ORE. A variety of haematite. (From a photograph.) has a very widespread distribution. Small quantities are found in Cornwall, Devon, and South Wales (Forest of Dean) ; but the largest English mines are in North Lanca- 174 DESCRIPTIVE MINERALOGY shire and Cumberland (Ulverston and Whitehaven). The chief foreign mining centres are France (Normandy and the Pyrenees), Spain (Bilbao, Almeria, Oviedo), Italy (Elba), Germany (Lahn and Dill districts, El- bingerode and Hiittenrode in the Harz), Southern Russia (Kvivoj-Rog), United States (Marquette, Gogebic, and Menominee ranges on the south side, and Vermilion and Mesabi ranges on the north side, of Lake Superior, Adirondack district, Clinton mines in Alabama), Cuba, Brazil (itabirite ores). Goethite. Hydrated oxide of iron : Fe 2 O 3 .H 2 O (iron 80*91 per cent.). Crystallizes in the rhombic system. Habit, columnar to acicular or capillary, with striated prism faces, or tabular parallel to the brachy- pinacoid (oio). Also in kidney-shaped or botryoidal masses with radial, fibrous, and concentric structures, or in fibrous or scaly aggregates. Colour, yellowish or reddish-brown to black. Translucent to opaque. Lustre on smooth faces, adamantine to metallic, also dull to silky. Streak, yellow to reddish-brown. Brachy- pinacoidal cleavage, perfect. Fracture, uneven. Brittle. Hardness, 5. Density, 3'8-4*4. Fusibility, 5-5*5. Yields water in the closed tube. Slowly soluble in concentrated hydrochloric acid. An ore of iron, but not so common as haematite or limonite. Occurs in Cornwall (Lostwithiel, Botallack, Redruth), Scotland, Westphalia, Nassau, Harz, Saxony (Schneeberg, Freiberg), Silesia, Bohemia (Przibram), Russia (Ekaterinburg), United States (Marquette, Mesabi, etc., in the Lake Superior district). ORES 175 Limonite, or brown iron ore. Hydrated oxide of iron: Fe 2 O 3 + wH 2 O. Does not occur crystallized, but has a crypto-crystalline to earthy texture, and forms nodular, botryoidal, kidney-shaped, or stalactitic masses, frequently with an internal fibrous structure. The amount of water is variable. Some authors use the water -content as a means of distinction between the following mineral species : turgite, with 5*3 per cent, of water, corresponding to 2Fe 2 O 3 .H 2 O ; limonite, with 14*5 per cent, of water, corresponding to 2Fe 2 O 3 .3H 2 O ; xanthosiderite, with 18*4 per cent, of water, corresponding to Fe 2 O 3 .2H 2 O; limnite, with 25*2 per cent, of water, corresponding to Fe 2 O 3 .3H 2 O. But it is somewhat doubtful whether these definite hydrates really exist. The different varieties of limonite have been classified by Tschermak, according to texture, as follows : fibrous limonite (including turgite) ; compact brown iron ore, in- cluding oolitic ironstone and the decomposition products of chalybite and pyrites; ochreous limonite, including xanthosiderite and the various ochres, umbers, and siennas; pitchy limonite (stilpnosiderite), characterized by a black colour, glazed surface, and conchoidal fracture ; earthy and sandy limonite, including limnite, bog iron ore, and lake iron ore, found as a brownish- yellow deposit from ferrous carbonate in marshes, wet moorland, and shallow lakes ; pisolitic and oolitic limonite, including the so-called "minettes " ; limonitic cement and impregnation, found in clays, sandstones, conglomerates, and as concretions. Limonite varies in colour from yellow to blackish- 176 DESCRIPTIVE MINERALOGY brown, and has a yellowish - brown streak. It is opaque, and has a dull to silky lustre. Its hardness is from I to 5*5, according to the variety, and its density from 3*3 to 4. Fracture, according to the variety, fibrous, conchoidal, or earthy. Brittle. Fusible with difficulty. Heated in the closed tube, yields water. Limonite is an important iron ore on account of its abundance and wide- spread distribution. The percentage of iron depends, of course, upon the amount of sandy and clayey im- purity. The considerable percentage of phosphoric acid depreciated the value of some of these ores until the introduction of the basic process. Limonitic ores occur in the British Isles : in South Wales (Forest of Dean) and Antrim. Large supplies are also derived from the oolitic iron ores of Jurassic age in North Yorkshire (Cleveland district), Northamp- tonshire, and Lincolnshire. Similar oolitic ores (the so-called " minette " ores) furnish large supplies in Lorraine, Luxemburg, and Belgium. Mention must also be made of the deposits, of recent age, of bog and lake iron ore of Silesia, the Banat, Fin- land, and Scandinavia (Smaland, Vestragotland, and Dalarne). Other important iron-mining centres where limonite ores are exploited are Prussia (Ilsede and Salzgitter, north of the Harz), Bavaria, Wiirtemberg, Spain (Lugo, Santander, Murcia, Oviedo), Russia (Nischne-Novgorod, Kaloga, and Kertch), United States (Appalachian Mountains and the Marquette, Gogebic, Menominee, Vermilion, and Mesabi ranges of the Lake Superior district). ORES 177 Chalybite, siderite, or spathic iron ore. Car- bonate of iron : FeCO 3 (iron 48*2 per cent.). Crystallizes in the hexagonal system, usually in flat rhombohedra with curved faces. Also occurs massive or mixed with clay (clay ironstone), and often contains carbonates of lime, magnesia, and manganese. Cleavage, rhombo- hedral. Colour, buff or fawn. Streak, white. Hard- ness, 3*5-4*5. Density, 3*7-3*9. Soluble in acids, with liberation of carbon dioxide, but not so readily soluble as calcite. Fusibility, 4*5-5. The nodules of clay ironstone, which are found in the shales of the Coal Measures, are termed sph&rosiderite. This variety of the ore forms the staple material of the British iron production. The varieties used in smelting contain from 25 to 35 per cent, of iron. The blackband ironstone is an impure carbonate of iron interstratified in thin seams in the Coal Measures of the North of England. It is especially valuable on account of its contained coaly matter. Clay ironstone is worked in all the principal coal- fields of the British Isles e.g., those of Scotland, Northumberland and Durham, Derbyshire and York- shire, North Staffordshire, South Wales and Coalbrook- dale. The Jurassic ironstones of Yorkshire, North- amptonshire, and Lincolnshire also consist largely of spathic ores (chalybite) in the deeper parts of the deposits. Other centres of the iron -mining industry where spathic ores are worked are France (Normandy and the Pyrenees), Spain (Leon), Germany (Sieger- land), Austria (Styrian Erzberg and the Huttenberger 12 178 DESCRIPTIVE MINERALOGY Erzberg of Carinthia), Hungary (Vares), United States (the Appalachian coalfields of Pennsylvania, Ohio, Kentucky, etc.). Chromite, or chrome iron ore. Oxides of iron and chromium : FeO.Cr 2 O 3 * (oxide of chromium 68 per cent.). Crystallizes in the regular system, but usually occurs massive, granular, or compact ; also in loose grains. Colour, iron black to brownish - black. Opaque. Streak, brown. Lustre, submetallic. Fracture, uneven. Hardness, 5*5. Density, 4*4. Infusible. Im- parts a characteristic green coloration to the borax bead. This mineral is valuable on account of its chromium content, which is required for a variety of purposes e.g., the manufacture of ferro-chrome for chromium steels, and of refractory furnace linings ; and the preparation of the compounds used as pigments, mordants, oxidizing agents, etc. The ore is widely distributed in serpentines, and as peripheral concentra- tions in the ultrabasic rocks (peridotites) from which the serpentines are derived. It is also found in sands. Examples of its occurrence are Silesia, Bohemia, Greece, Russia (Ural Mountains), New South Wales, New Zealand, British Columbia, United States, Trans- vaal, Rhodesia, New Caledonia, and Asia Minor, the two latter being the largest producers. Ilmenite, menaccanite, or titaniferous iron ore. Oxides of iron and titanium : FeO.TiO 2 , with or with- * Ferrous oxide is replaceable to some extent by magnesia (MgO), and chromic oxide by alumina (A1 2 O 3 ). ORES 179 out Fe 2 O 3 . Crystallizes in the hexagonal system, with rhombohedral symmetry. Isomorphous with haematite. Habit, tabular parallel to the basal plane, with dominant rhombohedral faces. Also occurs massive ; in granular and scaly aggregates ; and as rolled grains (inenaccanite sand). Colour, iron-black. Streak, black to brown. Opaque. Lustre, semi-metallic metallic on freshly fractured faces. Although there is no true cleavage, the mineral separates along basal and rhombohedral twin- ning planes. Fracture, conchoidal. Hardness, 5-6. Density, 4'5-5'3 The density increases with the Fe 2 O 3 content. As a rule non-magnetic. Infusible. Soluble when finely powdered in boiling hydrochloric acid, the solution heated with tinfoil giving a violet colour. A very common mineral, especially as an accessory constituent of igneous rocks ; also in sediments, schists, and as sand. Of no value as an iron ore. Original locality, the Ilmen Mountains in the Urals. A common companion of the diamond in the kimberlite pipes of South Africa. The black sands in the beach placers of the United States and New Zealand are rich in ilmenite. Iron Pyrites. Disulphide of iron : FeS 2 (iron 46*64, sulphur 53*36, per cent.). Crystallizes in the regular system. The cube, either alone or combined with the faces of the pentagonal dodecahedron and the octahedron, is the most frequent form. The faces of the cube are usually striated, in consequence of the tendency to oscillatory combination of that form with the pentagonal dodecahedron. The latter form also 180 DESCRIPTIVE MINERALOGY occurs alone, in which case its faces are striated by oscillation with the cube. Twinning parallel to a face of the rhombic dodecahedron. The crystals are of all sizes, and occur either singly or grouped. Nodular, botryoidal, kidney-shaped, com- pact masses, round pellets, and irregular grains all these forms are of common occurrence. The colour of unaltered pyrites is brassy yellow ; but the surface is frequently tarnished, and the mineral alters easily into a dark brown limonite, the change being one of oxida- tion and hydration. The streak is brownish - black. FIG. 89. CRYSTALS OF IRON PYRITES. Lustre, metallic. Opaque. Fracture, conchoidal to uneven. A cubic cleavage is scarcely perceptible. Brittle. Hardness, 6-6*5. Density, 4*9-5* i. The superior hardness is an important distinction from copper pyrites and from gold. Pyrites strikes fire with steel. Fusibility, 2*5-3. Yields a magnetic globule before the blowpipe. Decomposed by nitric acid, with separation of sulphur. Iron pyrites is very widely diffused through the crust of the earth, occurring both as an accessory constituent of rocks and * in veins and large masses, either in- dependently or as a very common companion of other ORES 181 sulphide ores ; but, in spite of its high percentage of iron, it is not used for the production of that metal. The chief use of pyrites is for the manufacture of sulphuric acid, for which purpose it is imported from Spain, Portugal, and Belgium, besides being exploited largely in British metalliferous mines. Pyrites is often associated with gold, and in such cases the latter is usually m FIG. 90. IRON PYRITES (AFTER J. M. BONTWELL, U.S. GEOL. SURVEY). mechanically included within its crystals (auriferous pyrites) . On account of its ubiquity, it is useless to give localities for pyrites : it is associated with marine and brackish water sediments of every age, appearing as grains disseminated through shales and clays or in seams and layers, as in the Coal Measures ; or, again, as con- cretionary nodules, as in certain zones of the Chalk or in the Oxford and Kimeridge clays. By oxidation it 182 DESCRIPTIVE MINERALOGY is converted into the oxides of iron, the liberated sul- phuric acid producing gypsum in calcareous clays, and alum shale in those free from lime. Marcasite, or cockscomb pyrites. Bisulphide of iron : FeS 2 (same composition as pyrites). Crystallizes in the rhombic system, with tabular habit parallel to the basal plane (ooi), or with dominant brachydomes ; or, again, prismatic, parallel to (no). Twinning parallel to the prism face, often repeated. Also occurs in botryoidal, nodular, and globular forms (sometimes with radial fibrous structure), or massive. Colour, brassy yellow. Metallic lustre. Opaque. Streak, dark greyish - green. Cleavage distinct, parallel to the prism (no). Fracture, uneven. Brittle. Hardness, 6. Density, 4-65-4-88. Behaviour before the blowpipe and with acid, same as pyrites. Occurrence similar to pyrites, but less widely dis- tributed. It is frequently present as concretions in clays, marls, and limestones (e.g , in the chalk of Dover and Folkestone). Pyrrhotite, or magnetic pyrites. Monosulphide of iron : FeS (iron 63*61 per cent.). Crystallizes in the hexagonal system, with tabular habit parallel to the basal plane. Twinning parallel to a pyramid face (1012). Usually occurs massive or granular. Colour, bronzy yellow to copper-coloured. Lustre, metallic. Streak, greyish -black. Basal cleavage, imperfect. Fracture, uneven. Brittle. Hardness; 3-4. Density, 4*5-4'6. Magnetic i.e., the fine powder is attracted by the ORES 183 magnet. Fusibility, 2*5-3. On charcoal yields a black magnetic globule. Soluble in hydrochloric acid, with separation of sulphur. Pyrrhotite is of common occurrence as an acces- sory constituent of igneous rocks, of limestones and of crystalline schists ; it is also found with other sulphides in ore veins, and in large independent lenticular masses in the crystalline schists (e.g., in the Huronian of the Appalachian Mountains of the Eastern United States). When it contains nickel (up to 5 per cent., average 3 per cent. nickeliferous pyrrhotite), it constitutes a valuable ore of nickel, as at Sudbury in Ontario, Canada,* at the Lancaster Gap mine in Pennsylvania, and at Erteli in Norway. In Rossland, British Columbia, it is associated with gold. ORES OF MANGANESE. Manganese is a widely-distributed metal, occurring as a constituent of at least a hundred different minerals. Only some half a dozen (mainly oxides and hydrates of manganese), however, are of importance as ores ; and these are all secondary compounds that owe their origin to the decomposing action of percolating alkaline and carbonated waters on primary manganese minerals in igneous and metamorphic rocks. As examples of such primary minerals, the manganese- pyroxene (rhodonite), the manganese-olivine (tephroite), and the manganese-garnet (spessartite) , may be quoted. * Sudbury is responsible for half the world's annual output of nickel. 