THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID QUARTZ CRYSTAL-NORTH CAROLINA Two fifths natural size. MINERALS, AND HOW TO STUDY THEM. A BOOK FOR BEGINNERS IN MINERALOGY. BY EDWARD SALISBURY DANA, YALE UNIVERSITY, NEW HAVEN, Author of a Text-book of Mineralogy, Sixth Edition of Dana's System of Mineralogy, etc. TlBKtb more tban 300 trilustratfons. SECOND REVISED EDITION FIRST THOUSAND. NEW YORK JOHN WILEY & SONS. LONDON: CHAPMAN & HALL, LIMITED. 1896. Copyright, 1895, BY EDWARD S. DANA. ROBERT DRUMMOND, KLECTROTYPER AND PRINTER, NEW YORK. BMTOH SCIENCES PEEEACE. THE author has occupied some hours, which could not be devoted to more serious labor, in preparing this little book, in the hope that it might serve to encourage those who have a desire to learn about minerals, and also to in- crease the number of those whose tastes may lead them in this direction. He shares with most teachers at the pres- ent time the conviction that the cultivation of the powers of observation is a most essential element in the education of young people of both sexes; he believes, further, that no subject is better fitted to accomplish this object and at the same time to excite active interest than that of Miner- alogy. The attempt has been made to present the whole subject in a clear, simple, and, so far as possible, a read- able form without too much detail and at the same time without cheapening the science. As the understanding of the different parts of the subject requires some preliminary knowledge of physics and of chemistry, a little elementary matter in these departments has been introduced. Much attention has been given to the illustrations, most of which have been made expressly for this book ; others (reduced in size) are taken from the sixth edition of the System of Mineralogy (1892); several have been borrowed iii iv PREFACE. from Tschermak's Mineralogy, and one from a work by Baumhaner. The correct representation of real crystals and of the actual specimens from the cabinet is a difficult matter, and in this the author has been so fortunate as to secure the services of the skillful wood-engraver Mr. W. F. Hopson of New Haven. Any suggestions which would tend to give this volume greater accuracy or usefulness will be always gratefully received. NEW HAVEN, July 1, 1895. TABLE OF CONTENTS. CHAPTER PAGE I. MINERALS AND MINERALOGY: INTRODUCTORY REMARKS. . 1 II. SOME PRELIMINARY HINTS AS TO How TO STUDY MIN- ERALS 8 Suggestions about making a Collection 11 III. THE FORMS OP CRYSTALS AND KINDS OF STRUCTURE 14 The General Characters of Crystals 14 The Systems of Crystallization 21 I. Isometric System 22 II. Tetragonal System 31 III. Hexagonal System 36 Rhombohedral System 39 IV. Orthorhombic System 41 V. Monoclinic System. 44 VI. Triclinic System 45 Irregularities of Crystals , 48 Distorted Crystals 48 Pseudomorphs 55 Groupings or Aggregations of Crystals . 56 Twin Crystals 57 Parallel Grouping , 60 Irregular Grouping 62 Structure in General 63 IV. THE OTHER PHYSICAL CHARACTERS OF CRYSTALS 70 1 . Characters depending upon Cohesion 70 * Cleavage 70 Fracture. 73 Hardness and Tenacity 74 v Vi TABLE OF CONTENTS. CHAPTER PAGE 2. Specific Gravity or Relative Density 79 3. Characters depending upon Light 88 Luster 88 Color 90 Transparency 92 Other Optical Characters 93 4. Characters depending upon Heat 95 5. Characters depending upon Magnetism 96 6. Characters depending upon Electricity 96 7. Taste and Odor 97 V. THE CHEMICAL CHARACTERS OF MINERALS 99 The Chemical Elements 100 The Chemical Formula, etc 104 Kinds of Chemical Compounds among Minerals 109 Percentage Composition 116 Classification 118 VI. THE USE OF THE BLOWPIPE 121 1. General Description of Apparatus 121 2. How to Use the Blowpipe 127 3. Examination in the Forceps 130 4. Use of the Platinum Wire 136 5. Use of Charcoal 140 6. Use of .the Closed and Open Tubes 147 7. Chemical Examination by Acids and other Reagents. 153 VII. DESCRIPTION OF MINERAL SPECIES , 158 VIII. THE DETERMINATION OF MINERALS 339 APPENDIX 365 GENERAL INDEX , 369 INDEX TO MINERAL SPECIES. ... 373 MINERALS, AND HOW TO STUDY THEM, CHAPTER I. MINERALS AND MINERALOGY. WE are to learn about minerals and how to study them; but, before we can fairly begin, we must understand clearly what substances we may call minerals, and what specimens have a right to a place in the collection that every one who wishes to become a mineralogist must make. We all know, in the first place, that minerals are the materials out of which the earth is built, and we often hear that division of nature to which they belong called the Mineral Kingdom, in distinction from the Animal and Vegetable Kingdoms, which embrace the animals and plants which live and grow upon the earth's surface. And here it is important to realize how little we can know by actual contact and direct observation about this earth, though we live upon it. It is possible, indeed, to measure its size and shape, to find out its density as a whole, to study its surface features and the changes which they have undergone; but of the materials of which it is made we can know little beyond those which form the surface upon which we walk, The miner digs down a 2 MINERALS, AND HOW TO STUDY THEM. little distance, and the artesian-well borer goes down still deeper, and we may have a chance to examine the spec- imens that their work brings up; or perhaps we can go down with the miner and see them in place. But the deepest mines descend to less than three quarters of a mile ; and though this seems deep to one who is let down a shaft in a bucket, it is but a little way compared with the whole distance to the earth's center, which would require a journey of nearly 4000 miles. Even the deepest artesian- well bor- ings hardly go down to the depth of one mile. Our knowledge, to be sure, is increased a little by the fact that we find now on the surface of the earth rocks made, as we have reason to believe, of materials brought up in a molten condition from great depths below. This is true of the lava thrown out by a volcano, and of such igneous rocks, for example, as form the Palisades along the Hudson River; and these occurrences give us some idea as to what kinds of matter there are, and in what condition, far below the surface. Further, we are able to weigh the entire earth, too, and find what its density is; and as this is nearly twice as great as that of the rocks on the surface, it gives a suggestion as to the heavy nature of the mineral material that must make up the interior. Thus the mineralogist is limited to the study of the lit- tle part of the crust of earth which he can reach with his hammer; and he cannot extend his collection much be- yond this, unless indeed he takes in some of those rare visitors from outer space called meteorites which once in a while tumble down to the earth, usually with a bright light and loud explosion, MINERALS AND MINERALOGY. 3 Now what does this study show of the hard rocky mate- rial of which the earth, so far as we can examine it, is made up; for example, of the sand of the seashore, the granite, the trap, the slate and marble of the hills ? We find, in the first place, that it in general consists of different kinds of substances, each one having certain peculiarities or characters of its own, by which it can always be recognized ; and it is to each of these individual kinds that the name MINERAL is given. Thus, more particularly, the sand of the seashore can be separated without much difficulty into various sorts of grains, each kind alike in chemical substance, as the chemist can prove in the laboratory, and with certain characters of hardness, density, luster, and color of its own, which enable us after a little practice to distinguish the different kinds with comparative ease. Most of the grains are alike clear and glassy, hard enough to scratch glass, and as we learri to know tnem better we call them quartz. There are also black grains; some of these are heavy and jump to a magnet, and often they are sorted out by the waves into little rifts on the white sand; these are called magnetite, or magnetic iron. There are other black grains, too, which the magnet does not attract, perhaps some red, glassy ones which are fragments of garnets, and, it may be, still others, depending upon where the sand comes from, and what kind of rock has been ground up by nature's mill and sorted out by the water to make the sand. If a piece of granite is taken, here too it is possible to distinguish several kinds of mineral substances, though it 4 MINERALS, AND HOW TO STUDY THEM. is not quite so easy to separate them. There are hard glassy grains with irregular surface, which, like the greater part of the sand-grains, are quartz. There are white or yellow or pale flesh-red fragments, also hard, though not so hard as the others, but which are sure to show one or two smooth surfaces of fracture: these are feldspar. Then there is the mica, more easily recognized still, which is either nearly white and silvery, or black (and sometimes both kinds), and which with a touch of the knife separates into very thin scales or leaves. Besides these there may be a little coal-black tourmaline, some bright red garnets, and other kinds which we shall learn later. If a cavity or open space in the granite can be found, here it is often possible to find the same kinds of substances, only larger and more distinct and very likely in the regular form which are called crystals. If, instead of a coarse-grained rock like granite, we ex- amine a fine compact orfe such as the trap-rock of the Palisades on the Hudson, it very probably appears all alike to the eye ; but if we crush some of it to powder, the magnet will pick out some magnetic iron, as from the seashore sand. Or the skillful mineralogist may make a slice thin enough to be transparent, so that he can study it under the micro- scope, and then recognize a variety of different minerals. In seams and cavities in these rocks other sorts are often found, not like those in the solid rock. Sometimes we find a rock, like the white marble of Vermont, which the examination of the chemist shows to be all of the same chemical substance, and which has throughout the same characters of hardness, density, color, MINERALS AND MINERALOGY. 5 and so on; then it is said to be a mineral itself, and not, like most rocks, a mixture of a variety of different min- erals. These different kinds of substances, then, which make up the rocky material of the earth so far as we can study it, and into which we can separate the seashore sand, the granite, and most other rocks, are called MIN- ERALS. Each one has, first of all, a definite chemical composition, wherever it is found. Moreover, if in the form of a crystal, it has a shape of its own, too, by which it may be distinguished; it has also certain characters of hardness and density, luster, color, transparency, and others. And because to it belong all these different char- acters, which distinguish it from other kinds, it is called a MINERAL SPECIES. It is the work of the mineralogist to study these min- erals; to learn all the different kinds; what the characters of each are; how they are classified and how distinguished from each other; how they occur in nature; and some- thing about their practical uses. All the knowledge which the many mineralogists have learned, after long years of patient observation and study, both in the field and the laboratory, has been arranged in systematic form and makes up the Science of Mineralogy. The question as to what particular minerals go together to make the different kinds of rocks, how these are formed, and what changes of position or of character they have experienced all these and other similar questions are re- ferred to the geologist. The science of the geologist, or geology, is much broader than mineralogy: it treats of 6 MINERALS, AND HOW TO STUDY THEM. the history of the earth and all the changes it has gone through; the different kinds of rocks; the way the moun- tains have been built up from them; the growth and de- velopment of different kinds of life from the earliest times down to the present. It was stated at the beginning of the chapter that min- erals belong to the MINERAL KINGDOM; but it is important to remember that all substances mineral in nature are not necessarily called minerals. The mineralogist, for example, usually excludes from his cabinet many mineral substances, such as the pearl of the oyster-shell and the shell itself, the lime of the bones of animals, and the opal-like form of silica secreted by the growth of plants, as the tabasheer found in the joints of the bamboo. In general mineral substances such as these, which are formed immediately by the processes of animal or vegetable life, are not called minerals. Further, the mineralogist does not, as a rule, admit among minerals gases like the oxygen and nitrogen which make up the air; and of the liquids he includes only the metal mercury, and perhaps also water. The many beautiful kinds of salts made by the chemist are also not called minerals. The rock salt or sodium chloride which is mined, sometimes in fine clear cubi- cal blocks, is the same sodium chloride which, as the table-salt of every-day life, is so commonly used. But the table-salt obtained from evaporating sea - water or the brines of salt- wells, or from the solution of crude rock salt, though when manufactured it may be formed in crys- MINERALS AND MINERALOGY. 