184 DESCRIPTIVE MINERALOGY The manganese thus derived is carried in solution as a bicarbonate, and reacting on rocks within the zone of weathering (in other words, in the presence of free oxygen) forms replacement deposits. The com- monest ores produced in this way are pyrolusitc (peroxide of manganese), psilomelane (hydrated oxide of manganese), and braunite (an oxide and silicate of manganese). Wad or bog-manganese is an indefinite mixture of various hydrated oxides that occur at the surface, especially as lateritic deposits. Other less frequent ores are manganiie (the hydrated sesqui- oxide) and rhodochrosite (the carbonate). Franklinite, which is an ore of zinc occurring at Franklin Furnace in New Jersey, contains sufficient manganese to make its extraction from the zinc-residuum profitable. Manganese is chiefly used for the preparation of certain alloys of iron and manganese (spiegeleisen and ferromanganese) which are used in the manufacture of steel ; but the oxide is also used in the manufacture of chlorine, bromine, and oxygen, as a dryer in paints and varnishes, and as a decolorizer of glass. Pyrolusite. Peroxide of manganese: MnO 2 (manga- nese 63*22 per cent.). When it occurs in crystals it is pseudomorphous after manganite, but it is mostly found as earthy masses, or in botryoidal, kidney-shaped, and nodular aggregates, with radiate fibrous structure. It also occurs as dendritic markings on the bedding and parting planes of rocks. Colour, steel grey to iron black and opaque, with dull, but sometimes silky, lustre. Streak, black, soiling the fingers. Hard- ORES 185 ness, 2*25. Density, 4*7-5. Infusible. Soluble in hydrochloric acid with evolution of chlorine. A widely-distributed mineral, and often an im- portant ore of manganese. It is frequently found as an alteration product of manganite and other manganese-bearing minerals, and largely as a replace- ment deposit within the zone of weathering, where, together with hydrated oxides of iron and alumina, it forms ores of lateritic origin. It thus often associates with psilomelane and with wad, but it may often be dis- tinguished from these rather similar oxides of manganese by the possession of a crystalline or fibrous structure. It is also of inferior hardness to psilomelane. Examples of occurrence are : Nassau, Moravia, Caucasus, United States, India, etc. Manganite. Hydrated sesquioxide of manganese : Mn 2 O 3 .H 2 O (manganese 62*5, water 10*23 P er cent.). Crystallizes in the rhombic system, with dominant prismatic habit (i 10) . The basal plane is striated parallel to the macrodiagonal. Twinning parallel to the brachy- dome (on). Also occurs in columnar to fibrous and radiating aggregates. Colour, dark steel grey to iron black. Opaque. Lustre, submetallic. Streak, dark brown. Brachypinacoidal cleavage, perfect. Fracture, uneven. Brittle. Hardness, 3-4. Density, 4'2-4'4. Infusible. Gives off water when heated in the closed tube. Often partially altered to pyrolusite. Manganite occurs in veins at Ilfeld in the Harz, and with other manganese ores in Nassau and at many other localities. 186 DESCRIPTIVE MINERALOGY Psilomelane. A mineral of rather indefinite com- position, but probably a hydrated manganese manganate in which a portion of the manganese is replaceable by barium and potassium, corresponding to the formula (H 2 ,K 2 ,Mn,Ba) 2 MnO 5 . The highest percentage of manganese possible, in accordance with the formula Mn 2 MnO 5 , is 67*35 P er cent. Psilomelane does not occur crystallized, being massive, botryoidal, reniform, or stalactitic. Colour, iron black to dark steel grey. Opaque. Streak, brownish-black, shining. Lustre, submetallic to dull. Fracture, uneven. Hardness, 5-6. Density, 3*7-4*7. Infusible. Soluble in hydrochloric acid, with evolution of chlorine. Psilomelane is widely distributed, and constitutes the most important ore of manganese, occurring both as a replacement deposit in rocks within the zone of weathering, and, together with the hydrated oxides of iron and aluminium, in ores of lateritic origin. It is mined in India, Russia (Caucasus), Spain, Brazil, and the United States. In India it constitutes, together with braunite, go per cent, of the manganese ore exported. Wad. The name given to an indefinite mixture of various oxides, chiefly of manganese, but also of iron, aluminium, barium, etc., together with a little silica and 10 to 20 per cent, of water. It occurs in amorphous earthy or compact masses or as incrustations. Colour, dull black to brownish-black. Streak, brownish-black to black. Usually very soft, soiling the fingers. Density, 3-4*3. Infusible. Soluble in hydrochloric acid. ORES 187 Wad is a widely-distributed and abundant surface deposit of manganese, and often still possesses the original slaty or schistose structure of the rocks which it has replaced. It is frequently associated with psilo- melane, but may be distinguished from it by its inferior hardness, and from pyrolusite by the absence of crystal- line or fibrous structures. Braunite. An oxide and silicate of manganese, corre- sponding to the general formula wMnMnO 3 + wMnSiO 3 . It may be regarded as an isomorphous mixture of manganese manganite and manganese silicate. The ratio of m : n is usually 3:1; but it may be also 7 : 2 or 4 : i (with m : n = 3 : i, manganese = 63*6 per cent.). Braunite crystallizes in the tetragonal system, with dominant pyramid of the first order (in). Also occurs massive and granular. Colour, dark brownish-black to steel grey. Opaque. Lustre, submetallic. Streak, dark brown. Cleavage parallel to the pyramid (m), perfect. Fracture, uneven to subconchoidal. Brittle. Hardness, 6-6*5. Density, 475-4*82. Slightly magnetic. Infusible. Soluble in hydrochloric acid, with evolution of chlorine, and yielding a residue of gelatinous silica. An important ore of manganese ; as at Ilfeld in the Harz, Telemark in Norway, and at many places in India, etc. Rhodochrosite, or dialogite. Carbonate of manga- nese : MnCO 3 (manganese 47*83 per cent.). Crystal- lizes in the hexagonal system, with rhombohedral symmetry, being isomorphous with calcite ; but usually 188 DESCRIPTIVE MINERALOGY occurs massive. Rhombohedral cleavage, perfect. Colour, pink, rose red and light brown. Lustre, pearly. Translucent. Streak, white. Brittle. Hardness, 3'5-4'5- Density, 3*45-3*6. Infusible. Soluble in hydrochloric acid, with effervescence. Distinguished from rhodonite (MnSiO 3 ), which it resembles in colour, by its inferior hardness and behaviour with hydro- chloric acid. Rhodochrosite is not widely distributed, but is an important ore of manganese when in sufficient quantity. It has been mined in the French Pyrenees and at a few other places. ORES OF BISMUTH, ANTIMONY, AND ARSENIC. Native bismuth is the principal source of the metal ; but the sulphides of the metal are also worked, and there are several compounds of bismuth with selenium, tellurium, silver, gold, copper, and lead, as well as oxides, carbonates, etc., that are of no importance as ores. Antimony occurs native ; but the principal supply of the metal is derived from the sulphide stibnite. It is also found in numerous sulphide compounds, such as antimonial tetrahedrite, pyrargyrite, etc., and in the oxidation zone as oxides and oxysulphides (senannon- tite, cervantite, and kermesite). Besides occurring in the native state, arsenic is found in two forms as sulphides, realgar and orpiment, and in numerous complex ores of silver, copper, lead, etc.; but the principal supply is obtained from the arsenosulphide of iron mispickel. ORES 189 Bismuth and antimony are chiefly valuable in the arts for their alloys with lead, tin, and copper. Bismuth is used in fusible alloys and for cliche metal ; antimony, in alloy with lead (antimonial lead), as type metal, babbitt metal, etc., and arsenic, for the manufacture of arsenious oxide (white arsenic). Native Bismuth. Bi. Crystallizes in the hexagonal system, but usually occurs in crystalline, granular, or scaly masses or as botryoidal incrustations. Colour and streak, silver white, generally iridescent at the surface. Opaque. Lustre, metallic. Basal cleavage, perfect. Brittle. Fracture, uneven. Hardness, 2. Density, 9'7'9'8. Fusibility, i. Soluble in nitric acid, the solution giving a white precipitate with water. Native bismuth, the most important source of the metal, occurs in association with cobalt and silver ores, as in Saxony (Schneeberg), Bohemia (Erzgebirge), Sweden (Dalarne), Bolivia (Tazna, Illampa), Ontario (Cobalt), New South Wales (Cobar). Bismuthinite. Sesquisulphide of bismuth: Bi 2 S 3 (bismuth 81*22 per cent.). Crystallizes in the rhombic system, with acicular habit. Usually massive, scaly, or fibrous. Colour, lead grey to tin white. Iridescent on the surface. Opaque. Streak, grey. Lustre, metallic. Brachypinacoidal cleavage, perfect. Hard- ness, 2. Slightly sectile. Density, 6*4-6*5. Fusi- bility, i. Yields a globule of bismuth in the reducing flame. Soluble in hot nitric acid, the solution giving a white precipitate with water. 190 DESCRIPTIVE MINERALOGY Bismuthinite occurs in Cornwall, Saxony (Schnee- berg), Hungary (Rezbanya), France (Meymac), Sweden (Riddarhyttan), Australia, Bolivia (Tazna, Chorolque), Utah (Beaver City). Native Antimony. Sb. Crystallizes in the hex- agonal system, with rhombohedral habit ; but usually occurs massive or as granular or botryoidal incrusta- tions. Colour, tin white. Opaque. Lustre, metallic. Basal cleavage, perfect. Brittle. Fracture, uneven. Hardness, 3. Density, 6*6-6*7. Fusibility, i. Volatile. Soluble in aqua regia. Native antimony occurs in Sweden (Sala), Germany (Andreasberg in the Harz), Bohemia (Przibram), Borneo (Sarawak), Chili (Huasco), Peru, New Brunswick (York County). Stibnite, antimonite, or antimony glance. Sesqui- sulphide of antimony : Sb. 2 S 3 (antimony 71*76 per cent.). Crystallizes in the rhombic system, occurring in diver- gent aggregates of long prismatic or acicular crystals, with pyramidal terminations. The prism faces are characterized by a well - developed vertical striation. Colour, lead grey. Streak, lead grey. Opaque. Lustre, metallic, especially bright on cleavage faces. Cleavage parallel to the brachypinacoid (oio), perfect. Fracture, subconchoidal. Slightly pliable. Hardness, 2. Density, 4*6-4*7. Easily fusible (fusibility on von Kobell's scale, i). Volatilizes. Decomposed by hydro- chloric acid. Stibnite occurs in ore deposits in association with ORES 191 galena, blende, cinnabar, and the gangue minerals barytes and quartz. It is of widespread occurrence in Europe e.g., Westphalia, Harz, * Hungary, Bohemia, Italy, Corsica, /AF\ ^vxS-^^n Sardinia, France, Algeria, Spain, Russia. The bulk of the world's supply, however, is now mined in Japan (Ichinokawa mines in the island of Shikoku), China, and Australia (New South Wales, Vic- FIG. 91. ANTIMONITE toria, and Tasmania). It is also A Pyramid ; w, prism ; b, macropmacoid. worked in the United States, Mexico, Peru, Borneo (Sarawak), and New Zealand. Mispickel, arsenical pyrites, or arsenopyrite. Arsenosulphide of iron : FeAsS (iron 34*34, arsenic 46*01, sulphur 19*65, per cent.). Crystallizes in the rhombic system, with prismatic habit parallel to (no), or brachydomatic parallel to (012). Twinning parallel to the macrodome (101) or to the prism (no). Also occurs massive and granular. Colour, tin white to steel grey. Opaque. Lustre, metallic. Streak, greyish- black. Cleavage parallel to the prism (no). Fracture, uneven. Brittle. Hardness, 5*5-6. Density, 5*9-6'2. Strikes fire with steel, with smell of garlic. Fusible before the blowpipe to a magnetic globule, with emission of arsenical fumes. Fusibility, 2. Decomposed by nitric acid, with separation of sulphur. Mispickel is a common companion of ores e.g., those of silver, cobalt, and nickel, in which it is associated with blende, galena, pyrites, and chalco- 192 DESCRIPTIVE MINERALOGY pyrite ; it also accompanies tin ore together with wolf- ramite, fluorspar, and quartz. Together with pyrites it is often associated with gold in gold-quartz veins. Mispickel is the chief source of the arsenic of com- merce, and is largely worked as an arsenic ore in Cornwall, at Schneeberg in Saxony, and in Ontario. Realgar. Monosulphide of arsenic : AsS (arsenic 70*08 per cent.). Crystallizes in the monoclinic system. Habit, short columnar with vertical striation. Also occurs massive or in granular aggregates, and as in- crustations. Colour, red to orange red. Streak of the same colour, but lighter in tint. Transparent to trans- lucent. Lustre, resinous. Clinopinacoidal cleavage, fair. Fracture, subconchoidal. Sectile. Hardness, 1-2. Density, 3*56. Fusibility, I. Volatile, with smell of garlic. Decomposed by aqua regia, with separation of sulphur. Realgar occurs in association with silver and lead ores: Harz (Wolfsberg), Hungary (Tajova, Nagy- banya, Felsobanya, Kapnik), Transylvania (Nagyag), Italy (Casa Testi, Vesuvius, Etna), Corsica, Chili? Peru, Bolivia. Orpiment. Sesquisulphide of arsenic : As 2 S 3 (arsenic 60*96 per cent.). Crystallizes in the rhombic system, with short columnar habit ; but occurs more frequently massive, in granular or fibrous aggregates, sometimes with a botryoidal surface. Colour, orange yellow. Streak, the same, but lighter. Translucent. Lustre, resinous. Brachypinacoidal cleavage, perfect. Flexible. ORES 193 Sectile. Hardness, 1-2. Density, 3*4-3*5. Fusibility, I. Very volatile. Soluble in aqua regia. Occurrence the same as realgar. ORES OF VANADIUM. Vanadium is found in dechenite (vanadate of lead PbO.V 2 O 5 ) and vanadinite (chlorovanadate of lead gPbO.3V 2 O 5 .PbCl 2 ), and other rare minerals occurring in Spain, Sweden, the Argentine, Mexico, and Colorado; but the only commercial source is an occurrence of vanadinite in Spain. It is used in the form of ferro- vanadium in the manufacture of steel alloys. The addition of from o'2 to 0*5 per cent, of vanadium to steel increases its tensile strength, its ductility, and its elasticity. Vanadinite. Chlorovanadate of lead : 9PbO.3V 2 O 5 . PbCl 2 (V 2 O 5 19*36 per cent.). Crystallizes in the hexagonal system, being isomorphous with pyro- morphite (chlorophosphate of lead) and apatite (fluoro- phosphate of calcium). Colour, red. Streak, nearly white. Lustre, resinous. Translucent. Cleavage, none. Brittle. Fracture, uneven. Hardness, 3. Density, 6*9 -7*1. Fusibility, 1*5. Partly soluble in acids. Occurrence as above. ORES OF TIN. The only important ore of tin is the oxide, cassit- erite, the sulphide, stannite, being of little practical value. A large proportion of the tin ore mined is 13 194 DESCRIPTIVE MINERALOGY from detrital deposits, in which cassiterite occurs as a sand in association with quartz, topaz, tourmaline, axinite, garnet, epidote, wolfram, scheelite, fluorspar, monazite, ilmenite, and magnetite, these minerals sharing with cassiterite the property of resisting de- struction by weathering. Tin ore is also found in the veins, chimneys, impregnations, etc., which occur at or near the margin of large granite intrusions, and are generally considered to be of pneumatolytic origin i.e., to have been derived from the granite magma through the agency of vapours and gases dis- solved in it at the time of intrusion, and given off during consolidation. In these original deposits cassiterite is associated with tourmaline, fluorspar, axinite, wolfram, and topaz, to all of which, on account of their peculiar composition, pneumatolytic origin is ascribed. Tin is of great use in the arts on account of its valuable alloys, such as soft solder (an alloy with lead), bronze, gun-metal, bell-metal, specular metal (all alloys with copper), Britannia metal (an alloy with antimony), and babbitt and other friction metals (alloys with anti- mony and copper). Its largest application is in the tin-plate industry, in which sheet iron is coated with a layer of tin. It is also used for tinning copper utensils, and in the form of tinfoil. The subchloride of tin (tin-salt) is used as a mordant in dyeing. It is a valuable reducing agent. Cassiterite, tinstone, or black tin. Dioxide of tin : SnO 2 (tin 78*82 per cent.). Crystallizes in the ORES 195 tetragonal system, the common form being the pyramid, combined with the prism, of the first order. The edges of both pyramid and prism are, however, some- times truncated by the faces of the pyramid and prism of the second order. The faces of the first pyramid show a horizontal, those of the prism a vertical, stria- tion. Habit, usually stout columnar. The crystals are often twinned, the twinning plane being a face of the deutero-pyramid (101). Repetitions of the twinning produce ring or star-shaped aggregates of three to five FIG. 92. CASSITERITE. s, Proto-pyramid ; m, proto-prism ; e, deutero-pyramid ; a, deutero- prism. FIG. 93. CASSITERITE TWINNED CRYSTAL). individuals. Tinstone also occurs massive in fibrous nodules (wood-tin) in grains dispersed through granite, or in pegmatite veins, but most frequently in loose rounded fragments, mingled with river gravels (stream- tin). Colour, usually brown or black, but yellow and grey tints also occur. Lustre, adamantine to resinous. Cleavage parallel to (100), imperfect. Fracture, sub- conchoidal to uneven. Brittle. Hardness, 6-7. Den- sity, 6*8-7'i. Infusible. Yields a globule of tin with carbonate of soda on charcoal. Insoluble in acids. 196 DESCRIPTIVE MINERALOGY Becomes coated with metallic tin when placed on zinc with hydrochloric acid. Cassiterite was formerly extensively mined in Corn- MAP OF THE TIN-MINING DISTRICTS OF THE FEDERATED MALAY STATES AND DUTCH EAST INDIES. wall, and tin ore is still a staple product of that county. The historical tin deposits of Altenberg and Zinnwald in Saxony also deserve mention ; but the main supplies are now derived (i) from the tin deposits (largely of ORES 197 alluvial origin) of the Malay Peninsula (Perak, Selangor, Nigri-Sembilan) and of the Dutch East Indies (Banka and Billiton) ; (2) from Australasia Queensland (Herberton), New South Wales (Vegetable Creek, etc.), West Australia, Tasmania (Mount BischofT and Briseis) and (3) from Bolivia (La Paz, Oruro, Potosi, and Chorolque). Other producers are Mexico, California, Dakota, Alaska (Seward Penin- sula), Japan, and China. An increasing production is being made in South Africa (Swaziland, Transvaal), and extensive alluvial deposits are being explored in Nigeria. Stannite, tin - pyrites, or bell - metal ore. Sul- phostannate of iron and copper : Cu 2 FeSnS 4 or SnS 2 . Cu 2 S.FeS (tin 27*68, copper 29*5, iron 13*02, per cent.). Crystallizes in the tetragonal system, with sphenoidal hemihedrism. Habit, pseudo-regular. Twin- ning axis the normal to (m). Generally occurs massive or in granular aggregates. Colour, steel grey to iron black. Streak, black. Opaque. Lustre, metallic. Basal and prismatic cleavages, indistinct. Fracture, subconchoidal to uneven. Brittle. Hardness, 3-4. Density, 4*3-4*5. Fusibility, 1*5. Gives off sulphur fumes when heated. Decomposed by nitric acid, with separation of sulphur and oxide of tin. Stannite is rarely of importance as an ore of tin. It occurs in Cornwall, Bohemia (Zinnwald), Bolivia (Oruro and Potosi), Dakota (Black Hills), Tasmania (Zeehan), Japan. 198 DESCRIPTIVE MINERALOGY ORES OF TITANIUM. Titanium occurs as an oxide (TiO 2 ) in the three minerals rutile, anatase, and brookite, of which the two first named are tetragonal, and the last rhombic. Its occurrence in ilmenite or titaniferous iron ore has been already dealt with on p. 178. The only important source of the metal is rutile, which can be reduced in the electric furnace by the aid of aluminium, the resulting ferrotitanium (10-20 per cent, of titanium) being used for increasing the strength of cast iron and of steel. Rutile. Dioxide of titanium : TiO 2 (titanium 60 per cent.). Crystallizes in the tetragonal system, in pris- matic forms similar to those of cassiterite. Twinned on the pyra- mid of the second order (101). Also occurs massive. Colour, reddish-brown. Streak, yellowish- brown. Lustre, metallic to ada- mantine. Translucent. Refrac- tive index high ; co = 2'6i6. Double FIG. 94. RUTILE refraction, strong, positive (e-o> = (REPEATED TWINNING). . 0*287). Prismatic cleavage, good. Fracture, uneven. Brittle. Hardness, 6*5. Density, 4*3. Infusible. Insoluble in acids. A good test for titanium is to fuse the mineral with sodium bisulphate, acidify the aqueous solution with sulphuric acid, and add hydrogen peroxide. A deep orange colour betrays the presence of titanium. Rutile is worked as an ore of titanium at Risor and other places in Norway, also in the United States. ORES 199 ORES OF MOLYBDENUM. Only two compounds of molybdenum are of com- mercial importance : the sulphide, molybdenite, and the molybdate of lead, wulfenite. The commercial ores should not contain less than 42 per cent, of the metal, and should be free from other metallic minerals. Molybdenum in the form of ferromolybdenum (con- taining from 75 to 87 per cent, of molybdenum) and nickel - molybdenum (containing 75 per cent.) is used for the manufacture of steel alloys. The effect of the addition of from 2 to 4 per cent, of molybdenum to steel is to increase the hardness, toughness, and elongation, without any corresponding deterioration when the steel is heated or welded. Molybdenite. Sulphide of molybdenum : MoS 2 (molybdenum, 59*96 per cent.). Crystallizes in the hexagonal system. Habit, six-sided tabular (basal plane, prism, and pyramid). Also occurs in scales and grains. Colour, lead grey. Streak, greenish -grey. Opaque. Lustre, metallic. Basal cleavage, perfect. Pliable. Sectile. Hardness, I. Greasy to the touch. Soils the ringers. Density, 47-4*8. Infusible. Heated in open tube yields sulphur fumes. Soluble in warm aqua regia. The powdered mineral, moistened with sulphuric acid and evaporated to dryness in a porcelain crucible, yields a characteristic blue colour. Molybdenite occurs in small quantities in many granites, pegmatites, gneisses, crystalline limestones, and schists. It is often associated with tin ore. 200 DESCRIPTIVE MINERALOGY Wulfenite. Molybdate of lead: PbMoO 4 (molyb- denum 26*2 per cent.). Crystallizes in the tetragonal system, with tabular habit. Colour, red. Streak, white. Translucent. Lustre, adamantine to resinous. Pyramidal cleavage, fair. Fracture, subconchoidal. Brittle. Refractive index, 2*402. Double refraction, strong. Hardness, 3. Density, 67. Fusibility, 2. Decomposed by hydrochloric acid. Wulfenite occurs in the oxidized zone of lead ores for example, in Utah, Nevada, Arizona, and New Mexico. ORES OF TUNGSTEN. Tungsten is obtained from the ores wolframite (tungs- tate of iron and manganese) and scheelite (tungstate of calcium), which are associates of cassiterite in nearly all tin-mining districts. The pure tungstate of manganese (hubneriU) and the pure tungstate of iron (ferberite), which are isomorphous with wolframite, are also known, and in places occur in sufficient quantity to be mined as ores.* The chief use for tungsten is in the manu- facture of steel alloys for lathe tools. A small percentage of tungsten increases the elastic limit and the tensile strength of steel. Tungsten steel is also self-hardening. On account of its high fusing-point (3080 C.), tungsten has also come into use as a filament in incandescent lamps. The world's production of tungsten ores is about 6,000 tons per annum, based on 60 per cent. ore. * A useful chemical test for tungsten is Digest the pulverized mineral in strong hydrochloric acid; the solution boiled with metallic zinc yields a blue coloration when tungsten is present. ORES 201 Wolframite. Tungstate of manganese and iron : (Fe,Mn,)WO 4 (WO 3 76 per cent.). Crystallizes in the monoclinic system. Colour, black. Streak, dark reddish-brown Lustre, metallic to adamantine. Opaque. Clinopinacoidal cleavage, perfect. Brittle. Fracture, un- even. Hardness, 5'5. Density, 7-3. Fusibility, 3. De- composed by hydrochloric acid. Wolframite is a common associate of tin ore, as in Cornwall, Spain, Bohemia, Straits Settlements, Queens- land, New South Wales, United States, Bolivia. Scheelite. Tungstate of calcium : CaWO 4 (WO 3 80 '6 per cent.). Crystallizes in the tetragonal system, with bipyramidal habit. Colour and streak, white. Lustre, vitreous to adamantine. Translucent. Index of refraction 1*919. Double refraction, strong. Py- ramidal cleavage, imperfect. Brittle. Fracture, uneven. Hardness, 4*5. Density, 6. Fusibility, 5. Decomposed by hydrochloric acid. Scheelite occurs in association with tin ore e.g., in Cornish, Bohemian, and Australian tin -mining districts. ORES OF URANIUM. The principal ores of uranium are pitchblende, torber- nite, and autunite. The rather doubtful mineral, carno- tite, appears to be a vanadium analogue of autunite. Uranium compounds are used in the coloration of glass and in porcelain - painting ; but they have recently acquired importance as the material from which radium compounds are prepared. 202 DESCRIPTIVE MINERALOGY Pitchblende, or uraninite. Oxide of uranium : UO.U 2 O 3 (contains lead and rare elements, including minute quantities of helium and radium). Crystallizes in the regular system, but usually occurs massive, botryoidal, or in grains. Colour, black, grey, or brown. Streak, greyish-black. Lustre, submetallic to resinous. Hardness, 5*5. Density, 9-97. Infusible. Soluble in dilute sulphuric acid. Pitchblende occurs in Cornwall in association with cassiterite ; also in Bohemia (Joachimsthal and Przi- bram), Saxony, Colorado (Gilpin County), and other places. Torbernite. Hydrated phosphate of uranium and copper: Cu(UO 2 ) 2 (PO 4 ) 2 .8H 2 O. Crystallizes in the tetragonal system, with tabular habit. Colour, emerald green. Transparent to translucent. Streak, pale green. Lustre, subadamantine to pearly. Basal cleavage, perfect (like mica). Non-flexible. Sectile. Hardness, 2-2*5. Density, 3*4-3*6. Fusibility, 3. Torbernite occurs in Cornwall, Bohemia (Joachims- thai), Saxony (Schneeberg). Autunite. Hydrated phosphate of uranium and calcium : Ca(UO 2 ) 2 (PO 4 ) 2 .8H 2 O. Crystallizes in the rhombic system, with tabular habit similar to torber- nite. Colour, citron to sulphur yellow. Translucent. Streak, yellowish. Lustre, subadamantine to pearly. Basal cleavage, perfect. Hardness, 2-2*5. Density, 3*05-3*2. Fusibility, 3. Autunite occurs in Cornwall. ORES 203 ORES OF OTHER RARE METALS. The minerals monazite and xenotime are esteemed is a source of ceria and thoria for incandescent gas nantles; but monazite alone occurs in sufficient quantity to be of economic importance. Columbite is ^alued as a source of tantalum for the manufacture of the metallic filaments of electric lamps, and zircon for the zirconium used in the electric lamps of the Nernst type. Monazite. Phosphate of cerium and lanthanum and didymium, with some thorium: (Ce,La,Di)PO 4 . Crystal- lizes in the monoclinic system. Colour, yellow to brown. Transparent to translucent. Lustre, resinous. Basal parting. Fracture, uneven. Hardness, 5-5*5. Density, 5'2-5'3. Index of refraction, i'8n. Double refraction, very high. Infusible. Monazite occurs in brown crystals in the granite of Arendal in Norway; but the commercial material is obtained from alluvial sands in Brazil (Prado), and North and South Carolina. Xenotime. Phosphate of yttrium : YPO 4 . Crystal- lizes in the tetragonal system. Colour, yellow to brown. Transparent to translucent. By alteration it becomes opaque. Lustre, resinous to vitreous. Prismatic cleavage, perfect. Fracture, uneven. Hardness, 4-5. Density, 4-55-5' i. Refraction and double refraction, very high. Xenotime occurs in granite and in sands, but less frequently than monazite. 