7 tals as fine as those found in the rocks, is not called a mineral, because not made by nature alone. So, too, the fine crystals of blue vitriol, or copper sulphate, made by the chemist, do not find a place in a mineral cabinet, though the much less fine specimens of the same material found in some of the Arizona mines do. In the same way, the crystals of the metals and of many interesting compounds formed in the metallurgical process of making iron or lead or zinc are called furnace-products, and not minerals. These substances, however, are all very inter- esting, and their study is a very important help to pure mineralogy. In recent years the chemist has busied him- self in imitating, so far as he can, the possible processes of nature, and thus making " artificial minerals." Kecently the diamond has been formed in minute crystals, also small but fine clear rubies, and so, too, quartz, feldspar, mica, and many common species. It must be acknowledged, however, that the specimens thus obtained in the laboratory are in most cases very minute and much less beautiful than those of nature; for the chemist in the laboratory has only a limited time for his experiments, and often must use violent means, as the great heat of a furnace./ while nature works slowly and gently. MINERALS, AND HOW TO STUDY THEM. CHAPTER II. SOME PRELIMINARY HINTS AS TO HOW TO STUDY MINERALS. A MINERAL, we have seen, is a substance formed by nature alone, a solid with one or two exceptions, and one having as a rule a definite form of its own and certain characters of hardness, density, luster, color, and still others, and, most important of all, a definite chemical com- position. The first group of characters, having to do with the form and structure and so on, are called PHYSICAL CHARACTERS, while those depending directly upon the composition are called CHEMICAL CHARACTERS. All of these will be described in some detail in subsequent chap- ters, but it is necessary first to gain a little knowledge as to how to study minerals, where the object is to learn as much as possible about each and to distinguish one kind from another. The mineralogist must first of all use his eyes and other unaided senses in studying minerals; in other words, he must gain all the information he can about minerals by looking at them and handling them. If he learns to do this wisely, he will be surprised to find how keen his senses become and how much he can find out. But as he gains in experience he will see that this only carries him to a certain point, and he should always recognize the im- HINTS ON THE STUDY OF MINERALS. 9 portance of confirming the conclusions reached by his eye and hand by more positive tests. Often, even in the case of the commonest species, the appearance may lead one who depends upon it alone quite astray. The old saying that "all is not gold that glitters," and the names ap- plied to certain common minerals of " fool's gold," " false galena," and others like them, express the result of experi- ence that the senses unassisted may readily be deceived. The trained eye of the mineralogist will show him, first of all, the form of the mineral, as to whether it has the regular geometrical shape of a crystal or not, or is simply granular, fibrous, and so on. It will show him whether it has the natural, easy, smooth fracture of many crystal- lized substances, called cleavage, or only the fracture of ordinary kinds. It will tell him, too, what peculiarities of luster the surface of a mineral presents, depending upon the way in which it reflects light, whether metallic, glassy, greasy, or silky, and so on; also what the color is, whether it is transparent or opaque, and many other points. The touch will show whether the "feel" is greasy, as is true of a few very soft minerals, or harsh, as are the majority. Again, a mass in the hand will often be recog- nized at once as heavy or light as compared with familiar substances of the same appearance. Thus the common minerals quartz, feldspar, calcite, have nearly the same density, and one can easily become so accustomed to them that a piece of gypsum seems light and one of barite (heavy spar) seems heavy. So a piece of the metal alu- minium seems very light because we instinctively compare 10 MINERALS, Afrt> HOW TO STUDY THEM. it with the other much denser metals which we are ac- customed to handle. The taste may sometimes tell, for instance, that rock salt is in hand, while the odor is occasionally a useful character, as the clayey odor of some minerals when breathed upon. But it requires some education and experience before the senses are so on the alert that all the characters noted are perceived at once and rightly estimated; this is what every one should strive for; and one of the great benefits to be derived from the study of mineralogy is that it culti- vates and stimulates the powers of observation. When the senses alone stop, simple tests to aid them come in. A touch upon the smooth surface of a mineral with the point of a knife serves to show whether it is relatively soft or hard. The color of the powder ob- tained by scratching with the knife or upon a plate of rough porcelain or ground glass, called the streak, is some- times quite different from that of the surface, and in such cases this is a very important character. Then come more careful tests : the determination of the relative density or specific gravity; the use of the blow- pipe, giving the comparative degree of fusibility; and a number of simple chemical trials, to show the presence of sulphur or arsenic, silver, lead or iron, barium or stron- tium. Then follow still other tests, till we come to the re- fined methods of the trained mineralogist with his beauti- ful goniometer for measuring angles, the microscope and optical instruments, the accurate chemical analysis and other means by which most of nature's secrets may be HINTS ON THE STUDY OF MINERALS. 11 learned and the characters of each mineral thoroughly studied. SOME SUGGESTIONS ABOUT MAKING A COLLECTION OF MINERALS. A very important matter in the study of minerals is the student's own collection; for every one who desires to really learn mineralogy must have a collection of his own to examine and experiment upon. It is very desirable that the school or college should have a larger cabinet for refer- ence and study, but this does not take the place of the in- dividual collection, which will be studied, arranged, labeled, and handled over and over again till every specimen is per- fectly familiar. Further, the student should obtain his specimens as far as he can by collecting for himself. No matter if he lives in a region that does not seem at first to afford very much, he can certainly find something that is worth keeping until he obtains better; and occasionally he will have the opportunity to make trips to some of the noted localities, where he can find more and a great variety. There is nothing more delightfully instructive and health-giving than to spend a day in the open air, with a good hammer in hand, a bag for the specimens, and plenty of soft paper (and perhaps some cotton) to wrap them up in. The hammer should be of hard steel that will not chip on the edges; it may weigh from a pound to a pound and a half, and the face should be square or slightly oblong and the edges sharp, while the back has the form of a 12 MINERALS, AND HOW TO STUDY THEM. wedge, as seen in Figure 1 (one-fourth natural size). A cold-ghisel or two, for working into cracks or crevices, will often be found useful; also a small light hammer with a sharp edge for trimming specimens. This will often do no damage, when a blow from a heavy hammer would shatter the specimen and destroy it. Do not break the crystals out of the rock, as a rule. A detached crystal of garnet is interesting when quite perfect, but in general the crystal is most interesting and instructive when in its own home. The seller of min- erals soon discovers this, and it is un- fortunately not an uncommon trick at some localities for instance in the Alps for the local collector working for his daily bread to exercise his ingenuity in mounting a loose crystal J4 natural size. in a mass of rock in which it never belonged, thus to increase the value of the specimen and deceive the unwary purchaser. The student is not advised to spend a great deal of money in buying specimens, particularly at any one time. Still it is less easy to collect personally now than it was years ago, and many students may not have opportunity to do the traveling that it requires: and even here the reward is often small, unless at a quarry or mine where work is being carried on all the time. HINTS ON THE STUDY OF MINERALS. 13 Hence a little money is by no means thrown away if judiciously expended from time to time, for it will serve to buy a few small characteristic specimens of the common species and pure fragments for blowpipe tests. Fine specimens, especially of the rarer species, are now very expensive, but sufficiently good ones of the . minerals it is important for the student to know well may be obtained, for very little money.* It is better to collect small specimens rather than large, as far as possible, such as will go in a little paper tray 2 inches square, or 2 by 3 inches, or at most 3x3. These trays are inexpensive and are very useful for the arrange- ment and preservation of a cabinet. If the specimens are placed loose in a drawer, it can hardly be opened a few times without throwing them into confusion, and sooner or later they will be badly injured. The sizes mentioned are the most useful, though 3x4 inches might well be added. A depth of half an inch is sufficient for the tray, but the drawers, if possible, should not be less than 2J or 3 inches deep. All the specimens in a collection should be care- fully labeled, particularly as regards the locality. * A list of the most important minerals is given at the close of the book, and those most useful for trial with the blowpipe are there indicated. 14 MINERALS, AND HOW TO STUDY THEM. CHAPTER III. THE FORMS. OF CRYSTALS AND KINDS OF STRUCTURE. THE principal characters of minerals, by which one species is distinguished from another, have been briefly alluded to in the preceding chapter. It is now necessary to study some of these characters more fully. First the PHYSICAL CHARACTERS will be considered. These include the form and structure, the cleavage, frac- ture, hardness, tenacity, elasticity; also the density; fur- ther, the color, luster, degree of transparency, and some few others. The present chapter is limited to a discussion of the crystalline form and the structure in general. THE GENERAL CHARACTERS OF CRYSTALS. If we examine the specimens of the different mineral species in a good cabinet, we see that many of them occur commonly in regular solid forms with smooth faces, which forms, as we study the subject further, we find to be char- acteristic of each individual species. These regular forms are called crystals. The cubes of fluorite (Fig. 2) or galena, the six-sided prisms of quartz (Fig. 3), the twelve- sided, twenty-four-sided, or even more complex forms of garnets, are common examples of crystals. Further, even when a specimen does not show this regular external form, there is usually a definite crystalline structure, which may THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 15 be shown in the easy fracture called cleavage, as that of calcite, or which may be indicated in other ways, as we shall soon learn. How is this regularity of form and structure to be explained ? First, we will speak of crystals. The physicist, as the result of his studies into the struc- ture of different kinds of matter, has concluded that every Fluorite, Group of Cubic Crystals. Quartz Crystal. body is built up of minute particles called molecules, which are much too small to be perceived even by the strongest microscope. In a solid body, as a lump of iron, ice, sulphur, he thinks of these molecules as bound to- gether by a strong force of attraction, called cohesion, so that it requires a hard blow or great pressure to change its shape. In a liquid body he thinks of them as free to move or roll over each other, so that the liquid takes at once the shape of the vessel in which it is contained, whatever that may be. In a gas, he believes that the molecules are sepa- 16 MINERALS, AND HOW TO STUDY THEM. rate from each other, a long distance in fact compared with their size, and that they are darting about very rap- idly, colliding against each other and any confining surface. The result of this is that the gas at once fills entirely a vessel into which it is introduced and presses against its sides; the pressure being simply the result of the bombard- ment of these little rifle-balls. The pressure of the external air, for example, is shown by the collapse of the cheeks when the air within the mouth is drawn away. The relation between these minute particles or molecules thus explains the condition of a body, as solid, liquid, or gaseous; for example, the distinction between ice, water, and steam. But more than this: When a liquid turns into a solid because the temperature falls, as when water freezes, or liquid sulphur or molten iron hardens on cooling, the force of cohesion comes into play to bind the particles together into a rigid mass. So, also, when by slow evaporation from a solution, as of salt or alum in water, the dissolving liquid is removed, the substance in solution also passes back into the solid form under the action of this same force of cohe- sion. Thus the solid is formed from the liquid by the action of the forces acting between these little particles. Further, if the molecules are all of one kind, as in a given chemical substance, and if there are no hindering causes, these molecules will build themselves up after some regular pattern and the external result is the geometrical form, which is called a crystal. It is somewhat as if the mole- cules were little building-stones, built up into a solid structure by forces acting between them and causing them THE FORMS OF CRYSTALS AltfD KINDS OF STRUCTURE. 