204 DESCRIPTIVE MINERALOGY Columbite. A niobo-tantalate of iron and man- ganese : (Fe,Mn)O.(Nb,Ta) 2 O 5 . Crystallizes in the rhombic system, in short, prismatic crystals terminated by the basal plane. Colour, iron black. Lustre, sub- metallic to adamantine. Streak, black. Opaque. Fracture, uneven. Hardness, 6. Density, 5'2-6'o. Fusibility, 5-5*5. Columbite occurs in granitic rocks in Sweden, West Greenland, Ilmen Mountains of Siberia, and in Brazil (Minas Geraes). Orthite, or allanite (a member of the epidote group). Silicate of calcium, iron and aluminium, with small proportions of cerium, lanthanum, didymium, yttrium and erbium. Crystallizes in the monoclinic system. Colour, black. Streak, grey. Opaque to translucent. Lustre, vitreous to resinous. Fracture, uneven to con- choidal. Hardness, 5'5-6. Density, 3-4. Fusibility, 2*5. Orthite occurs in granites, pegmatites and crystalline schists e.g., in Sweden, Siberia, and the United States. ORES OF ALUMINIUM. Aluminium compounds are among the most widely distributed of minerals ; thus, the felspars, micas, and most of the hornblendes and pyroxenes, which con- stitute the bulk of the igneous rocks, are all aluminium silicates ; and the same minerals, together with quartz and other less complex silicates of aluminium, compose the great group of clays, shales, and slates. The only compounds of aluminium, however, which can be con- ORES 205 sidered as ores-or, in other words, from which the metal -an under present economic conditions be profitably >xtracted-are bauxite (a hydrated oxide of aluminium) md cryolite (a double fluoride of aluminium and sodium). Bauxite is probably not a definite mineral, but rather a trade name given to any substance in which there are large quantities of aluminium hydrate, in contradistinc- tion to ordinary clay, in which the aluminium is com- bined with silica, and cannot be profitably extracted. Aluminium is esteemed on account of its low density, its rigidity, its malleability, and the fact that it takes a high polish. It is largely used, for instance, in the motor industry (for crank -cases, gear-boxes, radiators, etc.). Its high conductivity for electricity makes it a competitor with copper for the transmission of power. It also forms valuable alloys with nickel, copper, zinc, and magnesium. Bauxite. Hydrated oxide of aluminium: A1 2 O 3 . 2 H 2 0.* (aluminium 39 per cent.). Does not occur crystallized, but in concretionary, pisohtic, or earthy masses. Colour, white, yellow, brown, or red. Opaque. Lustre, dull or earthy. Hardness most variable. Density, 2-55. Infusible. Insoluble in acids. Bauxite derives its name from Baux, near Aries, in France, where it is largely mined as an ore of aluminium. The only other large producer * Holland regards bauxite as an intimate mixture of two mmerals-viz., gibbsite, having the formula A1 2 O , 3 H.O : and diaspore, with the formula A1 2 O 5 .H 2 O (" Rec. Geol. S vol. xxxii., p. 176). 206 DESCRIPTIVE MINERALOGY United States, where the ore is mined in Alabama, Arkansas, Georgia, and Tennessee. Cryolite. A fluoride of aluminium and sodium : Al 2 F 6 .6NaF (aluminium 12*85 per cent., sodium 3279 per cent., and fluorine 54*36 per cent.). Crystallizes in the monoclinic system. It forms tabular crystals or occurs massive. Colourless to greyish -white, yellow, or brown. Streak, white. Lustre, vitreous. Transparent to translucent. Index of refraction low (/9= 1*36). Double refraction, weak. Cleavage, basal and prismatic, perfect, the cleavage appearing nearly cubical on account of the close approximation to 90 of the angle between the two prism faces and between the latter and the basal plane. Fracture, uneven. Brittle. Hardness, 2*5. Density, 2*97. Fusibility, 1*5. Decom- posed by sulphuric acid. Cryolite occurs as a large bed or vein in gneiss at Ivigtut in Arksukjord (West Greenland). APPENDIX TO THE ORES. THE VEINSTONES OR GANGUE MINERALS. The ores are accompanied in their lodes and veins by a number of minerals, which, from an economic point of view, are either intrinsically worthless, or are regarded as valueless in comparison with the ore for which the particular vein is exploited. The commonest veinstones are quartz and its congeners jasper, opal, chalcedony; the carbonates of calcium calcite and APPENDIX TO THE ORES 207 aragonite ; the double carbonate of calcium and mag- nesium dolomite ; the carbonate of iron chalybite ; the sulphate of barium barytes ; the sulphates of calcium anhydrite and gypsum; the iron-oxides magnetite, hematite and limonite; the oxides of manga- nese wad, etc.; the sulphides of iron pyrites, marcasite, and pyrrhotite ; the arsenosulphide of iron mispickel ; and many other sulphides, which under more favour- able circumstances are themselves regarded as ores e.g., blende, galena, stibnite and molybdenite. Veins of pneumatolytic origin are characterized by the presence of tourmaline, fluorspar and topaz. Many rock-forming minerals also occur as veinstones: examples are, the felspars orthoclase and albite ; the hornblendes, pyroxenes and micas and their decomposition products chlorite, serpentine, sericite and talc; the zeolites, such as analcime, laumontite and prehnite ; lastly, the so-called accessory rock-forming minerals, such as garnet, apatite, and sphene. Descriptions of all these minerals are given in their appropriate chapters. CHAPTER III THE SALTS AND USEFUL MINERALS OTHER THAN ORES ALTHOUGH every combination of an acid with a base may be termed a salt, only the carbonates, sulphates, nitrates, chlorides, phosphates and borates of the alkalies and alkaline earths are included here.* These substances occur frequently as beds that have been deposited from the waters of a former lake or inland sea ; but many of them are found also as veins, either alone or forming the gangue material of ores of the heavy metals. Nitrates are formed by the decompo- sition of organic matter in saliferous and rainless dis- tricts ; while phosphatic deposits have in part been produced by the accumulation of bones or of the coprolites of fishes. An appendix to the chapter contains a description of the non-metallic elements sulphur and carbon. CARBONATES. A great number of carbonates occur as natural salts. Some of them namely, the carbonates of the heavy * The compounds formed by the union of some of the acids with the heavier metallic bases have been already referred to among the ores, 208 SALTS, ETC. 209 metals have been mentioned among the ores. The carbonates now to be described are the following : calcite,aragonite, dolomite, magnesite,witherite, strontianite, natron, and trona. Calcite. Carbonate of lime : CaCO 3 (lime 56, carbon dioxide 44, per cent.). Crystallizes in the FIG. 95. CALCITE: PRISMATIC FORMS. c, Basal plane; #, pyramid ; r, rhombohedron ; a, prism. hexagonal system, with rhombohedral hemihedrism. Common forms are flat or acute rhombohedra ; sharp- pointed scalenohedra (dog-tooth spar) ; prisms, terminated FIG. 96. CALCITE: RHOMBOHEDRAL FORMS. r t Rhombohedron (R) ; e, a more obtuse rhombohedron ( - \ R). FIG. 97. CALCITE. Twinned crystal, the twin-plane being a face of the rhombo- hedron, e(-\ R). either by the basal plane or by rhombohedral faces; and flat tables with dominant basal plane. There are several types of twinning : that in which the twin-plane 14 210 DESCRIPTIVE MINERALOGY is the obtuse rhombohedron (e), is the commonest, and is responsible for the polysynthetic lamellation char- acteristic of calcite when viewed in thin sections under the microscope. Colourless to white, yellow, red, and brown. Transparent to opaque. Lustre, vitreous. Streak, white. Index of refraction, moderate (co = 1*658). Double refraction, negative, very strong (to e = O'ij2). Rhombohedral cleavage, perfect. Fracture, conchoidal. Brittle. Hardness, 3. Density, 27. Infusible. Soluble, with effervescence in acids. FIG. 98. CALCITE : SCALENOHEDRAL FORMS. y, Scalenohedron (R 3 ). Calcite is a frequent veinstone, a common secondary constituent in rocks, and the chief component of meta- morphic limestones (marbles). The purest variety Iceland spar is used in the manufacture of optical instruments. Aragonite. Carbonate of calcium : CaCO 3 (lime 56, carbon dioxide 44, per cent.). Crystallizes in the rhombic system, in tabular and prismatic forms; its frequent pseudo-hexagonal habit is due to repeated twinning on the prism (m). Occurs also as fibrous SALTS, ETC. 211 aggregates. Colourless. Transparent. Lustre, vitreous. Index of refraction high (/3 = 1*682). Double refraction, negative, very strong (7 - a-=OT56). Brachypinacoidal and prismatic cleavages, imperfect. Fracture, sub- conchoidal. Brittle. Hardness, 3*5. Density, 2*9. Infusible. Soluble in acids, with effervescence. \ FIG. 99. ARAGOMTE. b, Brachypinacoid ; m, prism k, brachydome. FIG. loo. DIAGRAM ILLUSTRATING THE MODE OF TWINNING IN ARAGONITE : FOUR INDIVIDUALS TWINNED ON A FACE OF THE PRISM. The striation indicates the direction of the brachydiagonal. Aragonite occurs as a gangue material of ore deposits ; also as stalactitic incrustations, and as spheroidal con- cretions as in the Sprudelstein of Carlsbad in Bohemia. Dolomite, or pearl spar. Double carbonate of calcium and magnesium : CaMg(CO 3 ) 2 (lime 30*4, magnesia 217, carbon dioxide 47*9, per cent.). Crystallizes in the hexagonal system, with rhombo- hedral hemihedrism. Saddle-shaped crystals with curved faces are characteristic. Colourless to white. Transparent. Lustre, vitreous. Streak, white. Index of refraction high (a> = r682). Double refraction, negative, very strong (o> e = 0*179). Rhombohedral cleavage, perfect. Fracture, subconchoidal. Brittle. Hard- 212 DESCRIPTIVE MINERALOGY ness, 3*5. Density, 2-85. Infusible. Soluble with effervescence in warm acids. Dolomite occurs as a veinstone in association with ores, and massive in magnesian limestones. Magnesite. Carbonate of magnesium : MgCO 3 (magnesia 47*62, carbon dioxide 52*38, per cent.). Crystallizes in the hexagonal system, with rhombohedral symmetry. More usually, however, it occurs massive, in granular, fibrous, or compact aggregates. Colour- less, white, or yellow. Transparent to opaque. Rhom- bohedral cleavage, perfect. Hardness, 4-4*5. Density, 2*9-3. Infusible. Soluble in acids, with effervescence, when in the state of powder and warmed. Magnesite occurs as a decomposition product of ferro- magnesian silicates, and is found associated with ser- pentine, which is of similar origin. It is exploited for the manufacture of the sulphate of magnesia (Epsom salt). Witherite. Carbonate of barium : BaCO 3 (oxide of barium 77*7, carbon dioxide 22*3, per cent.). Crystal- lizes in the rhombic system. Habit, pseudo- hexagonal and bipyra- midal (see Fig. 101). Twinning on the prism. Also found massive or in FIG. IOI.-WITHERITE. botryoidal or kidney- Pseudo-hexagonal bipyramid com- posed of t, pyramid ; w, brachy dome . shaped aggregates. Colourless to white, grey or yellowish-grey. Transparent. Lustre, vitreous. SALTS, ETC. 213 Streak, white. Index of refraction, moderate (/3= 1*531). Double refraction, negative, weak. Brachypinacoidal and prismatic cleavages, imperfect. Fracture, uneven. Brittle. Hardness, 3-3*5. Density, 4*2-4*3. Fusibility, 2. Colours the flame green. Soluble in hydrochloric acid. Witherite accompanies certain ore deposits as a vein- stone, and is exploited to a small extent^ for use in the manufacture of plate-glass. Strontianite. Carbonate of strontium : SrCO ;{ (oxide of strontium, 70*17, carbon dioxide 29*83, per cent.). Crystallizes in the rhombic system, in varieties of habit resembling aragonite, but usually found in fibrous aggre- gations. Colourless to white, grey, yellow, brown, or green. Transparent. Lustre, vitreous. Streak, white. Prismatic cleavage, imperfect. Fracture, uneven. Hard- ness, 3*5-4. Density, 3*6-3*8. Infusible. Colours the flame red. Strontianite occurs as a veinstone, and is exploited for use in the refining of sugar. Natron. Hydrated carbonate of sodium : Na 2 CO 3 . ioH 2 O. Crystallizes in the monoclinic system, but usually found massive or as an earthy incrustation. Colourless to grey or white. Lustre, vitreous. Streak, white. Basal cleavage, imperfect. Hardness, 1-1*5. Density, 1*4-1*45. Fusibility, I. Colours the flame yellow. Soluble in water. Natron occurs as an evaporation product of the so- called soda-lakes in Africa and America, and is exploited for the manufacture of soda salts. 214 DESCRIPTIVE MINERALOGY Trona. Hydrated carbonate of sodium : Na 2 CO 3 . HNaCO 3 . 2H 2 O. Crystallizes in the monoclinic system, but usually occurs massive or as earthy incrus- tations. Colourless to white or grey. Transparent. Lustre, vitreous. Orthopinacoidal cleavage, perfect. Fracture, uneven. Hardness, 2*5-3. Density, 2* 1-2*15. Fusibility, 1*5. Colours the flame yellow. Soluble in water. Trona occurs as an incrustation from the evapora- tion of lakes ; e.g., at Borax Lake, San Bernardino Co., California. SULPHATES. Several important sulphates occur as natural salts. Of these, the sulphates of the heavy metals have been referred to among the ores ; but the following sulphates of the alkalies and alkaline earths are treated here : anhydrite, gypsum, barytes, celestite, mirabilite, epsomite, alum, and alunite. Anhydrite. Sulphate of lime: CaSO 4 (lime 41*16, sulphur trioxide 58*84, per cent.). Crystallizes in the rhombic system, in thick tabular crystals; but more often found massive, in coarsely crystalline aggre- gates resembling marble. Colourless, white, or bluish- white. Transparent to opaque. Pinacoidal cleavages, perfect. Fracture, uneven. Brittle. Hardness, 3-3*5. Density, 2*8-3. Fusibility, 3. Soluble in hydrochloric acid. Anhydrite occurs in beds associated with gypsum and rock-salt, as, for example, in the New Red Marl SALTS, ETC. 215 of this country, and in the Stassfurt salt deposits of Central Germany. Gypsum, or selenite. Hydrated sulphate of lime : CaSO 4 .2H 2 O (lime 32*5, sulphur trioxide 46*6, water 20*9, per cent.)- Crystallizes in the monoclinic system, usually in stout or slender prisms, but also in flat rhomboid tables, that owe their shape to the domin- ance of the clinopinacoid. Occasionally the crystals are lenticular, with curved faces. Also occurs in fibrous masses (satin spar) ; or granular (alabaster). Twinning FIG. 102. GYPSUM. /, Negative hemi-pyramid ; n, posi- tive hemi-pyramid ; f, prism ; b, clinopinacoid. FIG. 103. GYPSUM : CRYSTAL TWINNED ON THE ORTHO- PINACOID. on the orthopinacoid is common, producing a swallow tailed form. Usually colourless and water-clear, but also white or tinted. Streak, white. Lustre, vitreous. Transparent. Index of refraction, 1*522. Double refraction, moderate (7 a = o'oi). Clinopina- coidal cleavage, perfect. Sectile. Flexible. Hardness, 2. Density, 2'2-2'4. Fusibility, 3. Soluble in hydrochloric acid. Gypsum is a very common mineral occurring in beds and veins in marls and clays in the New Red Marl, 216 DESCRIPTIVE MINERALOGY Oxford Clay, Purbeck Beds, Gault, London Clay, etc. It is used to improve soils or to make plaster of Paris. The compact granular variety (alabaster) is in request for vases, statuary, etc. Barytes, or heavy spar. Sulphate of barium : BaSO 4 (oxide of barium 657, sulphur trioxide 34*3, per cent.). Crystallizes in the rhombic system, with tabular or prismatic habit, the former being due to the dominance of the basal plane (c) in combination with the prism (m), the latter to elongation either along the 6-axis with dominant macrodome (d), or along the a-axis with dominant brachydome (o). Also compact, granular, or fibrous. Colourless to reddish-white, grey, or yellow. Lustre, vitreous. Transparent. Streak, white. Index of refraction, high (/3= 1-637). Double refraction, positive, moderate (7 - a = croi2). Basal and pris- matic cleavage, perfect. Fracture, uneven. Brittle. Fusibility, 3. Colours the flame green. Not decom- posed by acids. Barytes often forms the gangue material of ores ; but it also occurs alone in veins, and is exploited for mixing with white lead. Celestite, or celestine. Sulphate of strontium : SrSO 4 (oxide of strontium 56*52, sulphur trioxide 43*48, per cent.). Crystallizes in the rhombic system, being isomorphous with barytes. Habit, prismatic, due to elongation along the 6-axis, with dominant macrodome ; or tabular, with dominant basal plane. Colourless to bluish-white or grey. Transparent. Lustre, vitreous. SALTS, ETC. 217 Streak, white. Index of refraction, high (/3= 1*624). Double refraction, positive, moderate (7 a = 0*009). Basal cleavage, perfect ; prismatic, good. Fracture, uneven. Hardness, 3-3*5. Density, '3*9-4. Fusible. Colours the flame red. Insoluble in acids. Celestite is exploited for the preparation of strontium compounds, which are used for fireworks. Mirabilite, or Glauber's salt. Hydrated sulphate of soda: Na 2 SO 4 ioH 2 O (soda 19*3, sulphur trioxide 24*8, water 55*9, per cent.). Crystallizes in the mono- clinic system, but occurs in Nature mostly in the form of an efflorescent incrustation. Colourless to white. Lustre, vitreous. Transparent. Streak, white. Pina- coidal cleavage, perfect. Hardness, 1*5-2. Density, 1*48. Fusibility, 1*5. Colours the flame yellow. Soluble in water. Occurs in the so-called soda-lakes of Wyoming and in Salt Lake, Utah. Epsomite, or Epsom salt. Sulphate of magnesium : MgSO 4 .7H 2 O (magnesia 16*26, sulphur trioxide 32*52, water 5 1*22 per cent.). Crystal- lizes in the rhombic system, with dominant prism (m) and brachydome (z), but occurs XT . ,1 r-i FIG 104 ARTIFICIAL CRYSTAL in Nature mostly as a fibrous 4 OF EpsoMITE . efflorescence. Colourless to white. Transparent. Lustre, vitreous. Cleavage, brachypinacoidal. Fracture, conchoidal. Hardness, 218 DESCRIPTIVE MINERALOGY 2-2*5. Density, i'7-r8. Fusibility, i. Soluble in water, to which it imparts a bitter taste. It occurs on the steppes of Siberia, in Catalonia, and as a deposit from the soda-lakes of Wyoming in the United States. It also exists in solution, as in the springs at Epsom and in sea water. Alums. The alums constitute an isomorphous group of sulphates of alumina and of the alkalies, magnesia, iron, and manganese, each member of which crystal- lizes in the regular system, with twenty-four molecules of water. Some of the more important members of the group are the following : Kalinite or potash-alum : K 2 S0 4 + A1 2 (S0 4 ) 3 +24H 2 0. Mendozite or soda-alum : Na 2 SO 4 + A1 2 (SO 4 ) 3 + 24H 2 O. Tschermigite or ammonia-alum : (NH 4 ) 2 S0 4 + A1 2 (S0 4 ) 3 These salts are easily soluble in water, and have a sweetish astringent taste. Although the alums exist in small quantities as natural efflorescences, the alum of commerce is chiefly prepared from the mineral alunite, or alum- stone, a hydrated sulphate of alumina and potash : (K 2 O.3A1 2 O 3 .4SO 5 .6H 2 O), and from aluminous shales containing pyrites, the latter mineral supplying the sulphuric acid. SALTS, ETC. 219 NITRATES. The important nitrates occurring as natural salts are those of the alkalies, potash, and soda viz., nitre and nitratine. They are used in the manufacture of nitric acid and of gunpowder, and are also of great value as artificial manures. Nitre, or saltpetre. Nitrate of potash: KNO 3 (potash 46*58, nitrogen peroxide 53*42 per cent.)- Crystal- lizes in the rhombic system, in silky needle-shaped prisms, but also occurs as an incrustation. Colour- less, white, or grey. Lustre, vitreous. Transparent. Cleavage, prismatic. Fracture, conchoidal. Hard- ness, 2. Density, 1*9-2*1. Fusibility, i. Easily soluble in water. Like all potash salts, imparts a violet coloration to the flame of a spirit-lamp or Bunsen burner. Nitre is found as an efflorescent crust, or mixed with the soil of certain rainless districts in Spain, Algeria, India, Quito and is used principally for the prepara- tion of gunpowder. Nitratine, soda-nitre, or Chili saltpetre. Nitrate of soda: NaNO 3 (soda 36*49, nitrogen peroxide 63*51, per cent.). Crystallizes in the hexagonal system, with rhombohedral hemimorphism ; but is usually found as an efflorescent incrustation. Colourless, but also white or grey. Lustre, vitreous. Transparent. Rhombo- hedral cleavage, perfect. Fracture, conchoidal. Hard- ness, 1-5-2. Density, 2*i-2'2. Deliquescent and easily 220 DESCRIPTIVE MINERALOGY soluble in water. Fusibility, I. In common with all soda salts, it colours the flame yellow. Nitratine is found mixed with clay and sand (caliche), on the rainless pampas of Chili (Tarapaca and Anto- fagasta) and Peru, and is exploited for the preparation of nitric acid and of nitre, and as a manure, being too deliquescent for the direct manufacture of gun- powder. CHLORIDES AND FLUORIDES. There are a great number of naturally - occurring chlorides. The following only are selected for de- scription : rock-salt, sylvite, sal ammoniac, carnallite, and the fluoride, fluorspar. Rock-Salt, or halite. Chloride of sodium : NaCl (sodium 39-3, chlorine 607, per cent.)- Crystallizes in the regular system, usually in cubes, but also occurs in granular masses and in fibrous aggregates. When pure, colourless or white, but often tinted yellow, red, blue, FIG. 105. ROCK-SALT. e tc., by the presence of a small Skeleton-cubes. quantity o f some impurity. Trans- parent to opaque. Lustre, vitreous. Streak, white. Cubic cleavage, perfect. Fracture, conchoidal. Brittle. Hardness, 2-2*5. Density, 2-1-2-3. Fusibility, 1-5. Soluble in water, and easily recognizable by its saline taste. Rock-salt is widely distributed among lacustrine SALTS, ETC. 221 deposits. Thus it occurs with gypsum in the Trias of Cheshire and Worcestershire; and there are thick deposits at Sperenberg near Berlin, at Wieliczka, in Austrian Poland, and at Parajd in Transylvania. In the Permian of Stassfurt, in Central Germany, it occurs in association with gypsum and numerous FIG. 106. ROCK-SALT CRYSTALS FROM STASSFURT (AFTER G. P. MERRILL). sulphates and chlorides of potassium and magnesium, the exploitation of which constitutes an important industry. Salt is also found as an efflorescence in the rainless districts of Chili and Africa, and as a deposit from brine in the " salt pans " of the Northern Transvaal. 222 DESCRIPTIVE MINERALOGY Sylvite, or sylvine. Chloride of potassium: KC1 (potassium 52*4, chlorine 47*6, per cent.). Crystallizes in the regular system, with cubic habit. Colourless. Lustre, vitreous. Transparent. Streak, white. Cubic cleavage, perfect. Brittle. Fracture, uneven. Hard- ness, 2. Density, 1*9. Fusibility, 1*5. Soluble in water, to which it imparts a saline, bitter taste. Sylvite occurs with rock-salt at Stassfurt in Central Germany, and at Kalusz in Galicia. Sal Ammoniac. Chloride of ammonium: NH 4 C1 (chlorine 66*26, nitrogen 26*25, hydrogen 7*49, per cent.). Crystallizes in the regular system, but usually in stalactitic aggregations or as an incrustation. When pure, colourless or white, but often stained yellow by chloride of iron. Hardness, 1*5-2. Density, 1*52. Soluble in water, to which it imparts a pungent saline taste. Sal Ammoniac occurs as an incrustation on lava at Vesuvius, Etna, and in other volcanic districts. Carnallite. Double chloride of magnesium and potash: MgCl 2 .KCl + 6H 2 O (MgCl 2 , 34*2, KC1, 26*8, H 2 O 39, per cent.). Crystallizes in the rhombic system. Colourless to white or red. Transparent. Lustre, vitreous. Fracture, conchoidal. Hardness, i. Density, 1*6. Fusibility, 1-1*5. Soluble in water, and deliquesces when exposed to air. Carnallite occurs at Stassfurt in Central Germany. Fluorspar or fluorite. Fluoride of calcium : CaF 2 (fluorine 48*72, calcium 51*28, per cent.) Crystallizes in SALTS, ETC. 223 the regular system, occurring in cubes and octahedra, sometimes in combination with the rhombic dodeca- hedron and icositetrahedron. Interpenetration twins FIG. 107, THE STASSFURT DEPOSITS. a, Rock-salt ; b, polyhalite ; c, kieserite ; d, carnallite ; e, kainite ; /, impervious clay ; g t anhydrite ; h, gypsum ; k, sandstone. common. The crystals are transparent, and have a glassy lustre. Colour, blue, violet, green, and yellow. FIG. 108. FLUORSPAR : CUBE WITH OCTAHEDRON. FIG. 109. FLUORSPAR : CUBE WITH ICOSITETRAHEDRON. Transparent. Lustre, vitreous. Streak, white. Index of refraction, low (/* = i'43). Octahedral cleavage, perfect. Fracture, subconchoidal. Hardness, 4. 224 DESCRIPTIVE MINERALOGY Density, 3-2. Fusibility, 3. Phosphorescent when heated. Decomposed by sulphuric acid. As a veinstone, fluorspar" is associated with tin ore in Saxony, Bohemia, and Cornwall, with lead ores in Derbyshire, Cumberland, and Northumberland, and with silver ores in Saxony, the Harz, and Norway (Kongsberg). The more beautiful varieties of the spar (" Blue John ") are used for the manufacture of orna- mental vases, while the commoner varieties are used as a flux in metallurgical processes, and for the prepara- tion of hydrofluoric acid. PHOSPHATES. The only phosphates of commercial importance are those of lime, known variously as apatite, phosphorite, coprolites, and guano. Apatite. Chlorophosphate of calcium, 3Ca 3 (PO 4 ) 2 . CaCl 2 , or fluorophosphate of calcium, 3Ca 3 (PO 4 ) 2 . CaF 2 . The fluorine variety contains 42*26 per cent, of phosphorus pentoxide, and 3*77 per cent, of fluorine ; while the chlorine variety contains 40*92 per cent, of the oxide, and 6-8 2 per cent, of chlorine. Crystallizes in the hexagonal system. Habit, short prismatic or thick tabular. The smaller crystals are composed of numerous pyra- mid and prism faces, usually terminated by the basal FIG. c, Basal plane ; x, proto SALTS, ETC. 225 plane. Large opaque crystals and compact or fibrous nodular masses (phosphorite) also occur. Colourless to white, but usually tinted green, blue, violet, or red. Lustre, vitreous to resinous. Transparent to opaque. Streak, white. Refractive index, moderately high (= 1*646). Double refraction, negative, weak (o> e = 0*004). Basal and prismatic cleavages, imperfect. Frac- ture, conchoidal. Brittle. Hardness, 5. Density, 3'i6-3*22. Fusibility, 5. Decomposed by hydrochloric acid. Mineral phosphates are exploited for use as fertilizers in Cornwall, Spain, Germany, Norway, Russia, the United States, and Canada. Many of these deposits occur as nodular concretions which have been formed by the concentration of phosphatic material of organic origin. Guano is a phosphatic deposit of recent organic origin, occurring in regions where abundant animal life and small rainfall combine to assist its accumulation, as in Chili, Peru, Bolivia and Africa. BORAXES. Two borates are here described viz., borax and boracite. Borax. Hydrated borate of sodium : Na 2 O.2B 2 O 3 . ioH 2 O (soda 16*23, boron trioxide 36*65, water 47*12, per cent.). Crystallizes in the monoclinic system, but usually occurs massive (tinkal). Colourless to white. Transparent. Lustre, vitreous. Streak, white. Index of refraction, low (/3=i*47). Double refraction, 15 226 DESCRIPTIVE MINERALOGY negative, weak (7 a= 0*004). Orthopinacoidal cleavage, perfect ; prismatic cleavage, good. Fracture, conchoidal. Brittle. Hardness, 2-2-5. Density, 1*7. Fusibility, 1-1*5. Soluble in water, to which it imparts a sweetish alkaline