17 to arrange themselves after a definite manner when they are free to do so. This regular building of the molecules, which, as has just been shown, may take place from a liquid, happens also, even more perfectly, when a solid is formed direct from a gas. Water vapor in the air, if cooled down suf- ficiently, is formed into the solid snow, and the little snow- crystals, that fall silently through the atmosphere and which we may catch on our coat-sleeve on a cold winter day, are often of wonderful regularity and beauty of form. The figure (4) gives some of the many forms of snow-crystals drawn by Scoresby in a visit to the Arctic 4. Snow-crystals. many years ago. So too, as will be described later, if a mineral containing arsenic is heated in a glass tube open at both ends, the arsenic driven off, uniting with the oxygen of the air, forms the vapor of oxide of arsenic; this is condensed a little higher in the tube, where it is cooler, and there deposited in minute spangling octahedral crys- tals (Fig. 6, p. 23), which are sometimes quite large and very perfect in form. It is not always easy to make good crystals, whether starting from a liquid or from a gas. This is true in part because we cannot give the time required for the perfect process, in part because there are other hindering con- 18 MINERALS, AND HOW TO STUDY THEM. ditions. But sometimes we can succeed well, and the growth of an octahedral crystal of alum in a strong solution can be watched from day to day and a large and fine crystal may be the reward of our skill and patience. In nature's laboratory the conditions are more favorable, particularly because there is never any limit of time, and the many beautiful and complex crystals of minerals with brilliant faces show the result. Even here, however, the building process often cannot go on freely, and imperfect crystals, or perhaps a mass with only a confused crystal- line structure and without distinct external form, may be all that is produced. The quartz, feldspar, and mica in the rock called granite have usually formed together in such a way that neither one has had an opportunity to build itself up into perfect crystals, and yet the student who understands the optical study of thin sections of a rock in polarized light can prove that each grain, formless though it may be externally, has all the internal molecular structure of the crystal. In a cavity in the granite we are not surprised to find crystals of quartz and feldspar, perhaps also of mica, as the cavity here means that each has had an opportunity to exercise its tendency to build itself regularly with something of the freedom which a perfect crystal requires. Another familiar example of crystallization is given by the ice covering a pond, which is as truly crystalline in structure as the perfect snow-crystal; but here there are no crystals, and it is easy to understand why. The slow dis- section of the mass, however, under the melting action of the sun reveals something of the regularity in the molec- THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 19 ular building, and the same thing is proved by an exami- nation in polarized light. Sometimes in tHe freezing of a little pool of water on a sidewalk the formation of the slender crystalline ribs of ice may be watched as they shoot out, forming a framework which may soon lose its distinctness as the entire surface is frozen over. We learn, then, that a crystal is the regular solid form which a chemical substance takes when it passes into the solid state from that of either a liquid or a gas, if under such conditions that the molecules are quite free to ar- range themselves according to the direction of the at- tractive forces acting between them. The crystal is, therefore, the outward expression of the structure in the arrangement of these molecules, and its form is for this reason the most important of all the physi- cal characters of a given species and the one which in gen- eral most definitely distinguishes it from others. It is interesting to note that a small crystal is just as per- fect and complete an individual as a similar one of great size; there is among the crystals of a given species no such connection between size, on the one hand, and age and maturity, on the other, that belongs to the individuals of a species in the animal and vegetable kingdoms. Some crystals are so minute as to be almost microscopic; others may be of enormous size, as the gigantic quartz crystals occasionally found in the Alps, or the equally large beryl crystals from New Hampshire. A cave opened a few years ago at Macomb, New York, contained 15 tons of great cubic crystals of fluorite ; another cave in Wayne County, Utah, contained a great number of enormous crystals of 20 MINERALS, AND HOW TO STUDY THEM. gypsum, some of them three feet or more in length. But the very small crystals and the like ones of enormous size are not essentially different except in this comparatively unimportant respect of magnitude. And yet there are many interesting points of resem- blance between crystals and living plants. Crystals grow as well as plants, and under favorable conditions so rapidly that the increase in size may be watched not only from day to day, but from hour to hour, or even from minute to minute. The complex forms that are built up especially in such cases of rapid growth are often wonder- fully plantlike in aspect. This is true, as every one has noticed, of the delicate frost-figures which form so quickly upon a window-pane or a paving-stone in winter; also, in other more permanent cases, the arborescent or dendritic forms of native gold or silver or copper are good examples of the same fact. The terms used in describing them are indeed given because of their resemblance to forms of vegetation. Furthermore, as a wounded plant tends to heal itself when, for example, a branch has been broken off, or as a slip or graft tends to develop a full individual; so, too, a broken crystal may be more or less healed, but in the last case the material which repairs the injury must be supplied from an outside source. Thus the silica to mend a broken quartz crystal must come from a foreign solution, and the crystal itself only directs the way in which the molecules of the solution are laid down; it is interesting, however, that the growth takes place more readily on a surface of fracture than on a natural crystalline face. In this way the grains of quartz in a sandstone, formless be- THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 21 cause only fractured fragments, often tend to build them- selves up into complete crystals. Still again, although a crystal never has an old age in the sense that this is true of a plant or an animal, it is nevertheless a fact that many crystals tend to change or decay as time goes on, if subjected, for example, to the corroding effects of some foreign solution. Even the beautiful gems, such as the sapphire, emerald, topaz, garnet, hard and comparatively insoluble as they are, have this liability to undergo what is called chemical decomposition, with the loss of all their beauty and a total change of chemical substance. This is spoken of again in a later part of the chapter, where pseud omorphs are de- scribed, but it is worth noting here because somewhat analogous to the change that old age brings to a living organism. THE SYSTEMS OF CRYSTALLIZATION. The forms of crystals are so varied and the difficulties in studying them minutely so great that we shall only attempt here to learn some of their simplest kinds. In the first place, it is important to understand that it can be shown that all crystals belong to one of six classes, or systems, which are named as follows: I, ISOMETRIC; II, TETRAGONAL; III, HEXAGONAL; IV, ORTHORHOMBIC; V, MONOCLINIC; VI, TRICLINIC. The characters of each system and the relations between them will be briefly mentioned after the chief forms in each have been described. 22 MINERALS, AND HOW TO STUDY T#EM. I. Isometric System. The principal forms of the Isometric System are the cube, octahedron, dodecahedron, the two trisoctahedrons, the tetrahexahedron, and the hexoctahedron. Cube. The cube has six equal faces, each one of which is a square, and the angle between any two faces is a right angle, or 90. It is shown in Fig. 5. Galena and fluorite often occur in cubes. Octahedron. A regular octahedron (Fig. 6) has eight like faces,* each a triangle with equal sides and three equal angles (each 60) ; the angle between any two adja- cent faces is 109 28'. Magnetite is often in octahedrons. Dodecahedron. The rhombic dodecahedron (Fig. 7) has twelve equal faces,* each of which is a rhomb with plane angles of 60 and 120, while the angle between two adja- cent faces is 120. This is a common form with garnet/ These forms may occur together on the same crystal. Thus crystals of galena often show the cube and octa- hedron together. Fig. 8 is generally described as a cube modified by an octahedron, and Fig. 9 as an octahedron modified by the cube. If a cube is cut out of a block and the solid angles sliced away carefully, the new surface making equal angles with the three cubic faces, the result is to give finally an octahedron. It is seen that the octa- hedral faces are little triangles on the solid angles of the * Octahedron is namsd from the Greek OKTGO, eight, and edpa, face, or the eight-faced solid. Dodecahedron is similarly named from dcodeKa, twelve, and edpa, face. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 23 cube and equally inclined to the three cubic faces. On the other hand, the cubic faces are small squares on the six solid angles of the octahedron. The angle between two adjacent faces of a cube and an octahedron is 125 16'. Figures 10, 11 show the cube and dodecahedron to- 5. gether, and Fig. 12 the octahedron and dodecahedron. Both the cube and octahedron have twelve similar edges, and these are cut off equally, or truncated, by the twelve faces of the dodecahedron. In Fig. 13 we have again a form (not shaded) resulting from the combination of the faces of the cube (a), octahedron (0), and dodecahedron (d). The angle between adjacent faces of the cube and dodecahedron is 135; be- tween those of the octahedron and dodecahedron it is 144 44'. Trapezohedron. A trapezohedron has twenty-four equal faces, each a four-sided figure or trapezoid. It is shown MINERALS, AKD HOW TO STUDY THEM. in Fig. 14, which is a common form with garnet. There may be a large number of different trapezohedrons, all having the same general form but differing in the angles between the faces. A similar remark may be made about each of the other type-forms of this system yet to be de- scribed. It requires much more study than is possible for the beginner to leaxn how these forms are mathematically distinguished from one another. Figures 16 to 19 show combinations of the trapezohe- 16. 17. dron (n or m) wiVh , the cube (a), octahedron (0), and dodecahedron (d). The last two are common forms with garnet. The trapezohedron is also called a tetragonal trisocta- hedron because its form suggests an octahedron in which three faces take the place of a single octahedral face, each of them being a four-sided figure or tetragon. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. *J5 There is also another trisoctahedron, called a trigonal trisoctahedron, shown in Fig. 15, which has, again, twenty- four faces, three of these also corresponding to an octahedral face ; but each is a three- sided figure (trigon) or an isosceles triangle. This form does not often occur alone, but may be seen on complex crystals of galena. Fig. 20 shows a figure of galena with the cube (&), octahedron (o), dodeca- hedron (d); also two different trigonal trisoctahedrons, lettered p and u. Tetrahexahedron. A tetrahexahedron (Fig. 21) is a twenty-four-faced solid,* each face an isosceles triangle and four together having the same position as the face of a cube. Fig. 22 shows a combination of the cube and a 21. 22. tetrahexahedron ; the latter is said to bevel the edges of the cube because the two planes are equally inclined to the two adjacent cubic faces. Hexoctahedron. A hexoctahedron (Fig. 23) is a forty- eight-faced solid; each face is a scalene triangle, and six faces have the same general position as a face of an octa- hedron, f * Named from rerpa, four, e, six, and edpa, face, f Named from e?, six,oKroo, eight, and ed pa, face. 26 MINERALS, AND HOW TO STUDY THEM. Fig. 24 shows a combination of the cube (a) with the hexoctahedron ; it is a common form with fluorite. Fig. 25 is a common garnet form, the dodecahedron (d) with the hexoctahedron, the latter beveling the edges of the former. Many of the figures thus far given and some of those which follow are shaded, so as to appear solid to the stu- dent learning about crystals for the first time. It is ob- 24. vious, however, that when the form is complex that the shading is impossible, and for the experienced crystallog- rapher it is quite unnecessary; hence these more complex figures are shown in line only. Through the rest of the book these line figures will be freely used. The student will soon find that they appear as solid to him as the others.* All the forms that have been mentioned belong to what is called the ISOMETRIC SYSTEM, in which the crystallog- rapher refers the planes to three equal- axes at right angles * It will be a great help to the student if he has a few models to handle. These are made in great perfection in wood and are not very expensive. The student may also try to cut them for himself out of soft wood or plaster-of-paris; even a potato can be employed for temporary use. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 27 to each other. The position of the axes passing through the centers of the crystals is shown in Figs. 26, 27, 28. It will be seen to be true of all these, as it is of all their combinations, that the arrangement of the faces is the same about each one of the six cubic faces, or in other words about the ends of the three axes. Another way of stating this is to say that all these isometric crystals have three equal planes of symmetry at right angles to each other.* These three equal planes of symmetry are planes parallel to the cubic faces and have a corresponding position in the other simple crystals or com- binations of them. Each plane of symmetry divideiTthe ideal crystal into two symmetrical halves, and here the three sets of halves made by the three planes parallel to each pair of cubic faces are identical; hence the planes of symmetry are said to be equal. The axes are the lines in which these three planes of symmetry intersect each other. * A plane of symmetry is a plane which divides the solid into equal halves such that if one half is placed against a mirror the re- flection completes the form. This is one form of the geometrical definition applying to an ideal crystal ; it will be explained later (p. 50) how this must be broadened to cover the crystallographic symmetry of actual crystals. 28 MINERALS, AND HOW TO STUDY THEM. A cube, as well as the other isometric forms mentioned, has also six other planes of symmetry passing diagonally through the opposite edges, and hence parallel to each pair of the dodecahedral faces. 29. 30. Fig. 29, of cuprite, and Fig. 30, of the rare species micro- lite, both drawn on a larger scale, are added to show some rather complex combinations of isometric forms. In Fig. 29 the cube (a) and the dodecahedron (d) predomi- nate; the faces of the octahedron (o) are small; n and ft are faces of two different trapezohedrons. In Fig. 30 the octahedron (o) predominates and then the cube (#), while the dodecahedron (d) is subordinate; the faces m belong to a trapezohedron, and p to a trigonal trisoctahedron. There are also several other forms belonging to the Isometric System, but which are described as half-forms, or forms in which only half of the faces in the correspond- ing whole form are present. To them the rules of sym- metry do not apply, but their faces are also referred to three equal axes at right angles to each other. The most important of these half-forms are the tetrahedron and pyritohedron. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 29 Tetrahedron. The tetrahedron (Fig. 31) is a form with four equal triangular faces, each of them an equilateral triangle. It is considered in Crystallography as the half- form of the octahedron, since half the faces of the octa- hedron, taken every other one, will if extended form a 31. 32, tetrahedron. Perhaps a study of Fig. 32 will make this clearer. The angle between two adjacent faces of a te- trahedron is 70 32'. Fig. 33 shows a combination of a cube (a) with the four faces of a tetrahedron (o). It is seen that the planes are present only on the alternate angles of the cube. In 33. 34. a, 35. Fig. 34, a combination of a cube and a tetrahedron, the latter predominates. Fig. 35 shows a combination of the tetrahedron before figured (o) with another similar form (lettered o x ) made up of the four remaining faces of the octahedron. It might be asked why this form cannot be regarded as an octahedron in which four faces are ac- 30 MINERALS, AND HOW TO STUDY THEM. cidentally larger (compare remarks ou p. 49) than the others; but this is impossible, for it can be proved, per- haps at once by difference of luster, that the eight faces are not all alike, but only four and four. This, however, is a somewhat difficult subject for a beginner. Pyritohedron. The pyritohedron (Fig. 36) is a twelve- sided solid, or dodecahedron, each face of which is a penta- gon, but not here as with the pentagonal dodecahedron of geometry a regular pen- tagon. In crystallography the name dodecahedron is usually given only to the rhombic dodecahedron described above (Fig. 7), and this form, the pyritohedron, takes its name from the species pyrite or iron pyrites, because of common occurrence with it. The pyritohedron is the half-form of the tetrahexahe- dron. If in combination with the cube, the solids in Figs. 37, 38 result; Fig. 39 is a combination of an 37. octahedron and pyritohedron. There are also other half- forms in the Isometric System, thus of the two trisoctahe- drons and the hexoctahedron; but they are not very com- mon and will not be described here. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 31 II. Tetragonal System. The chief forms of the Tetragonal System are the two square prisms and pyramids and the eight-sided prism and double eight-sided pyramid. 40. 41. Square Prism and Pyramid. One of the square prisms is shown in Fig. 40 and the square pyramid corresponding to it in Fig. 41, while Fig. 42 is a combination of the two forms. The square prism has, like the cube, angles of 90 be- tween the faces, but it differs from the cube because the four vertical faces are not like the two end faces, or basal planes as they are called. This is often shown in a crystal by the difference in the smoothness of the two kinds of faces; or there may be easy fracture, or cleavage, parallel to one set of planes and not to the other. The square pyramid looks somewhat like a regular octa- hedron, but here the faces are isosceles triangles (not equilateral) and the angle between two faces over a hori- zontal edge differs from that over one of the vertical edges in fact, either angle is characteristic of a given MINERALS, AND HOW TO STUDY THEM. species and differs from one species to another. There may be a great many square pyramids of the same type as this but differing in their angles and consequently flatter or sharper at the extremity. Fig. 43 shows an acute square pyramid, p 9 while Fig. 44 43. 44. represents another crystal of the same species (octahe- drite) in which this pyramid p is present but with it three others, z, i, v, each flatter or more obtuse at the summit than the others. There is also another square prism and another square 45. 46. ..__ pyramid diagonal to the set just described; they are shown in Figs. 45 and 46. Fig. 47 shows these two forms together. Taken alone these two forms cannot be dis- tinguished from the other two shown in Figs. 40, 41, but THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 33 49. 50. if they occur together the distinction is obvious. Thus Fig. 48 shows the two prisms on the same crystal, the faces of one truncating the edges of the other. Figs. 49 and 50 show the same two prisms in skeleton lines with the axes represented inside; it is seen that in the prism (Fig. 49) first mentioned often called the unit prism the horizontal axes join the middle points of the opposite edges, while in Fig. 50 they join the centers of the oppo- site faces. The latter form is often called the diametral prism, because the faces -are parallel to the axes or diam- eters. In Figs. 51 and 52 the two pyramids are again shown, 51. 52. 53. and here the position of the axes should also be noted. Fig. 51 is often called a unit pyramid or one of the unit series; Fig. 52 ,a pyramid of the diametral series. In Figs. 53, 54 the combinations of each square prism with the pyramid of the diagonal set are shown. Fig. 53 resembles a cube modified by an octahedron (Fig. 8, p. 23), but it differs from it in that the faces lettered p, while they make equal angles with the two adjacent faces a, make 34 MINERALS, A.ND HOW TO STUDY THEM. different angles with the basal plane or base c. The same statement could be made in regard to the form of Fig. 54. 55 > There is also an eight-sided prism made up of eight like faces, and it is shown on the complex crystal represented in Fig. 59; its faces are lettered h. Further, there is also a double eight-sided pyramid, as shown in Fig. 55. This is often called a zirconoid, because common with the species zircon. In Figs. 56, 57, 58, representations of crystals of zircon, the faces x (in part lettered) belong to a zirconoid or 57. 58. double eight-sided pyramid. The same is true of the faces lettered z in the figures 59, 60. Fig. 59 represents a complex crystal of wernerite, and Fig. 60 is a map of the top of the same crystal, or a basal section, as it is called. Note the prism and pyramid of the unit series, m and r, the prism and pyramid of the diametral series, a, e; also the eight-sided prism 7i and the double eight-sided pyramid z already referred to. All of the forms that have been described belong to THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 35 what is called the TETRAGONAL SYSTEM, in which the planes are referred to three axes at right angles, of which the two in the horizontal plane are equal, the third or ver- tical axis is longer or shorter. It will be seen by examining the figures, especially Figs. 58, 59, 60, that the grouping of faces is the same about the faces lettered a, but different from them about the face c, the basal plane. It will be seen, too, that about c the faces of the same kind are all arranged in fours or eights. In other words, all these tetragonal crystals have a pair 60. a" * d &X ^ \ m' \ 2' / r" e" r' e"' ,\ , r >n e r rt'"\. avi^v-X" z / "* ^VM of equal planes of symmetry parallel to the faces a and at right angles to each other. There is also another pair at right angles to each other parallel to the faces m. All these four planes meet in a common vertical line, which is called the vertical axis. There is, finally, a fifth plane of symmetry parallel to the top and bottom of the crystal, or the basal plane c, and hence at right angles to this vertical axis, but it differs from either of the other pairs. In the Tetragonal System there are also some half- forms but only one will be specially described. 36 MINERALS,, AND HOW TO STUDY THEM. Sphenoid. The sphenoid (Fig. 61) is a four-faced solid 61. looking like a tetrahedron, but differing from it since the faces are isosceles (not equilat- eral) triangles. It is described as the half- form of the square pyramid shown in Figs. 41 and 51. III. Hexagonal System. The chief forms of the Hexagonal System are the two six-sided prisms, the two double six-sided pyramids, and the twelve-sided prism and double twelve-sided pyramid. These will be briefly described first, and then the char- acteristic forms of the Khombohedral part of the Hexago- nal System will be mentioned. Hexagonal Prism and Pyramid. The hexagonal prism and pyramid are shown in Figs. 62 and 63, while Fig. 64 gives a combination of the two. The angles of the hexago- .nal prism are exactly 120, and the terminal face or basal plane is a regular hexagon. The faces of the hexagonal 62. pyramid are isosceles triangles, differing in angle accord- ing to whether the pyramid is more obtuse or acute. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 37 These angles, and the others that depend upon them, are characteristic for a given species. There is also another hexagonal prism and pyramid di- agonal to the others and looking much like them. These two sets correspond to the two square prisms and pyra- 65. mids of the Tetragonal System. Compare Fig. 65 of the unit prism and Fig. 66 of the second or diagonal prism and note the position of the axes shown in each; also the position of the axes in Fig. 67 of the unit hexagonal pyra- mid. Fig. 68 shows the combination of the unit hexago- nal prism and pyramid with the basal plane. Fig. 69, of a crystal of beryl, shows a combination of the unit prism and pyramid, m and p ; the diagonal prism and pyramid, a and s ; also the basal plane c. There is also a prism bounded by twelve similar faces, and a double pyramid bounded above and below by twelve triangular faces. This double twelve-sided pyramid is often called a berylloid, because common with crystals of beryl. Two berylloids are shown in Figs. 70, 71; the faces are lettered (in part) n and v respectively. Fig. 71 is an enlarged map, or basal section, of the top of a crystal much like that of Fig. 70. Note, also, on Fig. 71 the hex- 38 MINERALS, AND HOW TO STUDY THEM. agonal prism, m, and pyramids, u and p, of tiie unit series; the prism, a, and pyramid, s, of the diagonal series. These hexagonal forms belong to the HEXAGONAL SYS- 70. 71. m TEM, where the planes are referred to the four axes shown in Figs. 65, 66, 67, three in a horizontal plane equal and cutting each other at angles of 60, and the fourth vertical axis either longer or shorter. It will be seen that the faces are arranged in the same way about each face a and hence about each end of the horizontal or lateral axes, but differently about the basal face c, that is, at the extremity of the vertical axes. The faces are arranged about the face c in sixes or in twelves instead of fours and eights as in the Tetragonal System. All the hexagonal forms have three equal planes of sym- metry making angles of 60 with each other parallel to the faces m\ also another set of three, diagonal to the others and parallel to the faces a\ and a seventh plane parallel to the top or base of the crystal. There are several half-forms in the Hexagonal System, THE FORMS OF CRYSTALS AND KISTDS OF STRUCTURE. 39 but the only ones that will be described here are those of the Rhombohedral System. The RHOMBOHEDRAL SYSTEM is generally treated as a branch of the Hexagonal System. In the forms belonging to it the faces are in threes about the extremities of the vertical axis c, and there are only three vertical planes of symmetry making angles of 60 with each other inter- secting in this axis. The important forms are the rhom- bohedron and the scalenohedron. Rliombohedron. The rhombohedron is a six-sided solid, each face of which is a rhomb; it is shown in Figs, 72, 73, 74. There may be a great many rhombohedrons, as shown 72. 