FIG. in. BORAX. o, Orthopinacoid : m, prism, taste. Borax is found as a chemical deposit on the shores of lakes in Thibet, California, and Nevada, and is exploited for its antiseptic properties. Boracite. Chloroborate of magnesium : 6MgO. 2B 2 O 3 .MgCl 2 (magnesia 26*9, boron trioxide 62*5, chloride of magnesium 10*6, per cent.). Crystallizes apparently in the regular system, with tetrahedral development. According to a recent view, the crystals are considered to be compound twins of rhombic or of monoclinic individuals. Colourless to white. Lustre, vitreous. Transparent to trans- lucent. Streak, white. Index of refraction, moderate (ft = 1-667). Double refraction, moderate FIG. 112. BORACITE. ( 7 -a = 0'OIl). Tetrahedral a > Cube : rf > rhombic dodeca- hedron ; t, tetrahedron. cleavage, imperfect. Fracture, conchoidal. Brittle. Hardness, 7. Density, 2'g-3. Fusibility, 3. Soluble in hydrochloric acid. Boracite is found enclosed in gypsum and anhydrite at Luneburg and Segeberg, and in carnallite at Stass- furt in Germany. OTHER USEFUL MINERALS 227 Streak, Double APPENDIX. OTHER USEFUL MINERALS. Sulphur, S. Crystallizes in the rhombic system, with bipyramidal habit. Also occurs in nodular, kidney- shaped or stalactitic masses or as a mealy deposit or incrustation. Colour, pale yellow to dark brown. Lustre, resinous to adamantine. Transparent white. Index of refraction, high (^=2*04). refraction, very strong (7 a= 0*290). Basal and prismatic cleavage, very imperfect. Fracture, conchoidal. Brittle. Hardness, r'5-2'5. Density, 1*9. Easily fusible. Volatile and combustible. Insoluble in acids. Sulphur is produced by sublima- tion in volcanic districts, by deposi- tion from sulphurous springs, or by the alteration of beds of gypsum. It occurs in deposits of industrial importance in Sicily and Japan. Graphite, Carbon, C. Crystallizes probably in the monoclinic system, although its six-sided crystals have a decidedly hexagonal habit. Most frequently it occurs massive or in scales disseminated through rocks. Colour, black. Lustre, metallic. Streak, grey. Opaque. Unctuous to the feel, soiling the fingers. Pliable. Hardness, 0*5-1. Density, i'9-2'3. Infusible. Insoluble in acid. Graphite occurs in scales disseminated through lime- stone, slate, gneiss, mica-schist, or in larger pocket-like deposits and veins, and it is exploited for " black- FIG. 113. SULPHUR; RHOMBIC PYRAMID. 228 DESCRIPTIVE MINERALOGY lead " or plumbago. It is found in deposits of indus- trial importance in Borrowdale, near Keswick, in the Lake District, in Finland and Siberia, in Austria, in India and Ceylon, and in the United States and Canada. Coal, mineral pitch (asphaltum), bitumen, mineral oil (petroleum), mineral wax (ozokerite), mineral resin (amber), are other mineral substances of which the chief con- stituent is carbon. They are not, however, properly speaking, mineral species, since they have neither an unvarying chemical composition nor definite physical properties. Petroleum consists of an homologous series of hydrocarbons of the general formula CH 4 + nCH 2 , which in Nature occur mixed in all proportions. By fractional distillation it can be separated into heavy lubricating oils, light oils, spirits (petrol, benzol, etc.). CHAPTER IV GEMS UNDER the head of gems are included certain rather rare minerals which are used for the purpose of orna- mentation and personal adornment. The two chief physical features in a precious stone are hardness and brilliancy. While a sufficient degree of the former insures durability by preventing deteriora- tion under wear, it is the latter which determines the beauty of the gem-stone. But brilliancy is produced by a combination of properties which may be variously developed in a stone. These properties are the pellu- cidity, colour, refractive index, dispersive power, and pleochroism. The pure and delicate colours possessed by gems are a great feature of their beauty ; and in many cases the jeweller's names are founded entirely on differences in colour. Thus the ruby and the sapphire are red and blue varieties respectively of the mineral corundum, and the emerald and the aquamarine are green and blue varieties of beryl. Although most gems are perfectly limpid, a few are only translucent or opaque (opal, turquoise, etc.). The dispersive power (see p. 47) 229 230 DESCRIPTIVE MINERALOGY imparts the quality of emitting brilliant flashes of variously coloured light, which determines the " fire " of a gem. Many gems possess in greater or less degree the property of transmitting differently coloured light when viewed in different directions. This is a phe- nomenon of pleochroism (see p. 54). Among the minerals mentioned in this chapter will be found several that have been described in other parts of the book. Thus some varieties of quartz, felspar, and olivine, which are among the commonest rock-forming minerals, are used as gem-stones. Diamond, Carbon, C. Crystallizes in the regular system, the usual habit being the octahedron alone, or FIG. 114. CRYSTALS OF DIAMOND. this form in combination with the rhombic dodecahe- dron and other regular forms. The faces are generally curved, producing rounded crystals. Twinning on the octahedron. Colourless ; but also yellow, straw-coloured, or brown, and very occasionally green and blue. Trans- parent. Lustre, adamantine. Index of refraction, very high (//. = 2*417). Dispersion, very strong. Octahedral cleavage, perfect. Fracture, conchoidal. Brittle. Hard- ness, 10. Density, 3*52. Infusible. Insoluble in acid. Burnt in oxygen, yields carbon dioxide. Diamonds are found in alluvial sands, in association GEMS 231 FIG. 115. THE CULLINAN DIAMOND. Half natural size. with other precious stones, and with gold and platinum in the Deccan of India, Brazil (Minas Geraes), Borneo, Sum- atra, and Australia. The chief source of the present supply is at Kimberley, in South Africa, where stones of the first water and of good size are found imbedded in a dark-coloured ultrabasic volcanic breccia (kimberlite), filling volcanic vents or " pipes." Corundum. Oxide of alu- minium, A1 2 O 3 (aluminium 52*9 per cent.). Crystallizes in the hexagonal system, with rhombohedral symmetry. Habit, tabular, pris- matic, or pyramidal, the first -named being deter- mined by the dominance of the basal plane. Hori- zontal striation on pyramid and prism faces. Also occurs in rounded crystals and as rolled pebbles and grains in the so-called " gem-sands." Colourless when pure. Generally coloured : blue (sapphire), red (ruby), yellow (oriental topaz), grey, purple (oriental amethyst), or green (oriental emerald). Streak, colourless. Lustre, vitreous to adamantine or resinous. Transparent to translucent. Index of refraction high (w = 1768). Double refraction, negative, moderate (o> - e= 0*008). No true cleavage. Separation (due to twinning) parallel to the basal plane and to the rhombohedron r (1011). Frac- ture, conchoidal. Brittle. Hardness, 9. Density, 3*9-4'!. 2&2 DESCRIPTIVE MINERALOGY Corundum occurs as an accessory constituent in igneous rocks rich in alumina, both as an allogenic and as an autogenic product. It also occurs in granite contact zones, especially in argillaceous limestones. The dark granular variety, known as emery, occurs in admixture with iron ores (magnetite and haematite) in association with schists and gneiss on the island of Naxos. A massive variety of red corundum is exploited for use as a polishing material at Salem in Madras. FIG. 116. CORUNDUM: CHARACTERISTIC CRYSTALS. d, Basal plane; r, rhombohedron (R) ; o, rhombohedron (-2R); n, deutero-pyramid (|P2) ; e, deutero-pyramid (f P2) ; I, deutero- prism. Rubies occurring in a matrix of limestone are worked in Burmah. Many of the best gems are found loose in gravels and sands, as in Ceylon (rubies), Siam (sapphires and rubies), Montana (sapphires). Spinel. A double oxide of aluminium and magne- sium : MgO,Al 2 O 3 (magnesia 28-2, alumina 71*8, per cent.). Crystallizes in the regular system. Habit, octa- hedral. Twinned on the octahedron. Colour, red. Streak, white. Lustre, vitreous. Transparent. Index of re- fraction, high (yii=r7i5). Octahedral cleavage, im- perfect. Fracture, conchoidal. Brittle. Hardness, 8. GEMS Density, 3*5. Infusible. Soluble with difficulty in sulphuric acid. Spinel occurs in crystalline limestones and dolomites, also in igneous and metamorphic rocks. It is of frequent FIG. 117. SPINEL: OCTA- HEDRON. FIG. 118. SPINEL: CRYSTAL TWINNED ON AN OCTAHEDRAL FACE. occurrence in sands e.g., the so-called " gem sands " of Ceylon. Beryl. Metasilicate of beryllium and aluminiurn : 3BeO.Al 2 O 3 .6SiO 2 (oxide of beryllium 14, alumina 19, silica 67, per cent.). Crystallizes in the hexagonal system, forming long six-sided prisms, terminated by the basal plane. In addition pyramidal faces are some- times developed. The prismatic planes usually show a vertical stria- tion. Colourless to white, yellowish or greenish white, pale pink, honey- yellow, and various shades of green and blue. The emerald-green to apple-green varieties are known as emerald, while the blue or sea-green varieties are distinguished as aqua- marine. Lustre, vitreous. Transparent to translucent. FIG. 119. BERYL. c, Basal plane ; P, pyramid ; a, prism ; r, deutero-pyramid (2P2). 234 DESCRIPTIVE MINERALOGY Streak, white. Index of refraction, moderate (&> = I "584). Double refraction, negative, weak (&> e=o'oo6). Basal cleavage, distinct. Fracture, conchoidal to uneven. Brittle. Hardness, 7*5-8. Density, 2'6-2'S. Fusibility, 5'5. Insoluble in acids. Beryl is found imbedded in granite and pegmatite veins ; also in mica-schist, limestone, and clay slate. The best-known localities are the Ural Mountains (Mursinka, Ekaterinburg, and Miask), the Altai, Colombia (Muzo, Cosquez and Somondoco mines), and New South Wales (Emmaville). Garnet. The garnets are silicates of aluminium, iron, manganese, chromium, calcium, and magnesium, having the general formula 3MO.R 2 O 3 .3SiO 2 , in which M stands for metals like calcium, magnesium, etc., forming prot- oxides, and R for metals like aluminium and chromium, forming sesquioxides. According to the variation of MO and R 2 O 3 , the following varieties may be dis- tinguished : Constituent Bases. Name. Colour. Density. Index of Refraction. MO. R 2 3 . Grossularia Almandine Pyrope CaO FeO FeO, MgO, CaO AUO, ' Pale green AUG. Claret A1. 2 O,, Cr 2 O, | Blood red 3-4-3-6 3-9-4-3 3*7-3 '8 i'747 i 807 1*745 Melanite CaO Fe 2 O 3 I Blackish-. \ brown ' 3-6-4-3 1-856 Spessartine MnO A1 2 O 3 {reith} 4-0-4-3 .'Sic Uvarovite CaO Cr 3 O 3 Green 3'4~3'5 1*838 Other existing varieties are isomorphous mixtures of these. They crystallize in the regular system, the most GEMS 235 common habit being the rhombic dodecahedron, with edges sometimes truncated by icositetrahedral faces. Colour, as above. Streak, white. Lustre, vitreous to resinous. Transparent to opaque. Index of refrac- tion, as above. Rhombic dodecahedral cleavage, im- perfect. Fracture, uneven. Hardness, 6*5-7. Density, as above. Brittle. Fusibility, 3 for all varieties except uvarovite, which has a fusibility of 6. Attacked with difficulty by hydrochloric acid. The garnets are frequent accessory constituents of FIG. 120. GARNET: COMMON FORMS, RHOMBIC DODECAHEDRON, ICOSITETRAHEDRON, ALONE AND IN COMBINATION. igneous rocks, such as granite, microgranite, aplite, trachyte, and andesite. They are also found in gneisses and other crystalline schists, and in ultrabasic rocks, such as peridotites, eclogite, serpentine, kimberlite, etc. They are a frequent constituent of the so-called "gem sands." It would serve no purpose to give localities for so common a mineral ; but the occurrence of pyrope in Bohemia, of melanite at Frascati, near Rome, and uvarovite in the Ural Mountains, may be mentioned. Topaz. Fluorsilicate of aluminium : A1(F.OH) 2 . AlSiO 4 (alumina 55*44, SiO 2 32*61, fluorine 20*65, per 236 DESCRIPTIVE MINERALOGY cent.)