73. here, differing in angle and consequently more or less obtuse or acute. The rhombohedron looks somewhat like a cube if the cube is placed with the line joining two opposite angles vertical; in fact, the cube comes between the obtuse and acute rhombohedrons, having an angle of just 90. The rhombohedron may be regarded as a half-form of the hexagonal pyramid, but this subject is a rather diffi- cult one and cannot be followed up here. Scalenohedron. The scalenohedron (Fig. 75) is a twelve- sided solid, looking a little like a double six-sided pyramid, 40 MINERALS, AND HOW TO STUDY THEM. but the faces are scalene triangles and the edge is zigzag, up and down, like that of a rhombohedron, instead of hori- 75. zontal as in the pyramid. Moreover the angles between the faces over the edges which meet in the vertex are only alike every other one in other words, there are two sets of three each, those of one set more obtuse than those of the other. The two hexagonal prisms before described and the hexagonal pyramid of the diagonal series also belong to the Rhombohedral System. The number of species crystallizing in the rhombohe- dral division of the Hexagonal System is very large, and some of them, as, for example, calcite, are very highly complex. In the figures of calcite given here, 76 to 80, 76. 77. 79. the faces r,f, e belong to different rhombohedrons; v to a scalenohedron ; m is the unit hexagonal prism ; c the basal plane. Fig. 81 represents a more complex crystal, also of cal- cite, and Fig. 82 gives a basal projection of it. Here there are several rhombohedrons, r, e, 5 * 10 = 60.68. OO.O The percentage composition of sodium chloride is, there- fore: Na 39.32 Cl 60.68 100.00 Again, the formula of stibnite, Sb 2 S 3 , means that two atoms of antimony (Sb) unite with three of sulphur (S). But the atomic weights of antimony and sulphur are 120 and 32 respectively. The molecular weight is, therefore, equal to 2 X 120 + 3 X 32 = 240 + 96 = 336. Hence in 336 parts, 240 are antimony and 96 sulphur, and to find the amount of each in one hundred parts we have the proportions inn 336 : 240 = 100 : 71.43, or = 71.43. ooo 336 : 96 = 100 : 28.57, or 96 * 10 = 28.57. odo The percentage composition is, therefore: Sb 71.43 S 28.57 100.00 Again, the formula of one kind of garnet is Ca 3 Al 2 Si 3 0,, or, as it may be written, 3CaO.Al a 3 .3SiO a . Taking the 118 MINERALS, AND HOW TO STUDY THEM. second form and finding the atomic weights for each ele- ment from the table, adding them together for each group of atoms and multiplying by the factor given, we have: 3CaO = 3(40 + 16) = 3 X 56 = 168 A1 2 3 = 2 X 27 -f 3 X 16 = 54 + 48 = 102 3Si0 2 = 3(28 + 2 X 16) = 3 X 60 =180 450 Hence again, by the rule of proportions : 450 : 168 = 100 : 37.33 450 : 102 = 100 : 22.67 450 : 180 = 100 : 40.00 The percentage composition is, therefore : Lime, CaO 37.33 Alumina, A1 2 3 22.67 Silica, Si0 2 40.00 100.00 If desired it would have been as easy to deduce the amounts of the elements Ca, Al, Si, present, but, as stated on p. 115, it is more convenient to use the oxides instead. CLASSIFICATION. There are various methods of classification that may be adopted for minerals. The strictly scientific way is to arrange similar compounds together, that is, first, the native elements; then the sulphides, the oxides, the carbonates, and so on. These are further classified by the relationships which a study of the elements and of the crystalline forms of their compounds makes known. THE CHEMICAL CHARACTERS OF MIKERALS. 119 For example, the following minerals being all carbonates are, on a strictly scientific method, placed in the same general division: Calcite, calcium carbonate, CaC0 9 . Dolomite, calcium-magnesium carbonate, CaMg(C0 3 ) a . Magnesite, magnesium carbonate, MgC0 8 . Siderite, iron carbonate, FeC0 3 . Ehodochrosite, manganese carbonate, MnCO s . Smithsonite, zinc carbonate, ZnC0 3 . Further, they are all placed side by side in the same group, called the Calcite group, because they have the same general crystalline form and very nearly the same angles, e.g., all show rhombohedral cleavage with the angle varying from 105 to 1.07. This is called, therefore, an isomorphous group, having like form * and analogous composition. Another series of minerals, also in the same division of carbonates, form a second isomorphous group, the Aragon- ite group: Aragonite, calcium carbonate, CaC0 3 . Witherite, barium carbonate, BaC0 3 . Strontianite, strontium carbonate, SrC0 3 . Cerussite, lead carbonate, PbC0 3 . A third case is the Barite group of sulphates : Barite, barium sulphate, BaS0 4 . Celestite, strontium sulphate, SrS0 4 . Anglesite, lead sulphate, PbS0 4 . Also, a little less closely related, Anhydrite, calcium sulphate, CaS0 4 . * Isomorplious is from zVoS, like, and fj.oprj, form. 120 MINERALS, AND HOW TO STUDY THEM. The Galena group (galena, argentite, etc.), the Apatite group (p. 113), the Feldspar group, the Mica group, are other examples. These and many besides are described in an advanced work on mineralogy. Another method of classification is to place together the different compounds of each metal, as all the compounds of iron, all those of silver, and so on. Still another way would be to put the metallic ores together, the gems, and so on. Of these and still other different methods, the most satis- factory for us is the second, as further explained on a later page (p. 158 et seq.). It will be noticed, in the cases of the two groups taken for illustration, that the same composition, calcium carbonate, CaC0 3 , belongs to two minerals, calcite and aragonite. These are regarded as distinct species because they have a different crystalline form and different physical characters, e.g., specific gravity. What is true of this chemical com- pound is true of a number of others. Among minerals, such compounds are said to be dimorphous or to have two forms. THE USE OF THE BLOWPIPE. CHAPTER VI. THE USE OF THE BLOWPIPE. 1. GENERAL DESCRIPTION OF APPARATUS. THE chemist in the laboratory, as has been already ex- plained, can subject a mineral specimen to a process of analysis, and in this way discover, first, what simple sub- stances or elements it contains, and, second, in what pro- portion by weight they are present; in other words, he can analyze it. This has been done many times in the case of all the minerals we know, and the result has been to show what the composition of each species is, and by what for- mula this can be expressed. It is obvious that this method of complete analysis is the only satisfactory way to gain a complete knowledge of the chemical nature of a given mineral. But the work of the chemist is slow and laborious, and it is often important to be able to learn something about the composition of a mineral more quickly and by an easier method. This can be done by the blowpipe, supplemented by some simple chemical tests; and any one who is supplied with a few tools, and who has the patience to learn to use them, can accomplish it. The results of this blowpipe analysis, taken in connection with the study of the physical charac- ters of a given specimen, almost always suffice to enable a mineralogist who has a fair amount of knowledge and ex- 122 MINERALS, AND HOW TO STUDX THEM. perience to determine what it really is, even if at first it was entirely unknown. The following list includes the articles that are most es- sential for this work : 1. Lamp. 2. Blowpipe. 3. Platinum-pointed forceps. 4. Charcoal. 5. Platinum wire. 6. Glass tubes. 149. Also (7) a few chemical reagents as explained beyond. After some words of explanation about each of these, several other appliances which it is also convenient to have will be mentioned. 1. Lamp. The most convenient form of lamp is a Bun- sen gas-burner (Fig. 149) ; it is provided with a special jet (b in the figure). This burner can be connected with any ordinary gas-jet by a rubber tube, so as to be placed on the table for use. In the Bunsen burner proper, that is, when the jet b is not inserted, the gas mingles in the tube with the air which enters at a, and they together burn at the top in a very hot flame, but one which gives very little light and which deposits no soot upon a surface of cold glass or porcelain. This flame is used by the chemist in the lab- oratory, and also by the mineralogist in heating glass tubes as described beyond. Instead of the Bunsen burner an alcohol lamp may be employed, and in fact was long used by the early chemists; Alcohol, however, is a very inflammable substance, so that THE USE OF THE BLOWPIPE. 123 its use requires much care. One precaution also must be observed with the Bunsen burner: it is best not to turn the flame down low (unless the end of the tube is covered with a cap of wire gauze), for if this is done the flame is liable to " snap down/' that is, the gas may ignite within the tube just above a (Pig. 149). It then burns with a feeble yellowish flame, yielding a disa- 150. greeable odor, and the tube becomes im- mediately very hot. This is dangerous, not only because a severe burn may result from touching the tube, but, still more, because if left a few moments the rubber tube may be melted, the gas ignite from it, and a serious fire be caused. Hence it is better never to go out of the room and leave a Bunsen flame burning even for a few minutes. In a laboratory where there is a slate table this precaution is not so important. When the jet b is inserted in the tube of the Bunsen burner the air-supply a from the openings below is cut off and the gas now burns at the top with the usual yellow flame, here flattened by the shape of the jet; the convenient flame for" ordinary use is about one and a half inches in height. This is the flame to be used with the blowpipe. Instead of this gas-flame a good stearine candle will answer the purpose sufficiently well, or an oil lamp with a suit- able burner. 2. Blowpipe. A common form of blowpipe is shown in 124 MINERALS, AND HOW TO STUDY THEM. Fig. 150. It may be very simple and inexpensive, but should have an air-chamber, , to collect the condensed moisture from the breath. A separate tip (&), either of brass or platinum, with a fine hole, is often used, but it is not absolutely necessary. The essential thing is that the hole, whether in the tip or the tube itself, should 151. be large enough and not too large, and also that it should be round and true, so that a moderate pres- sure of air shall suffice to blow a clear blue flame (see Fig. 153). A trumpet-shaped mouthpiece (c) is usu- ally furnished, but some prefer to dispense with it. 3. Forceps. A pair of steel forceps (Fig. 151) is needed, and it is desirable that they should be nickel- plated to prevent rusting. . One end has platinum points at d, self-closing by a spring, so that the piece of mineral to be heated, placed between them, is firmly supported. At the other end are ordinary forceps for picking up small fragments; this end should never be inserted in the flame. A caution in regard to the use of the platinum points is given on p. 130, for, though infusible, they can be easily injured. 4. Charcoal. Several pieces of charcoal are needed. These are most conveniently rectangular in shape (see Fig. 156) and about four inches long, an inch wide, and three fourths of an inch thick. The charcoal must burn without snapping and must leave very little white ash. It is so difficult to obtain really good charcoal that it is well worth while to purchase a few pieces expressly pre- pared for the purpose, and with care one piece will last for THE USE OF THE BLOWPIPE. 125 many experiments, the surface being rubbed clean, as by a file or knife, after each use. 5. Platinum Wire. A few inches of platinum wire, of the size designated No. 27, usually sold for this purpose, are needed ; directions for its use are given on a later page. In addition to the wire, a small piece of platinum foil is sometimes useful. 6. Glass Tubes. Some tubes of rather hard glass are required; it is convenient to have two sizes, with bores of one sixth and one quarter of an inch, but one will suffice. The larger size can be cut into pieces about five inches in length; the tube will break easily if a single scratch is first made with the edge of a three-cornered file. These tubes are to be used as open tubes, as explained later. Again, pieces a little longer, say six inches, and of the size with the smaller bore, may be taken and held with .the middle point in the hot part of the Bunsen flame. When the glass is soft, draw the two ends apart by a quick motion (without twisting), and then heat each long tapering end in the flame and pinch it off short while hot, using for this the steel end of the forceps. In this way two dosed tubes will be made from each piece; a considerable number should be made and kept in a closed box for use. A tube must be clean inside and out, and should not be used twice. 7. Fluxes and other Chemical Reagents. The chemical reagents needed are influxes * borax (sodium tetraborate), soda (sodium carbonate), and salt of phosphorus, or micro- cosmic salt (phosphate of soda and ammonia). Each of * So called because they help in the melting or fusion of the sub- stance under examination. 126 MINERALS, AND HOW TO STUDY THEM. these may be kept in a round wooden pill-box, or in a small bottle with a glass stopper. A little potassium bisulphate, to be kept in a glass bottle, is occasionally needed. Small bottles of hydrochloric, nitric, and sulphuric acids are also useful, and one of a solution of cobalt nitrate; these bottles may conveniently have a glass dropping-tube with a bulb in the place of the ordinary glass stopper. Test-paper is also required, cut up into small strips, both turmeric-paper and blue litmus-paper. The yellow tur- meric-paper is turned brown by an alkali, such as soda, while the blue litmus-paper is turned red by an acid or acid fumes, as of sulphur dioxide in the open tube. Eed litmus-paper turns blue with an alkali, but the turmeric- paper is better. In addition to the above, the following articles will be found very convenient, though not all of them quite so es- sential : A small hammer having a square face with sharp edges; also a steel anvil an inch or two long* 152. A horseshoe magnet (Fig. 152), the place of which may be taken by a magnetized knife- blade. A small agate mortar and pestle; also a steel diamond mortar (one in which the pestle fits tightly) in which a hard mineral can be pulverized without loss of the fragments. A pair of cutting pliers. A three-cornered file. A few small watch-glasses are convenient; also several small dishes of glass or porcelain (smooth butter-plates are THE USE OF THE BLOWPIPE. 127 very good) to hold the fragments of the mineral under ex- amination; several test-tubes; a porcelain dish, or casser- ole, in which a substance can be heated with acid. Also, if chemical tests proper are to be tried, a wash-bottle (for distilled water), a bottle of ammonia, and some filter-paper. Before beginning to experiment it is best to put a thick sheet of cardboard, covered each time with a fresh piece of white paper, upon the table and place the lamp upon this. A slate or a sheet of plate glass is even better than the cardboard. The student must remember also that the acids men- tioned are powerfully corrosive in their action, staining and finally destroying any fabric, as clothes or the carpet, which they are allowed to touch.* Moreover, the fumes from the acids when hot are injurious; for any extended series of strictly chemical trials it is almost essential, there- fore, to have some of the conveniences of a laboratory. Still another caution is needed : do not put away a piece of charcoal after use until it is quite certain that no fire lingers in it. 2. How TO USE THE BLOWPIPE. The first thing in the use of the blowpipe is to learn to blow a hot, steady flame. Place the tip of the blowpipe close to or just within the flame as shown in Fig. 153, directing it slightly downward, and blow through the tube. The blast of air will direct the flame into a thin cone, and with * In case of accident the effect of the acid can often be neutralized by the prompt application of ammonia or carbonate of soda, which may afterward be washed out with a little water. 128 MINERALS, AND HOW TO STUDY THEM. a little practice a clear blue flame quite free from yellow will be the result. This flame is much hotter than the or- dinary gas-flame, and when the blowpipe is in skillful hands it is hot enough to melt a fine platinum wire. The hottest part is just at the extremity of the blue flame (shaded in Fig. 153). It seems difficult at first to blow a continuous steady flame, but it is really very easy. It is only necessary to continue slowly to breathe through the nose while the pressure of the cheeks upon the reservoir of air kept all the time in the mouth prolongs the blast. This pressure need not be great not enough to tire the cheek-muscles sensibly except after a long time; if fatigue soon comes, it is because the student is unskillful or has a bad blowpipe. It is not wise, however, to give too much thought to the learning of the art of steady blowing; this will come quickly with practice. At the same time it will not do to be careless about the character of the flame; the stu- dent is ready to go on when he can take a thin sliver of orthoclase and without great difficulty melt the edges. An important distinction must be made between the reducing flame and the oxidizing flame. The flame in general consists of two parts : the inner blue cone, and the outer almost invisible envelope extending far beyond. In the former the gas is only partly burned; there is a de- ficiency of oxygen, and a substance which at that tem- perature can part with its oxygen is reduced. Here the reducing effect is to rob of oxygen, as when oxide of nickel, NiO, is changed to metallic nickel (Ni); or iron sesqui- oxide (Fe,0 3 ) is changed to iron protoxide (FeO). THE USE OF THE BLOWPIPE. 129 In the outer part of the flame, on the other hand, there is an excess of oxygen from the surrounding air, and the tendency is to give oxygen, or to oxidize. Here the lower oxide of manganese, MnO, is changed to the higher oxide, Mn,0 3 . This distinction between the action of the two parts of the flame is very important in a certain class of experi- ments. The student must notice further that to blow a good strong oxidizing flame the tip of the blowpipe should be placed just inside the gas-flame, as indicated in Fig. 153; the flame is then free from any yellow, and the sub- 153. 154. stance under experiment is to be held well beyond the end of the blue cone, at d. For a good reducing flame, on the other hand, the tip should be a little outside of the gas-flame (Fig. 154), so that a little yellow follows the flarne down, above the blue cone; the substance is held at d, ivithin the blue cone, and best more or less surrounded by the yellow flame. The experiment described on p. 138 with manganese will show the learner with what success he is following the direc- tions here given. In the following pages the different methods of exami- nation with the aid of the blowpipe are described fully. The student should take them up in order, going through 130 MINERALS, AND HOW TO STUDY THEM. as many as possible of the trials with the minerals sug- gested and endeavoring to obtain the results described as closely as he can. It is essential that the material used for the experiments should be pure. 3. EXAMINATION IN THE EOKCEPS. A small fragment of a mineral, held in the platinum points of the forceps, may be tested to see whether it can be melted, and, if so, whether easily or with difficulty. At the same time it may be observed that the mineral imparts a color to the flame which will give information as to its composition, while other phenomena, as detailed below, may also be noted. And here a few important suggestions must be made. It is very necessary to remember that while platinum can- not be injured by the heat of the blowpipe flame, nor attacked by the ordinary acids used by the chemist, it may yet be easily injured. A mineral containing antimony or arsenic, if fused in the forceps, may destroy the platinum points, for these metals form a very fusible alloy with platinum. Hence it is desirable to try minerals about which there is question especially a mineral with metallic luster in the closed tube or on charcoal first, and if there are fumes given off, caution is needed. In any case it is a good rule never to let the fused part of the mineral fragment come in contact with the plati- num; for it may adhere to the points in an inconvenient way, even if not capable of doing any permanent harm, and thus much time be wasted in cleaning them. THE USE OF THE BLOWPIPE. 131 Take now a little sliver, if possible with a thin edge, of a piece of barite or heavy spar; place it between the platinum points, letting the edge project well beyond them; blow a clean blue flame with the blowpipe, and just in front of this (in the oxidizing flame, see Fig. 153) insert the min- eral. It will be seen to melt rather easily to a white opaque glass; at the same time the flame beyond will be streaked with a pale yellowish green, which is character- istic of the element barium. Further, if the fused end, after it has cooled, be placed upon a piece of moistened turmeric-paper, it will be seen to turn it brown, showing the presence of an alkaline earth. If a piece of a barite crystal is taken, it is very likely to break violently into fragments when the flame is thrown upon it. This is called decrepitation itnd is not uncom- mon, especially with crystallized minerals. It can often be prevented by heating the fragment quite slowly at first, but in some cases it is necessary to begin by reducing the mineral to a fine powder, then mix it with a drop of water in the agate mortar, and finally support the thick paste so formed on a loop at the end of the platinum wire. Scale of Fusibility. The method of experiment de- scribed gives in the first place an approximate determina- tion of the melting-point or degree of fusibility. The following scale is used to define the fusibility of the differ- ent minerals : 1. Stibnite (must be heated on charcoal) : fusible in the ordinary gas-flame even in large fragments. 2. Natrolite : fusible in fine needles in the ordinary gas- flame, or in larger fragments in the blowpipe-flame. 132 MINERALS, AND HOW TO STUDY THEM. 3. Almandite, or iron-alumina garnet: fusible to a glob- ule without difficulty with the blowpipe, if in quite thin splinters. 4. Actinolite : fusible to a globule in thin splinters. 5. Orthoclase : thin edges can be rounded without great difficulty. 6. Bronzite : fusible with difficulty on the finest edges. The following list gives the names of some minerals, most of them common, with the degree of fusibility of each according to this scale. It is repeated here that for miner- als with metallic luster the trial should be in charcoal. Stibnite, galena 1. Cryolite, apophyllite, pyromorphite 1.5 Amblygonite, witherite, prehnite, arsenopyrite 2. Rhodonite, analcite 2.5 Gypsum, barite, celestite, fluorite, epidote 3. Oligoclase , 3.5 Albite , . . . . 4. Apatite, hematite, magnetite 5. Bronzite 6. Infusible : quartz, calcite, topaz, sphalerite, graphite. It may be interesting here to add the temperatures (in degrees Centigrade) at which the prominent metals fuse, that is, pass from the solid to the liquid state : Mercury 39 Antimony . . . 450 Silver 1020 Tin 230 Copper 1090 Bismuth 320 Gold 1100 Lead 330 Iron 1500 Zinc.. . 420 Platinum,. . 2000 THE USE OF THE BLOWPIPE. 133 The student must be warned that the method of express- ing the fusibility of a mineral, by referring it to the scale given, is not exact. The results obtained in different cases will depend upon the size and shape of the fragment taken, the conductivity for heat, also obviously upon the skill of the experimenter. Flame -coloration. Besides the fusibility, this experi- ment with a fragment of barite in the forceps serves to prove the presence of barium by the color given to the flame. It is found that a considerable number of sub- stances are characterized in the same way, hence the flame coloration becomes a simple and important means of quali- tative blowpipe chemical analysis. Color of the Flame. The following is a list of the colors likely to be observed and the substances to which they are due: f Carmine-red.. . . Lithium. RED . . . < Purple-red Strontium. I Yelloivish red, .Calcium. YELLOW Sodium. r Yellowish green . Barium. I Sislcine-green . . . Boron. GREEK. HOW TO StUDY THEM. chalcopyrite), also a few other compounds, as cuprite, are treated with nitric acid, Separation of Sulphur. A number of sulphides, as for example pyrite, dissolve in nitric acid with the separation of particles of sulphur which usually cling together and float on the liquid. It may be added that this is also true of chalcopyrite, or copper pyrites, but this, like other cop- per sulphides, gives a green solution which turns a deep fine prussiari blue when ammonia is added in sufficient quantity to dissolve the precipitate that forms at first. Separation of Tin Dioxide. When metallic tin is treated with nitric acid, tin dioxide (SnO,) is formed, which sepa- rates as an insoluble white powder, Separation of Silica. A number of silicates dissolve in hydrochloric acid with the separation of the silica, some- times as a powder, sometimes as ft slimy mass. Other sili- cates dissolve entirely ; but if the solution is gently heated until part of the liquid has been evaporated off, a thick jelly is finally formed, so that the test-tube can be partially in- verted without its flowing out. Such silicates are said to gelatinize with acid. This is true of calamine and a num- ber of the zeolites; chabazite, on the other hand, is de- composed with the separation of slimy silica, Difficultly -ftolulle or Insoluble Minerals. A large num- ber of minerals, even when pulverized, dissolve very little or not at all in strong hot acid. Quartz and corundum, for example; also the silicates, orthoclase, topaz, and many others, even when finely pulverized and long heated in strong acid, are not at all or only very slightly attacked. 'I'll': question whether there has been partial solution is not THE ISK OF THE BLOWPIPE. 15? always easy to answer, but can be decided if the liquid takes a distinct color, or more fully by filtering off the liquid from the undecomposed mineral, and then adding to it a few drops of ammonia, which, in general, will cause the bases which have gone into solution to separate as precipi- tates. To explain the various ways simple, too, many of them in which the bases present in the solution can be identified would take us too far into the subject of Chemis- try. Do not forget, however, the test foi copper just men- tioned (p. 156), or that for silver given on an earlier page (p. 145). Further, attention may be called to the fact that, as a test for sulphuric acid or a sulphate, the addition, to a solution containing them, of a little barium chloride will cause a heavy white precipitate of barium sulphate to form. 158 MINERALS, AND HOW TO STUDY THEM. CHAPTER VII. DESCRIPTION OF MINERAL SPECIES. THE following chapter gives descriptions of all the com- mon species of minerals, with remarks, more or less brief, about many of those which are rarer. The system of classification is that spoken of on p. 120, in which the different compounds of the same metallic element are grouped together. The Silicates, however, many of which are complex in composition, containing more than one metal, are, with the exception of a few valuable ores, most conveniently included in a common section at the close of the chapter. The several characters for each mineral species are enumerated in the following list : Crystalline system ; the characteristic angles and the common form, or habit, of the crystals; also the structure of the crystalline aggregates and massive varieties. Cleavage; also fracture and tenacity. Hardness (H.). Specific gravity (G-.). Luster, color, streak, degree of transparency. Other physical characters, as magnetism, etc. Chemical composition and blowpipe characters.