- Crystallizes in the rhombic system. Habit, prismatic, terminated by the basal plane alone or together with faces of various brachydomes, and those of the proto-pyramid. One end alone shows perfect development, the faces constituting the opposite termi- nation being usually rudimentary. The prisms show a vertical striation. Colourless, wine-yellow, brown, or tinted blue, red, or green. Lustre, vitreous. Trans- parent to translucent. Index of refraction, 1*62. Double refraction, moderate (7 a = 0*009). Basal cleavage, M FIG. i2i. TOPAZ. P, Basal plane ; s and 0, pyramids ; x, brachypyramid ; M , prism ; /, brachyprism ; n andjy, brachydomes. perfect. Hardness, 8. Density, 3'4-3'6. Infusible and unattacked by acids. Topaz occurs in drusy cavities of granitic rocks or as loose and rolled pebbles. Well-known localities are St. Michael's Mount in Cornwall, the Mourne Mountains in Ireland, Saxony (Schneckenstein), Brazil (Minas Geraes), United States, Siberia, Ceylon. Tourmaline. Hydrated borosilicate of sodium, magnesium, and aluminium. According to Clarke, the tourmaline series consists of salts of an acid which GEMS 237 may be represented by the formula Al 6 (SiO 4 ) 6 (BO 2 ) 2 . BO 3 H 2 .H 12 . Crystallizes in the hexagonal system, with rhombohedral symmetry and hemimorphic develop- ment. Habit, prismatic, with rhombohedral termina- tions, and the prisms vertically striated. Also occurs in fibrous veins and stellate aggregates. Colour, usually black or dark brown, but various tints of red, blue, and green, are common, and even colourless crystals are known. Transparent to translucent. Lustre, vitreous. Index of refraction, fairly high ( = 1*64). Double refraction, negative, strong (o)-e= 0*017). Strongly dichroic, with marked absorption of the ordinary ray. Rhombohedral and prismatic cleavages, imperfect. Fracture, subconchoidal. Brittle. Hardness, 7. Den- sity, 3-3*2. Fusibility, 3-5. Insoluble in acids. Tourmaline occurs in the contact zones of granite and in pegmatites. It is also a frequent constituent of sands and sandstones. The best gems come from Brazil, Ceylon, Siberia, and North America. Zircon. Silicate of zirconium : ZrO 2 .SiO 2 (ZrO 2 67-2, SiO 2 32*8, per cent.). Crystallizes in the tetragonal system, being isomorphous with cas- siterite and rutile. Habit, prismatic or pyramidal. The prisms are of both orders; the pyramid, of the first order only. Colour, usually dark brown, also orange, yellow, FIG. 122. ZIRCON. red (jacinth), pale green, and grey p > Pyramid; m, prism; J J a, deutero-prism. (jar goon}. Streak, colourless. Lustre, adamantine. Transparent to opaque. Index of refrac- 238 DESCRIPTIVE MINERALOGY tion, high (co=J'g^). Double refraction, positive, very strong (e o> = 0*062) . Dispersion, strong. Pyramidal and prismatic cleavages, imperfect. Fracture, conchoidal. Brittle. Hardness, 7*5. Density, 47. Infusible. Only attacked with difficulty by hot sulphuric acid. Zircon is a constituent of most granites, syenites, etc., and of the sands derived from them. The gems are derived from Ceylon, New South Wales, and Queensland. Sphene, or titanite. Titanosilicate of calcium : CaO.TiO 2 .SiO 2 (oxide of titanium 40*8, oxide of cal- cium 28*6, silica 30*6, per cent.). Crystallizes in the monoclinic system. Habit, cuneate. Colour, olive- brown. Transparent. Lustre, adamantine. Streak, white. Index of refraction, high (ft = 1*894). Double refraction, positive, very strong (7 a = 0*121). Dis- persion, very strong. Prismatic cleavage, distinct. Fracture, subconchoidal. Hardness, 5. Density, 3*5. Fusibility, 3. Decomposed with difficulty by hydro- chloric acid. Sphene occurs in many igneous rocks and crystalline schists, also in limestones. Its inferior hardness mili- tates against its employment as a gem. Turquoise, or callaite. A hydrated and basic phos- phate of alumina: A1 2 (OH) 3 PO 4 .H 2 O. Only known massive, occurring as veins and nodules in igneous rocks. Colour, cerulean or peacock blue. Opaque. Lustre, resinous. Fracture, conchoidal. Hardness, 6. Density, 2*7. GEMS 239 The turquoise used in jewellery is found near Nishapur in Persia, and in the Burro and Jarilal mountains in New Mexico. Chrysoberyi. A double oxide of beryllium and alu- minium, BeO.Al 2 O 8 (oxide of beryllium 19*8, alumina 80-2, per cent.). Crystallizes in the rhombic system, in forms similar to those of olivine. Often in twins and triplets, the latter termed alexandrite, which was once considered to be a distinct mineral. Habit, short prismatic or thick tabular, with vertical striations. FIG. 123. CHRYSOBERYL. a, Macropinacoid ; b, brachy- pinacoid ; i, brachydome. FIG. 124. COMPOUND TWINNED CRYSTAL (ALEXANDRITE). Colour, yellowish-green to olive-green. Lustre, vitreous. Transparent. Streak, colourless. Index of refraction, high (/3=i'75). Double refraction, positive, moderate (7-a=o'oog). Pleochroic. Cleavage, imperfect. Frac- ture, conchoidal. Brittle. Hardness, 8*5. Density, 3*7. Infusible. Insoluble in acids. Chrysoberyi is known to the jewellers under the name of the cafs-eye (cymophane) and oriental chrysolite, and is chiefly found in Brazil, Ceylon, Ural Mountains, and in Connecticut in the United States. 240 DESCRIPTIVE MINERALOGY Peridot. A beautiful transparent, deep olive- green variety of olivine (see p. 106), is used as a gem-stone. Opal. Hydrated silica: SiO 2 +^H 2 O. Amorphous, occurring in botryoidal or stalactitic masses. Colour, bluish, yellowish, or milk white. Translucent. Streak, white. Lustre, vitreous or resinous. Index of refrac- tion, low (/x = 1*455). The " fire " of " precious opal " is the result of reflection and diffraction of light from internal surfaces. Fracture, conchoidal. Brittle. Hardness, 6. Density, 2*1. Infusible, but yields water on heating. Soluble in potash. Hyalite is a transparent, colourless variety of opal. Chalcedony. Silica : SiO 2 . Occurs in concretionary, botryoidal, or stalactitic masses which have an internal fibrous and crystalline structure. Colourless to white. Transparent to translucent. Lustre, resinous. Fracture, uneven to splintery. Hardness, 7. Density, 2'6-2'6/j.. Carnelian is a red variety of chalcedony ; Sard, a brownish-red variety ; Plasma, a leek-green variety ; Chrysoprase, an apple-green variety ; Agate and Onyx are banded and variegated varieties which occur as the infilling of amygdaloidal cavities in certain basic lavas ; Sardonyx is a variety of onyx which contains layers of carnelian or sard. Quartz. Certain coloured varieties of quartz (see p. 84) are used for decorative purposes. Jasper is an opaque red-coloured variety in which there is much admixed iron oxide ; Amethyst is a violet- GEMS 241 coloured quartz often used as a gem-stone; Smoky Quartz, Cairngorm, Rose Quartz, are smoke-coloured yellow and pink varieties of quartz; Tiger's Eye is a golden - yellow replacement by quartz of fibrous crocidolite. Felspar. Some varieties of felspar (see p. 86) are used as gem-stones. Moonstone is an opalescent variety of orthoclase found in Ceylon; Amazon-stone is a green variety of micro- cline ; Labrador Spar is an iridescent variety of labra- dorite, which when polished exhibits a magnificent play of colours. The iridescence is due to interference produced by a lamellar structure. INDEX ACANTHITE, 143 Acicular habit, 4 Actinolite, 79, 105 Adamantine lustre, 42 Adularia, 88, 90 Agate, 240 Alabaster, 215, 216 Albite, 79, 86, 92, 207 Alexandrite, 72, 239 Alkali-micas, 101 Allanite, 204 Almandiiie, 78, 234 Alum, 58, 218 Aluminium, 64, 121, 204 Alumstone, 75, 218 Alunite, 75, 218 Amazon-stone, 241 Amber, 228 Amesite, 80, no Amethyst, 85, 240 Oriental, 231 Ammonia-alum, 218 Amorphous bodies, 35 State, 3 Amphiboles, 83, 108 Analcime. 81, 112, 113, 207 Anatase, 67, 72, 198 Andalusite, 77, 113, 114 Andesine, 87 Angle, Law of Constant, 8 Anglesite, 75, 156, 153 Anhydrite, 74, 207, 214, 226 Anisotropic media, 43, 44 Annabergite, 75. 161, 164 Anomite, 101 Anorthic system, n, 23 Anorthite, 78, 79, 86, 94 Anorthoclase, 88 Antimonite, 70, 190 Antimony, 69, 120, 188 Native^ 190 Antimony glance, 190 Apatite, 76, 193, 207, 224 Apophyllite, 112 Aquamarine, 229, 231, 233 Aragonite, 66, 75, 207. 209, 210 Argentite, 69, 143, 144, 146 Ar senates, 75 Arsenic, 64, 69, 120, 188 Arsenical Pyrites, 191 Arsenide group, 69 Arsenious oxide, 189 Arsenopyrite, 70, 191 Arsenosulphide group, 70 Asbestos, in, 241 Asphaltum, 228 Asterism, 43 Asymmetric system, n Atacamite, 72, 142 Augite, 79, 10 1 n., 103, 105 Autunite, 76, 201, 202 Avanturine, Lustre of, 43 Axes, 8 Optic, 48 Dispersion of, 52 Plane of, 51 of elasticity, 50, 51, 53, 54 of symmetry, 6 Axial angle, Apparent, 52 n. Colours, 54 Axinite, 82, 116, 194 Azurite, 74, 140 BABBITT METAI,, 194 Barium, 64 Sulphate, as Veinstone, 207 242 INDEX 243 Barytes, 75, 216 Bauxite, 73, 204 Baveno type of Twinning, 88, 89 Becke's method, 53 n, Bell-metal ore, 197 Benzol, 228 Beryl, 79, 233 Biaxial Crystals, 5 1 Biotite, 81, 100, 101 Bismuth, 69, 144, 188 Native, 188, 189 Bismuthinite, 70, 189 Bitumen, 228 Blackband Ironstone, 177 "Blackjack," 155 Black Oxide of Copper, 132 Black sands, 179 Black Tin, 193, 194 Blende, 69, 120, 154, 207 " Blue John " (fluorspar), 224 Bog-manganese, 184, 186 Boracite, 76, 225, 226 Borates, 76, 225-6 Borax, 76, 225 Bornite, 70, 129, 135, 138 Borosilicates, 81 Botryoidal, 4 Brachy-axis, 19 Brachy-pinacoid, 21 Braunite, 73, 184, 187 Braurts solution, 6 1 Brilliancy of Gems, properties causing, 229 Brittle Silver Ore, 140 Brittleness, 39 Bronzite, 42, 43, 56 Brookite, 67, 73, 198 Brown Iron Ore, 168, 175 Brucite, 72 Bytownite, 87 CADMIUM, 120, 155 Cairngorm, 85, 241 Calamine, 74, 154, 158, 160 Calaverite, 122, 126 Calcite, 66, 67, 74, 83, 95, 107, 109, 209 Calcium, 64 Carbonates, 210 as Veinstones, 206 Fluorophosphate of, 193 Calcium (continued) : Sulphates, as Veinstones, 207 Tungstate of, 200, 201 Callaite, 238 Capillary habit, 4 Carbon (see Diamond and Graphite), 64, 208, 230 Carbonates, 65, 74, 208 Carlsbad type of Twinning, 89 Carnallite, 71, 222, 226 Carnelian, 240 Carnotite, 201 Cassiterite, 73, 193, 194 Cat's-eye, 239 Celestite or Celestine, 74, 216 Centro symmetry, 6 Law of, 24 Cerium, 203, 204 Cerussite, 74, 80, 152 Cervantite, 188 Chalcanthite, 75, 129, 142 Chalcedony, 73, 85, 206, 240 Chalcocite, 69, 129, 133, 135 Argentiferous, 143 Chalcopyrite, 70, 120, 128, 129, 134. : 58 Chalybite, 67, 74, 168, 177, 207 Change or play of Colours, 43 Chatoyancy, 43 Chemical Change, 68 Chessylite, 74, 129, 140 Chiastolite, 114 Chili Saltpetre, 219 Chloanthite, 144, 161, 162 Chlorargyrite, 149 Chlorides, 64, 220 Chlorite, 80, 83, 100, 109, no, 207 Chloritoid, 81 Chlorobromide of Silver (see Embolite), 143 Chromates, 74 Chromite, 72, 121, 164, 169, 178 et alibi Chrysoberyl, 72, 239 Chrysocolla, 80, 129, 141 Chrysolite, 107 Oriental, 239 Chrysoprase, 85, 240 Chrysotile, no Cinnabar, 69, 127, 128 244 INDEX Classification by Chemical Composition, 68 et seq. of Crystals, 9 Clay(s), 66, 83, 204 Clay Ironstone, 177 Slate, 234 Cleavage, 35 Clino-axis, 22 Clinodome, 88 Clino-pinacoid, 22, 88, 104 Coal, 228 Cobalt, 165 Cobalt-bloom, 165, 167 Cobalt Glance, see Cobaltite Cobaltite, 70, 144, 165, 166 Cockscomb pyrites, 182 Cohesion, Molecular, 35 Columbite, 77, 203, 204 Columnar habit, 4 Compound Crystals, Twinning in, 33 Concentric Laminated arrange- ment, 5 Conchoidal Fracture, 38 Conductivity Heat, 55 Electricity, 56 Conglomerate beds, Gold from, 122 Couseranite, 98 Constant Angle, Law of, 8 Copper, 64, 68, 129 Grey, see Tetrahedrite Native, 129 Ruby, see Cuprite Sulphides, Separation, 60 Copper glance, 69, 133 Copper Pyrites, 70, 120, 128, 129, 134, 158 Coprolite, 224, 225 Cordierite, 80, 114, 115 Axial colours of, 54 Corundophilite, no Corundum, 73, 114, 231 Coyellite, 129, 133;*. Critical angle, 45 Crocidolite, 241 Cross-shaped twins, 32 Cryolite, 71, 204, 205, 206 Crystal Axes, 8 Crystalline Matter, 3 Rocks, 83 Crystals Classification of, 9 Cubic System of, 1 1 External habit, 3-4 Internal structure, 4-5 Faces of, 5 Mixed, 67 Positive and Negative, 5 1 -52 Regular system of, n Simple, Twinning of, 33 Symmetry of, 5 Cube form, 11,12 Four-faced, 14 Pyramidal, 14 Cubic System, 1 1 Cubical cleavage, 37 Cupric Oxide, 132 Cuprite, 72, 129, 131 and ;/. Cuprous Oxide, 131 Curve of Hardness, 39 Curved faces, 5 Cymophane, 239 DATOWTE, 82, 117 Dendritic habit, 4 Density, 60 of Gems, 62, 63 Rock-forming minerals, 62 Dechenite, 193 Detrital deposits, 1 19 Deutero-prism, 17 Deutero -pyramids, 16 Diallage, 105 Diallogite, 187 Diamond, 64, 69, 230 Diaspore, 204 ;/. Didymium, 204 Dihexagonal prism, 1 7 Dihexagonal pyramids, 17 Dimorphism, 67 Diopside, 78, 104 Dioptase, 141 Dioxide group, 73 Dipyre, 98 Dispersion of Optic axes, 52 of Rays, 45 Dispersive power in Gems 229 Disulphide group, 70 Dodecahedron, see Rhombic INDEX 245 Dolomite, 67, 74, 83, 107, 109, 209, 211 Domatic cleavage, 37 Domes, 20 Double Refraction and Polariza- tion, 47 Drusy faces, 5 Ductility, 39 42 Elasticity, 39 Axes of, 51,53, 54 Ellipsoid of, 53 Electric Properties,^ 55 Electro-magnetic Separation, 57 Electrostatic separation, 56 Electrutn, 68 Elementary substances, 64 Elements, Surface tension in, 59 Ellipse >& Ellipsoid of Elasticity, 53 of Fresnel, 51 Triaxial, 35 Embolite, 71, 143, 149 Emerald-nickel, 164 Emery, 232 Enargite, 71, 129, 138 Eustatite, 78 Epidote, 80, 95, 104, in, 112 Epimorphs, 61 Epitritoxide group, 72 Epsomite, 75, 212, 217 Epsom Salt, 75, 212, 217 Erubcscite, 70, 138 Erythrite, 75, 165, 167 Esmarkite, 116 Even Fracture, 38 Extraordinary Ray, 48, 49 FACES of Crystals, 4 Fahl ore, 129, 139 Fayalite, 77, 106 Felspar, 66, 79, 83, 86, 204, 207, 230, 241 Microcline, 91 Oligoclase, 87, 94 Orthoclase, 86, 88, 91, 94, 207 Plagioclase, 86, 94 Felspathic rocks, 109 Felspathoid group, 95 Ferberite, 200 Ferro-magnesian Micas, 100 Ferromolybdenum, 199 Ferro titanium, 198 Fibrolite, 114 Fibrous Iron ore, 175 Fluorides, 64, 220, 222 Fluorine, 64 Fluorite, see Fluorspar Fluorspar, 71, 194, 207, 222, 224 Form, 6 Forsterite, 77, 106 Fracture, 35, 38 Franklinite, 72, 159, 160, 184 Freieslebenite, 71, 143 Fundamental Intercepts, 26-8 Fusibility, 55 A, 69, 120, 135, 143, 149, 151, 155, 207 Argentiferous, 142, 151 Gangue Minerals, 206 Garnet, 78, 115, 194, 207, 234 Garnierite, 161, 164 Gems, 229 et seq. Density, 62, 63 Physical features, 229 Colour, 41 Gem-sands, Ceylon, 233, 235 Geniculated forms, 32 Genthite, 164 Gersdorfite, 161, 163 German silver, 155, 161 Gibbsite, 73, 204 n. Gigantolite, 116 Glaucodot, 165, 167 Glauber's salt, 217 Glimmering' lustre, 42 Glistening lustre, 42 Globular form, 4 Goethite, 73, 168, 174 Gold, 64, 68, 118, 122 Alloys of, 125 Associated minerals, 120, 123 Alluvial, 119, 123 Drift, 123 Dust, 124 Free, 120, 122 246 INDEX Gold (continued} : Native, 122, 124 Mustard, 124 Paint, ibid. Sponge, ibid. Nuggets, 124-5 Placer, 124 Beach placer, 124 Pyritic, 123, 181 Telluride of, 122, 126 Gold Amalgam, 122, 125 Gold fields, principal, 124 Gold and Silver, Tellurides of, 123, 126, 127 Goniometer, reflecting, 5 & ., 9 Granular structure, 4, 5 Graphic Tellurium, 126 Graphite, 69, 227 Gravitation methods of separa- tion, 60 Gravity concentration, 63 Gravity, Specific, 60 Greasy lustre, 42 Grey Copper ore, 139 Grossularia, 234 Guano, 225, 225 Gypsum, 75, 109, 207, 214, 215, 226, 227 HABIT of Crystals, 4 Hackly, Fracture, 38 Haematite, 73, 168, 169, 172 Itabarite ores, 174 Kidney ore, 172 Micaceous, 172 Veinstone, 207 Hair-like habit, 4 Halite, 220 Halogens, 64 Haloid, compounds of silver, Hardness, 35, 39 Average, scale of, 40 Curve of, 39 Relative scale of i of Gems, 229 Haiiyne, 78, 97 Heart-shaped twins, 32 Heat- Conductivity of,f55 Specific, 55, Heavy Metals, Sulphides, Sur- face Energ} 7 , 59 Heavy solutions, 61 Heavy spar, 216 Hemihedrism, u, 29 Hemimorphism, 30, 31 Hemimorphite, 79, 159 Hemi-pyramid term, 22, 88, 104 Heulandite, 81, 113 Hexagonal system, 10, 15, 44 Hexakis-octahedron, u, 14 Holohedral, 22, 31 n. Hornblende, 79, 101 n., 103, 204 Actinolite, 105 Fibrous, in Veinstone, 207 Horn Silver, 71, 143, 149 Horseflesh ore, 138 Hiibnerite, 200 Hydatogenetic ore-deposits, 150 Hyalite, 240 Hydrates, 65 Hypersthene, 78 ICEI/AND Spar, 210 Cleavage, 35 ! Icosi- tetrahedron, n, 13 Idocrase, 81, 116 Ilmenite, 73, 178, 194, 198 Incrustation, 68 Index of Refraction, 44 Determination of, 53 n, Intercepts, Fundamental, mul- tiples of, 26-8 Rational, Law of, 20 Iridescence, 43, 241 Iridium, 121 Iridosmine, 68 Irisation, 43 Iron (see also Haematite), 64, 68, 168 Output, 169 Sources, 168 et seq. Arsenosulphide of, as Vein- stones, 207 Carbonate of, as Veinstone, 207 Native, 169 Ores, 168 Brown, 175 Chrome, 178 Ochreous, 175 INDEX 247 Iron (continued] : Ores (continued) : Oolitic, 175, 176 Pisolitic, 175 Reserves of, 169-70 Spathic, 177 Titaniferous, 178, 198 Oxides, 65, 168 as Veinstone, 207 Pig-, 168 Pyrites, 129, 135, 168, 179 Forms, 30 Salts, 66 Sand, 119 Sulphides of, as Vein- stones, 207 Tungstate of, 200 Ironstone, Blackband, 177 Clay, 177 Isomorphism, 67 Isomorphous Mixed Silicates. 