* * These last are also called pyrognostic characters because depend- ing upon the application of heat (nvp, fire); this word is often coo tracted to Pyr, DESCRIPTION OF MINERAL SPECIES. 159 The order in the above list is that which is at once the most convenient and scientific. In the account given of each species in the following pages, however, it is not at- tempted to adhere to this order strictly, as would be done in an advanced scientific work. On the contrary, so far as is possible in the brief space available, the aim is to make this account readable and to call attention especially to the characters most easy or most important to remember. Further, in the description of many species no men- tion is made in regard to certain characters, which are relatively unimportant in these particular cases. Thus if the cleavage is not mentioned, it is because it is either not observed or too imperfect to be an important character. So, too, nearly all minerals are brittle, hence it is unneces- sary to repeat this word in each case; but if the mineral is not brittle but malleable or sectile, this is stated and to be carefully noted. Again, if the streak is not given, it is to be understood to be white or nearly white, like that of most non-metallic minerals, even when the mineral itself in the mass has a deep color. Also all minerals if having a metallic luster are opaque. The localities of the species are men- tioned, if at all, very briefly. The student will find it easier to remember the charac- ters of the different minerals, and a help in other ways, if, after studying the descriptions in the book and comparing them with such specimens as he has access to, he will make a brief tabular list of the characters for each species, some- thing like that on the following page. It is very easy to arrange a note-book (conveniently of the square letter size) for this purpose by ruling a series of 160 MINERALS, AND HOW TO STUDY THEM. Diamond. Graphite. Galena. Sphalerite. Cryst.sy stem & common form Cryst. & mass . Isometric octahedron Hexagonal tabular Foliated Isometric cube Granular- cleavable Isometric tetrahedral Granular- cleavable Cleavage Octahedral Basal Cubic Dodecahedral Hardness, etc. . Gravity 10! 3.5 1-2! flexible 2.2 2.5-3 75! 3,5-4 4 Luster Adamantine Metallic Metallic Color Colorless, yel'w Black Lead-gray Yellow brown black, etc. Streak Comp White Carbon Black Carbon Dark gray PbS White to brown ZnS Pyr, etc Infusible Infusible Easily fusible Infusible parallel vertical columns, and the trouble of writing the list of characters over each time may be avoided if they are written on the edge of the first left page and the corresponding strip from a sufficient number of the sheets following neatly cut off. A little contraction of some common words will save space : hardness is often indicated by the letter H.; specific gravity by G.; yellow may be written yiv, and so on. When a character is particularly important it may be underscored or followed by an exclama- tion point. It is not worth while to repeat in tabular form the entire description in the text; a little experience will soon show how much may be advantageously written down. It will also be a useful exercise to fill out a similar column, so far as the individual case allows, for any species from the specimen itself, and then it may be compared with the description in the book, or the list in the stu- dent's note-book made out from the book. If the species was not known at first, this list of characters will often suffice to enable the student to determine it. DESCRIPTION OF MINERAL SPECIES. 161 It is not necessary to learn by sheer effort of memory all the characters from the book at once; this would be diffi- cult and tiresome ; the most important can be learned (and first the chemical composition), while the knowledge of most of the physical characters is rather to be acquired gradually by the repeated handling of the specimens themselves. The following is a summary of the species included in the pages which follow,* arranged, except for the silicates, under the prominent element of which they are com- pounds. Many other species are mentioned briefly in the text, though not included here. The student should read again the brief statements in regard to the classification of the chemical elements and the prominent groups of chemical compounds given on pp. 109 to 116. It may be interesting here to recall the old alchemistic method of designating the chief metals by referring them to one of the members of the Solar System, as follows : the Sun, gold; the Moon, silver; the planet Mercury, mercury; Venus, copper ; Mars, iron ; Jupiter, tin ; Saturn, lead. Other prominent metals are platinum, zinc, and aluminium. CARBON : Diamond. Graphite. SULPHUR: Native Sulphur. HYDROGEN: Ice (and water). * A list is given in the Appendix of those of the species here enumerated which it is most important for the student to have in his collection. 162 MINERALS, AND HOW TO STUDY THEM. ARSENIC : Native Arsenic. Kealgar and Orpiment, Arsenic sulphides. ANTIMONY: Native Antimony. Stibnite, Antimony sulphide. BISMUTH: Native Bismuth. MOLYBDENUM: Molybdenite, Molybdenum sulphide. GOLD: PLATINUM; SILVER: MERCURY: COPPER: LEAD: Native Gold. Sylvanite, Gold telluride. Native Platinum. Native Silver. Argentite, Silver sulphide. Pyrargyrite, Sulphide of silver and anti- mony. Proustite, Sulphide of silver and arsenic. Cerargyrite, Silver chloride. Native Mercury. Cinnabar, Mercury sulphide. Native Copper. Chalcocite, Copper sulphide. Bornite and Chalcopyrite, Sulphides of copper and iron. Tetrahedrite, Sulphide of antimony and copper. Cuprite, cuprous oxide. Malachite and Azurite, Carbonates of cop- per. Dioptase and Chrysocolla, Silicates of cop- per. Native Lead. Galena, Lead sulphide. DESCRIPTION OF MINERAL SPECIES. 163 Jamesonite and Bournonite, Sulphides of antimony and lead. Pyromorphite, Lead phosphate. Mimetite, Lead arsenate. Vanadinite, Lead vanadate. Cerussite, Lead carbonate. Anglesite, Lead sulphate. Also Crocoite, Lead chromate; Wulfenite, Lead molybdate, etc. TIN: Cassiterite, Tin dioxide. TITANIUM: Kutile; also Octahedrite and Brookite, all alike Titanium dioxide, TiO a . URANIUM : Uraninite. Torbernite, Autunite, Uranium phosphates. IRON: Native Iron. Pyrrhotite, Iron sulphide. Pyrite and Marcasite, Iron disulphide. Arsenopyrite, Iron sulph -arsenide. Hematite, Iron sesquioxide. Magnetite, Magnetic iron oxide. Franklinite, Iron-zinc-manganese oxide. Chromite, Iron-chromium oxide. Limonite, Hydrated iron oxide. Siderite, Iron carbonate. Also Columbite, Iron niobate (columbate) and Wolframite, Iron tungstate ; Triphy- lite, Phosphate of iron and lithium. NICKEL: Millerite, Nickel sulphide. Niccolite, Nickel arsenide. 0nthite and (jarnierite. Nickel silicates. 164 MINERALS, AND HOW TO STUDY THEM. COBALT: Linnaeite, Cobalt sulphide. Smaltite and Cobaltite, Arsenides of co- balt. Erythrite, Cobalt arsenate. MANGANESE: Pyrolusite and Manganite, Oxides of man- ganese. Rhodonite, Manganese silicate. Rhodochrosite, Manganese carbonate. ZINC : Sphalerite, Zinc sulphide. Zincite, Zinc oxide. Willemite and Calamine, Zinc silicates. Smithsonite, Zinc carbonate. ALUMINIUM: Corundum, Aluminium oxide. Spinel, Oxide of magnesium and alumin- ium. Cryolite, Fluoride of aluminium and so- dium. Turquois and Wavellite, Aluminium phos- phates; Amblygonite, Phosphate of aluminium and lithium. CALCIUM : Fluorite, Calcium fluoride. Calcite and Aragonite, Calcium carbonates. Apatite, Calcium phosphate. Anhydrite, Calcium sulphate. Gypsum, Hydrated calcium sulphate. Scheelite, Calcium tungstate. MAGNESIUM: Brucite, Hydrated magnesium oxide. Magnesite and Dolomite, Magnesium car- bonates. Boracite, Magnesium borate, DESCRIPTION OF MINERAL SPECIES. 165 BARIUM: Barite, Barium sulphate. Witherite, Barium carbonate. STRONTIUM: Celestite, Strontium sulphate. Strontianite, Strontium carbonate. SODIUM and POTASSIUM: Halite or Rock Salt, Sodium chloride. Borax, Sodium borate. Sylvite, Potassium chloride. SILICON: Quartz, Silicon dioxide. Opal, Hydrated silicon dioxide. SILICATES : * Feldspars : Orthoclase (and Microcline), Albite, Anorthite ; also Oligoclase, Labradorite. Pyroxene (Diopside, Salite, Augite, etc.). Amphibole or Hornblende (Tremolite, Actinolite, Asbestus, etc.). Beryl. Garnet (Grossularite, Almandite, etc.) Micas: Muscovite, Biotite, Phlogopite, Lepidolite. Chlorites : Clinochlore, etc. Chrysolite. Zircon. Scapolite. Vesuvianite. Epidote. Tourmaline. Topaz. * The composition of the following minerals is in many cases too complex to be given briefly here. 166 MINERALS, AND HOW TO STUDY THEM. Titanite or Sphene. Andalusite, Sillimanite and Cyanite. Staurolite. Talc. Serpentine. Datolite. Prehnite. Apophyllite. Pectolite. Zeolites : Thomsonite, Natrolite, Analcite, Chabazite, Stilbite, Heulandite. CARBON. Diamond. Carbon, C. The DIAMOND is usually found in distinct isolated crys- tals, most of them very small, but sometimes as large as a robin's egg or even larger. The crystals are commonly octahedrons, though less often some of the other forms of the isometric system (p. 22) are observed. The natural crystals before cutting " rough diamonds " they are called frequently have rounded edges and curved faces, or the faces show little pits like the etchings spoken of on p. 64. This is illustrated in Fig. 157, while Fig. 158 shows a hex- octahedron with convex faces. There are also forms with irregular structure, occasionally as round as peas, and one peculiar kind is massive and dull black in color. The crystals have perfect cleavage parallel to the octa- hedral faces, which the lapidary makes use of to bring a stone into the form best suited for cutting. The hardness i)ESCRIPTiOK 01? MINERAL SPECIES. 167 is 10, or higher than that of any other species, and the specific gravity is also high, 3.5 (see p. 84). The luster is very brilliant and of the peculiar character named (from this species) adamantine; the brilliancy of the diamond, however, is much greater when cut with many facets than in the natural crystals. The most highly prized stones are colorless and clear as water (then said to be " of the first water") ; a pale yellow color is very common, and some- times other colors, in pale shades, as green, pink, and blue, are observed ; rarely it is black and dull. ^ . The diamond consists of pure carbon, and has thus the 157. 158, same composition as a piece of charcoal. It is infusible as is charcoal, and is not acted upon by acids; but it is unlike charcoal in that it does not burn. When heated very hot, however, as in the electric arc, it is slowly consumed, form- ing, like burning charcoal, carbon dioxide or carbonic-acid gas (C0 2 ). Heated out of contact with the oxygen of the air it is converted into a mass resembling coke. The diamond has been found mostly in gravel deposits, or the rocks formed by their consolidation, and but little is known about its real home or the way it was made. For- merly it was obtained in great quantities in India; later Brazil afforded many of the gems, but both these countries 168 MINERALS, AND HOW TO STUDY THEM. now yield comparatively few. The great region which pro- duces the diamond at the present time is in South Africa, some eight hundred miles from Cape Town, where it occurs along the Vaal River, and in larger quantities especially in the neighborhood of Kimberly, in peculiar oval regions called " pans." Here the diamond mines have been worked for about twenty-five years and a vast number of stones have been found and brought to market. Think of more than eight tons of diamonds obtained during this time! Everybody knows of the use of the diamond for jewelry, for which its brilliancy, hardness, and comparative rarity peculiarly fit it. Many of the great diamonds of the world have a long and fascinating history of their own, which would fill a large volume if all were told of the way in which they have repeatedly changed hands until they have come into the possession of royalty, as the famous " Kohinoor " among the crown jewels of England, or the " Florentine " of Austria and the great "Orlov" diamond of Russia. Diamonds are also used for cutting glass and, in the form of powder, in grinding diamonds and other hard gems. The black coal -like diamonds, set in a collar and rotated rapidly by machinery, as a diamond drill, cut quickly through the hardest rocks, leaving a core behind, which is raised at in- tervals ; a well-boring is thus easily made. Graphite or Plumbago. Carbon, C. GRAPHITE, or Plumbago as it is often called, is usually found in massive forms which may be separated easily into thin leaves or plates and hence are said to be foliated; sometimes also it is finely granular and compact. Rarely DESCRIPTION OF MINERAL SPECIES. 