79 Isotropic, 43, 44 Itabarite ores, 174 JACINTH, 237 Jade, 38 Jargoon, 237 Jasper, 85, 206, 240 , 75, 218 Kaolin, 91, 95, 109 Kaolinite, 75, 81, 83, in Kaolinization, 90, in Kerargyrite, 71, 143, 149 Kermesite, 188 Kidney ore, 172 Klein's solution, 61 Krennerite, 122, 126 Kyanite, 77, 114 LABRADOR Spar, 241 Labradorite, 87 Lamellar Twinning, 33 Lateritic deposits, 184 Laumontite, 112, 113 Laws of Centra-symmetry, 24 Constant Angle, 8 Rational intercepts, 20 Refraction, 44 Symmetry, 8 Lead, 64, 68, 149, 150 Leafy form, 4 Lepidolite, 100 Leptochlorites, 109 Leucite, 79, 96 Light, Phenomena relating to, 35, 4i, 54 Lime, 1 66 Phosphates of, 224 Lime-felspar, 79, 86, 94 Lime-mica, 80, 100 Limestone, 65 Argillaceous, gems in, 232 Contact-altered, 98 Crystalline, gems in, 233 Graphite in, 227 Magnesian, 214 Matrix of Rubies from, 232 Metamorphic, 106 Limonite, 73, 168, 175, 207 Linnseite, 161, 165 Lithium-mica, 100 Loadstone, 172 MACRO-AXIS, 19 Macro-pinacoid, 21 Magnesia, 66 Magiiesian-Iron mica, 80 Magnesian mica, 80 Magnesite, 74, 209, 212 Magnetic Iron -ore, see Mag- netite Magnetic Permeability, 57 Properties, 55, 57 Magnetite, 72, 168, 171, 194, 207 Malachite, 74, 129, 140 Malay States and Dutch East Indies, Tin - mining Districts in, Map, 196 Malleability, 39 Mammillated form, 4 Manganese, 64, 183 Manganese-garnet (Spessartite), 183 Manganese-olivine (Tephroite), 183 Manganese - pyroxene (Rhodo- nite), 183 Manganite, 184, i8g Manebach type of Twinning, 90 Marcasite, 70, 128, 168, 182, 207 Margarite, 80, 100 248 INDEX Marialite, 79 Massive Structure, 5 Meionite, 78, So Melaconite, 72, 129, 132 Melanite, 234, 235 Melilite, 97 Menaccanite, 178 Mendozite, 218 Mercury, 62, 64, 68, 127 et seq. Sulphide, see Cinnabar M elaborates, 65 Metacinnabarite, 127 Metalli^ Lustre, 42 Metasilicates, 65, 78-9 Metasomatic deposits, 150, 154 Meteorites, 169 Mica, 66, 83, 98, 204, 207 Percussion figure, 38, 100-1 Microcline, 86, 91-2 Microperthite, 94 Millerian system of Symbols, 24 Millerite, 70, 161, 193 Mineral Oil, 228 Pitch, 228 Resin, 228 Wax, 228 'Minettes,' 175, 176 Minium, 150 Mirabilite, 75, 217 Mispickel, 70, 120, 128, 144, 165, 188, 189, 191, 207 Mizzonite, 98 Molecular Cohesion, 35 Molybdates, 77 Molybdenite, 70, 199 Molybdenum, Ores of, 199 Monazite, 73, 194, 203 Monoclinic System, n, 22, 44 Monosulphide group, 69 Monoxide group, 72 of Lead, see Galena Monticellite, 77, 106 Moonstone, 241 Morphological Characters, 3 Mossy habit 4 Muscovite, 80, 91, 99, 100 NATIVE ELEMENTS, List of, 68-9 Native Metals and Alloys, List of, 68-9 Natrolite, 81, 112 Natron, 74, 209, 213 Naumanrfs Symbols, 24 Needle-shaped habit, 4 Nepheline, 77, 95, 112 Nephrite, 106 Niccolite, 69, 144, 161, 162 Nickel, 64, 161 Nickel-bloom, 75, 164 Nickelmolybdenum, 199 Nitrate of soda, 2 1 9 Nitrates, 65, 76, 219 Nitratine, 76, 219 Nitre, 58, 76, 219 Nodular form, 4 Norway, Copper deposits in, 136 Nosean, 78, 97 Noumeite, 164 OBLATE SPHEROID, 49 Ochres, 175 Octahedral cleavage, 37 Octahedron, u, 12, 29 Pyramidal, 14 Six- faced, 14 Odour, 58 Oil, Mineral, 228 Oligoclase Felspars, 87, 94 Olivines, 77, 83, 106 Onyx, 240 Opacity of Gems, 229 Opal, 73, 240 Opalescence, 43 Optic axis, 48 Dispersion of, 52 Plane of, 5 1 Optical properties of Crystals 34 Ordinary Ray, the, 48, 49 Ores, \\%etseq. Deposits Detrital or Placer, I 19 formed in situ, 118 Primary, 119 Secondary, oxidized, 119 Orpiment, 70, 188, 192 Orthite, 80, 204 Ortho-axis, 22, 53 Orthoclase, 79, 86, 88, 94, 207 Orthochlorites, 109 Orthodome, 88 Orthopinacoid, 88, 104 INDEX 249 Orthorhoinbic System, 19 Orthosilicates, 65, 77 Oscillation of faces, 5 Osmium, 121 Osmium-indium, 121 Ottrelite, 81 Oxides, 64, 118 Oxidized ores, Copper from, 129 Oxy-acids, 64 Oxychlorides, 72 Oxygen, 64 Compounds of Metals with, 72 Salts of the Metals, 74 Oxy-salts, 64 Ozokerite, 228 PALLADIUM, 121 Paragon ite, 80, 100 Parameters, 20 Parametral form, 20 Paramorphs, 68 Pearceite, 71, 143 Pearlspar, 211 Pearly, lustre, 42 Pellucidity of Gems, 229 Penninite, no Pentagonal Dodecahedron, 29 Pentlandite, 161 Percussion figures, 38 Peridot, 105, 107, 240 Perth ite, 94 Petroleum, 228 Petzite, 122, 127 Phillipsite, 112 Phlogopite, 81, 100, 101 Phosphates, 65, 75-6, 224 Phosphorite, 225 Physical Properties of Minerals, 34 Piedmontite, 80 Pinacoidal cleavage, 37 Pinacoids, 19, 21 Finite, 116 Pitchblende, 201, 202 Pitch, Mineral, 228 Placer deposits, 119 Gold, 124 Sands, Platinum from, 121 Plagioclase Felspars, 86, 94; see also under Albite and Anorthite Plagioclase Felspars (continued) : Decomposition products, 95 Twinning in, 33 Formulae, 86, 87 Isomorphous series, 94 Planes Basal, 16, 88 of Composition, 32 Minimum cohesion, 36 ' Optic axes, 51 Parting, 38 Symmetry, 6-7, 9 Twinning, 31 Platinum, 64, 68, 120 Native, 121 Spongy, surface energy of, 59 Platy habit, 4 Plasma, 240 Pleochroism, 54 of gems, 229, 230 Plumbago, 228 Pneumatolytic veins, 194, 207 Polybasite, 71, 143, 148 Positive and Negative Crystals, 49 andn. Polymorphism, 66 Poly synthetic Twinning, 5, -3 Polarization and Double Refrac- twn, 47 Potash, Nitrate of, 219 Potash-alum, 75, 218 Potash-felspar, 86, 88, 91 Potash-mica, 80, 99 Praseolite, 116 Prehnite, 80, 207 Principal Axis, 15 Prisms, 16, 17, 88, 104 Dihexagonal, 17 Prismatic cleavage, 37 Habit, 4 Prochlorite, no Prolate Spheroid, 49 Proto-prism, 16 Proto-pyramid, 16 Proustite, 71, 143, 147 Psilomelane, 184, 186 Pseudo -hexagonal symmetry, 33 Pseudomorphism, 67 Pseudomorphs, 67, 68, 112 Psilomelane, 73 Purple ore, 138 250 INDEX Pyramidal cleavage, 37 Pyramids Dihexagonal, 17 of the First and Second orders, 16 Pyrargyrite,7i, 143, 147, 188 Pyrites, 70, 128 Arsenical, 191 Auriferous, 123, 125. 18 in Chalk, 182 Cockscomb, 182 Copper, 120, 134 Cupriferous, 129 Iron, 129, 135, 168, 179 Magnetic, 182 Tin, 197 Pyritic Ores, 123 Pyroelectricity, 31, 57 Pyrolusite, 73 184 Pyromorphite, 76, 154, 193 Pyrope, 78, 234, 235 Pyroxenes, 83, 101 and ., 105, 204 Rhombic, 105 Pyrrhotite, 70, 168, 82 Nickeliferous, 161, 162, 183 QUARTZ, 63, 73, 83, 84, 95, 204 Milky, 85 Rose, 41, 85, 241 Sceptre, 85 Smoky, 85, 241 euartz group, 84-6 uartz Veins, Auriferous, 122 Quartz Veinstone, 86, 207 RADIUM, source of, 201 Rational Intercepts, Law of, 20 Ray, Extraordinary, 48 Ordinary, 48 Realgar, 70, 128, 188, 192 Retger's solution, 61 Red Copper ore, 131 Red ochre, 173 Redruthite, 133 Reflection, Total, 46 Refraction, 43 Double, and Polarization, 47 Refraction (continued) : Index of, 44, 45, 55 . Index of, determination of, 53 In Relation to Critical Angle, 46 Law of, 44 Refractive index of Gems, 229 Regular System, n Ellipsoid in, 34-5 Isotropic, 44 Symbols, 26 Reniform, 4 Replacement, 67 Resin, Mineral, 228 Rhodium, 121 Rhodochrosite, 74, 184, 187 Rhodonite, 78, 183 Rhombic dodecahedral Cleavage, Rhombic Dodecahedron, n, 12 Pyroxenes, 105 Rhombic System, n, 19, 44 Ellipsoid in, 35 Symbols, 27 | Rhombohedral cleavage, 37 System, n i Rhombohedt on, 27, 30 ! Rock-crystal, 85 ! Rock-forming Minerals, 83 Rock salt, 71, 109, 214, 220 Rubellan, 100 Ruby, 229 Ruby Silver ores Dark, 147 Light, 147 Colour, 44 Rutile, 67, 73,115, 198 SAI, AMMONIAC, 71, 220, 222 Salt, see also Epsom, Glauber, & Rock-Salt Stassfurt deposits, 215 Salt Lake, Utah, 217 Saltpans, 221 Saltpetre, 76, 219 Chili, 219 Salts, 227 Sand, Cassiterite as, 194 Menaccanite, 179 Sanidine, 88, 90, 91 INDEX 251 Sapphire, 229, 231, 232 Asterism, 43 Sard, 240 Sardonyx, 240 Satin Spar, 215 Saussurite, 95 Scalar Properties oj Crystals, 34. 35 Scale no hedron, 30 Scapolite group, 79, 98 Scheelite, 76, 194, 200, 201 Schiller lustre, 43 Schists, 95, 106, 204 Corundum in, 232 Crystalline, 98. 114, 1 15 et alibi Gems in, 238 Scolecite, 112 Secondary Minerals, 83, 107 et seq. Sectility, 39 Selenite, 215 Senarmontite, 188 Sericite, 201 Serpentine, 80, 83, 107, no, 164, 178, 212 Scsquioxide group, 73 Sesquisulphide group, 70 Shining lustre, 43 Siderite, 74, 177 Siennas, 175 Silica, 65, 240 Silicates, Isomorphous, Mixed, 77 79 Metasilicates, 78-9 Ortho silicates, ib. Silky lustre, 42 Sillimanite, 114, 115 Silver, 62, 64, 68, 142 Chloride of, 149 Chlorobromide of, 149 Haloid compounds of, 143 Iodide of, 143 Native, 144, 145 Ores, 142 Sulphide of, 146 and Gold, Tellurides of, 126, 127 Silver-lead and Copper, Sulph- antimonites & Sulph- arsenates of (table], H3 Smaltite, 69, 144, 166 Smithsonite, 154 Soapstone, in Soda, 66 Soda-alum, 218 Soda-felspar, 79, 86, 88, 92 Sodalite, 78, 96 Soda-mica, 80, 100 vSoda-nitre, 76, 219 Soda Salts, 213 Solids, Surface Energy in, 58, 59 Sonstadfs solution, 61 Spartalite, 72 Spathic ores, 30, 168, ,77 Specific gravity, 60 Heat, 55 Spelter, 154 Output of, 155 Sperrylite, 69, 120, 122 Spessartine, see Spessartite Spessartite, 78, 183, 234 vSphaerosiderite, 177 Sphalerite, 156 Sphene, 77, 238 Spinel, 232 Splendent lustre, 42 Splintery Fracture, 38 Spodumene, 79 Sprudelstein, 211 Stalactitic habit, 4 Stannite, 193, 197 Stassfurt deposits, 215; see Fig. 104, 223 Staurolite, 80, 115 Steatite, in Stephanite, 71, 140, 143 Stibnite, 128, 188, 190 Stilbite, 8 1 Stilpnosiderite, 175 Streak, 41 Sttiated faces, 5 Strontianite, 74, 209, 213 Stromeyerite, 69, 143 Structure, 4, 5 Subconchoidal Fracture, 38 Submetallic lustre, 42 Sulph-acids, 65 Sulphantimonites and Sulph- ar^eniates of Silver and Copper, 143 252 INDEX Sulphates, 65, 74-5, 214 et seq. Hydrated, 75 Sulphides, 64 of Copper, 60 of Lead, see Galena Primary, of Metals, 120 Rhombic, of Copper, 133, 143 Silver, 143 Sidpho-salts, 64 Sulphur, 64, 69, 208, 227 Compounds, 58 Sulphur Salts, 64, 70-1 Sulphuretted Hydrogen, Odour, 58 Sulphurous Acid, Odour, 58 Surface Energy (Tension] in Liquids, 59 Solids, 58, 59 Sylvanite, 70, 122, 126 Sylvite or Sylvine. 71, 220, 222 Symbols, 24 Millerian System, 24 Naumann's, 24 Symmetry of Crystals, 5 Angles of, 7 Axes, 6 Binary, 16 Senary, 16 Law of, 8 Modifications in, 28 Physical properties in relation to, 34 Plane of, 6, 7, 9 Parallelism of, 7 Pseudo-hexagonal, 33 TABULAR Habit of Crystals, 4 Talc, 8 1, 83, 91, 109, in, 207 Taste, 58 Telluride of Gold, 122, 126 Tellurium, Graphic, 126 Tenacity, 35, 38 Tennantite, 71, 139 Tenorite, 121, 132 Tension, Surface, 59 Tephroite, 77, 106, 183 Tetragonal System, 10, 17, 44 Symbols, 27 Tetrahedrite, 29, 71, 139, 143 Antimonial, 188 Tetrahedron, 29 Tetrakis - hexahedron, 11, 14, Thermal Properties, 53 Tile ore, 131 Tin, 64, 193 Tenacity, 39 Uses, 194 Pyrites, 197 Stream, 119, 195 Tinkal, 225 Tin ores, 194 Tinstone, 193, 194 ; Titaniferous iron ore, 178, 198 Titanite, 238 Titano-silicate, 77 Topaz, 77, 117, 194, 207, 235 Oriental, 231 Torbernite, 76, 201, 202 Total Reflection, 46 Tourmaline, 82, 115, 194, 236 j Translucency of Gems, 229 Tremolite, 79, 106 Triakis-octahedron, n, 13 Triclinic System, 1 1 , 23, 44 Axes of Elasticity, 54 Ellipsoids in, 35 Trigonal System, 1 1 Trimotphism, 67 Trona, 209, 214 Tschermigite, 218 Tungstates, 76 Tungsten, 200 Turgite, 175 Turquoise, 76, 238 Twinning, 31 Forms produced by, 323 Types of Baveno, 88, 89 Carlsbad, 89 Manebach, 90 Plane of, 32 Polysynthetic, 5 UMBERS, 175 Uneven Fracture, 38 Uniaxial Crystals, 47 Uranite, 201, 202 Uranium (see also Autunite and Torbernite, 201 Uvarovite, 78, 234, 235 INDEX 253 VAXA DATES, 75-6 Vanadinite, 76, 193 Vanadium, 193 Vector properties, 34 Veinstone or Gangue Minerals, 91, 206 Vesuvianite, 81, 116 Vicinal faces, 5 Vitreous lustre, 42 WAD, 184, 186, 207 Warmth, Effect of on Sym- metry, 34 'Water of Constitution,' 65, 79 of Crystallization,' 65, 81 Wave Surface, 49 Wavellite, 76 Wax, Mineral, 228 Weathering, 90, 120, 194 et alibi Wernerite, 98 Wetting angle, 60 White Arsenic, 189 Willemite, 77, 154, 158 Wiry habit, 4 Witherite, 74, 209, 212 Wolfram, 77, 194 Wolframite, 200, 201 Wollastonite, 78, 103 ;/. Wood-tin, 195 Wulfenite, 77, 199, 200 XANTHOSIDERITE, 175 Xenotime, 75, 203 YTTRIUM, 204 ZAFFER, 165 Zaratite, 161, 164 Zeolite Group 81, 83, 112 Zinc, 64, 119, 154 Deposits, forms of, 154-5 Zinc-blende, 156 Zinc Spar, 158 Zincite, 72, 154, 159, 160 Zinnwaldite, 100, 101 Zircon, 73, 115, 203, 237 Zirconium, 203 Zoisite. 86, 95 Zones, 9 BILLING AND SONS, LTD., PRINTERS, GUILDFORD, ENGLAND A LIST OF BOOKS PUBLISHED BY WHITTAKER & CO. 2 White Hart Street, Paternoster Square, London, E.C, A complete catalogue giving fall details of the following hooka will be sent post free on application. s. d. ADAMS, H. Practical Trigonometry,, for the use of Engineers . . . . . . net 2 6 ALEXANDER, J. Model Engine Construction . net 5 ALLSOP, F. C. Practical Electric Light Fitting . .50 ARNOLD, J. O., and IBBOTSON, F. Steel Works Analysis net 10 6 ASHWORTH, J. R. Magnetism and Electricity . .26 Heat, Light and Sound . . net 2 ATKINS, E. A. Practical Sheet and Plate Metal Work net 6 BAMFORD, H. 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