169 the plates are distinct and separate and show a regular six- sided outline, whence it is referred to the hexagonal system ; it is then seen to have perfect basal cleavage. It is sectile and so soft as to make a mark on paper and to feel greasy to the hand ( H. = 1 to 2), and its specific gravity is only 2.2. It has a metallic luster and an iron- black or steel-gray color and streak; it is perfectly opaque. Graphite has the same composition as the diamond, con- sisting also of nearly pure carbon ; it is, however, a differ- ent substance in its physical characters and is hence a dis- tinct mineral. Note that they differ in crystalline form; also the diamond is hard and heavy, while graphite is soft and light. It is also infusible like the diamond, and is not at- tacked by acids, but may be converted into carbon dioxide (C0 2 ) by heating to a very high temperature in the air. Graphite is commonly found in the crystalline rocks called gneiss, sometimes scattered in scales, but occasionally in large beds that can be mined ; it is also found in scales in crystalline limestone, and is often formed in an iron fur- nace. It is largely mined at Ticonderoga, N. Y. ; also in Eastern Siberia, and in Ceylon. Graphite is the so-called black lead of our " lead-pencils " (but it is only like lead in its color), and would be mined for this purpose if for no other. It is used as an excellent lubricator because of its smooth soapy character when pul- verized ; also, mixed with clay, for making crucibles because it is infusible and not affected by the heat of an ordinary furnace ; in electroplating because it is a conductor of elec- tricity. 170 MINERALS, AND HOW TO STUDY THEM, CARBON is also the element which forms the essential part of the different kinds of coal and of mineral oil or petro- leum. Anthracite, the coal of eastern Pennsylvania, contains 85 to 95 per cent of carbon and has a bright shiny surface and conchoidal fracture; it burns with a pale feeble flame without smoke. Bituminous coal is black to dark brown in color, often dull and with a pitchy luster; it contains less carbon than anthracite (usually 75 per cent) and more hydrogen and oxygen; it burns with a yellow smoky flame. Brown coal, or lignite, has a brown color, dull luster, often retains the structure of the original wood and contains still less carbon, sometimes only 50 per cent. These different kinds of coal and others related to them, though of great economic value, are not properly mineral species, since they have no definite chemical composition. The same remark applies to asphaltum, bitumen, mineral wax or ozocerite, the many kinds of mineral resins including amber, and finally min- eral oil or petroleum, all of which consist chiefly of carbon. The element carbon is also present in the large group of minerals called carbonates, of which calcite, including com- mon limestone, is much the most important. SULPHUR. Native Sulphur. S. SULPHUR is another of the chemical elements occurring in nature. It is found in crystals of the orthorhombic system ; a common form is an acute rhombic pyramid (Fig. 159) with terminal angles of 106| and 85. Figs. 160, 161 DESCRIPTION OF MINERAL SPECIES. 171 are also common forms. It also occurs in masses and in powder. It is soft (H. = 1.5 to 2.5) and, though brittle un- der the blow of a hammer, is easily cut by the knife; the specific gravity is about 2. It has a resinous luster and a bright sulphur-yellow color and streak. The crystals are often clear and transparent. It consists of pure sulphur, and is remarkable among minerals because when heated it takes fire and burns with a pale blue flame, giving a gas (sulphur dioxide, S0 2 ) which has a very suffocating odor familiar to all who use sulphur matches. Sulphur is for the most part found in volcanic regions, as in Sicily and the Sandwich Islands; also in beds associated 159. 160. 161. with gypsum. It is used for making sulphur matches; it is one of the three substances of which gunpowder is made (with charcoal and niter) ; it is used in preparing the rubber gum for overshoes and other purposes; also in making sulphuric acid and in other ways. It is in fact a most important mineral. Sulphur also occurs abundantly in nature, not as an ele- ment, but in combination with the metals forming the very large and important class of sulphides, as lead sulphide, PbS, the mineral galena. It also forms the acid, sulphuric 172 MINERALS, AND HOW TO STUDY THEM. acid, H 2 S0 4 , the salts of which are the important class of sulphates, as barium sulphate, BaS0 4 , the mineral barite. Ice. Hydrogen oxide, H 2 0. Although it cannot be preserved in a mineral cabinet, ICE, the solid form of water, is as truly a mineral as diamond or quartz. It occurs in crystalline forms of the hexagonal type, often of great complexity and beauty, as seen in snow-crystals. These, as stated on p. 17, are formed in the atmosphere direct from the water vapor. Some of the forms are shown in figure on p. 17. The ice-grains that make the pellets of hail, not infrequently occurring with summer thunder-storms, are also occa- sionally in clusters of crystals, somewhat resembling the hexagonal pyramids of quartz, though this is the exception; generally there is simply a concentric con- cretionary structure. The ice of the pools and ponds is always crystalline, though it is usually only in the first stages of the process of freezing that the crystals are separately visible. This process of solidification goes on, as every one knows, at a temperature of 32 Fahrenheit (0 Centigrade). The hardness of ice near the freezing- point is 1.5, but this increases at lower temperatures. The specific gravity is about 0.92, so that it floats in the water with a little more than nine tenths of its bulk submerged. Water expands, therefore, largely on freezing and exerts a great force on confining surfaces. One consequence ol this is the breaking of vessels, water-pipes, etc., when the water they contain is frozen. In nature ice is on account of this DESCRIPTION OF MINERAL SPECIES. 173 property a powerful agent in pulling rocks to pieces, the water creeping into the cracks, especially into the narrow ones, by capillarity, and when it solidifies the rock masses are slowly but surely wedged apart. Water consists chemically of hydrogen and oxygen, combined in the ratio of 2 : 1 by volume, or 11.1 : 88.9 by weight. TELLURIUM. Carbon in its two forms, the diamond and graphite, and sulphur belong, as was stated on p. 102, to the non- metals among the chemical elements. Intermediate be- tween them and the true metals, like gold and silver, come several elements which occur in nature, namely, tellurium, arsenic, antimony, and bismuth. TELLURIUM has a bright tin-white color and a metallic luster, though, unlike the true metals, it is rather brittle; it is occasionally found in Colorado. This element is of little economic importance, but is interesting because it is the only one with which gold occurs combined in nature, in some of the rare minerals called tellurides (see p. 181). ARSENIC. ARSENIC is found occasionally as a mineral and then called NATIVE ARSENIC. It has a metallic luster and tin- white color, but soon tarnishes on the surface to a dull dark gray; it is also brittle. It generally occurs showing a fine granular structure when fractured, and the masses commonly have a reniform or botryoidal surface. Arsenic is used with copper and tin to form the alloy called speculum metal, useful for metallic mirrors because 174 MINERALS, AKD HOW TO STUDY THEM. of the brilliant surface it takes when polished. The lead employed for making shot contains a small amount of arsenic. The compounds of arsenic find various uses, as pigments, (sulphide); as a preservative; a poison for in- sects (white arsenic and Paris green); also in dyeing, medicine, etc. Kealgar, Orpiment. Sulphides of Arsenic. Two important but rather rare minerals containing ar- senic are the sulphides, Realgar, AsS, and Orpiment, As 2 S 3 . REALGAR is found in transparent monoclinic crystals and massive forms of a beautiful aurora-red color. It is soft and sectile (H. = 1.5-2) and has a specific gravity of 3.5; the luster is resinous. Its composition is AsS, or arsenic monosulphide, which gives the percentage com- position : Sulphur 29.9, arsenic 70.1 = 100. OKPIMENT, named from the Latin auripigmentum, or Gold pigment (also called King's yellow), is of a beautiful golden yellow. It is generally found in masses showing a foliated structure and with one perfect cleavage so that it can be split off into thin flexible leaves. Distinct crystals of orpiment are very rare; they belong to the ortho- rhombic system. It is soft (H. = 1.5-2), sectile, and the specific gravity is about 3.5. The composition is As 2 S 3 , or arsenic trisulphide, which gives: Sulphur 39.0, arsenic 61.0 = 100. Its behavior in the closed and open tubes is mentioned on p. 150; on charcoal it -is all volatilized, giving the characteristic garlic odor of arsenic and white fumes of the oxide (As,0 3 ). Arsenic is present in a great many other minerals. It DESCRIPTION OF MINERAL SPECIES. 175 forms with the metals a series of compounds called arsen- ides, of which arsenopyrite and cobaltite are examples. It forms with sulphur a number of compounds of the metals, as proustite. There are also a series of salts called ar 'senates, one of which is the lead arsenate mimetite; an- other is the cobalt arsenate, erythrite. White arsenic, or the " arsenic of the druggist," is the oxide, As 2 3 , which occasionally occurs as a mineral (then called ARSENOLITE). It is formed whenever metallic ar- senic or an arsenide is roasted in the air. In the open tube it is often obtained in spangling octahedral crystals (see pp. 17 and 150). ANTIMONY. ANTIMONY, like bismuth, is usually included among the metals, for it has a high metallic luster, although its structure is crystalline and it is quite brittle. It is a very easily fusible metal and is useful in the arts because of the alloys which it forms with lead and tin, to which it imparts greater hardness and durability. Thus type-metal is an alloy of one part of antimony to three or four of lead. Britannia metal, often used as the base of plated silver- ware, is an alloy of antimony with brass, tin, and lead. Babbitt metal, used for bearings, is another alloy of anti- mony with tin and copper. Tartar emetic, used in medi- cine, is tartrate of antimony and potassium. Native Antimony, Sb. NATIVE ANTIMONY is a bright tin-white mineral with metallic luster, and commonly showing brilliant cleavage 176 MINEEALS, AND HOW TO STUDY THEM. surfaces; rhombohedral crystals are rare. Its hardness is 3 to 3.5, and the specific gravity 6.7. It is not a common mineral, but is found in New Brunswick in some quantity, in California and elsewhere. Heated on charcoal it fuses and goes off entirely in white fumes of the trioxide, Sb 2 3 ; in the open tube dense white fumes of this oxide are also deposited (see p. 151). Stibnite, or Antimony Glance. Antimony Sulphide, Sb 2 S 3 . STIBNITE, the sulphide of antimony, is its commonest and most important ore. It is found in prismatic crystals of the orthorhombic system, often spear-shaped at the ends (Fig. 162). These crystals are frequently acicular and arranged in radiating groups, or again they may be very large; the mines in Japan have afforded specimens magnificent in size and brilliancy of luster. The crystals have very perfect cleavage, parallel to one vertical edge, and the surfaces formed by this are smooth and highly polished. Besides the prismatic crystals, stibnite also occurs in massive forms, generally columnar in structure and then also showing the perfect cleavage ; but also sometimes compact and granular and then the cleavage is not apparent. The hardness is only 2, so that it is scratched by the nail and leaves a mark on paper; it is quite sectile. It is not, however, to be confounded with graphite, which is much more soft and greasy in feel and marks the paper without the slightest tendency to tear it. The specific DESCRIPTION OE MINERAL SPECIES. 177 gravity of stibnite is about 4.6. The luster is metallic and on a fresh surface particularly a cleavage surface it is very brilliant, as already noted. The color is a bluish gray, but less blue than galena, with which it is sometimes confounded (but note the difference in cleavage); the streak is nearly black. Stibnite is the sulphide of antimony (antimony tri- sulphide), Sb 2 S 3 ; this gives the percentage composition: Sulphur 28.6, antimony 71.4 = 100. Heated on charcoal it fuses very easily and gives off fumes of the oxide of antimony (Sb 2 3 ), which form a thick coating at a little distance; after a few moments the fragment is entirely volatilized. If the reducing flame is thrown for a moment on the coating, it is burned off with a greenish-blue flame. In the open tube, heated slowly, the same dense deposit or sublimate is formed in the cold portion ; this is powdery and not readily volatile like the somewhat similar white oxide of arsenic. In the closed tube a dark red sublimate of antimony oxysulphide is formed (cf. p. 151). Antimony also enters into a number of other minerals, as pyrargyrite, or dark red ruby-silver; also tetrahedrite, or gray copper, jamesonite, bournonite, etc. These are further mentioned under the metals of which they are compounds ; for a description of the other related minerals reference must be made to larger works on mineralogy. BISMUTH. BISMUTH is silver-white in color with a reddish tinge and has a bright metallic luster; it is rather brittle and shows a crystalline structure "with perfect cleavages; it is, 178 MINERALS, AND HOW TO STUDY THEM. however, nearer to the true metals than either arsenic or antimony. Native bismuth is a rare mineral, and its com- pounds, chiefly among the sulphides, are also too rare to be particularly mentioned here. The sulphide of bismuth, or bismuthinite, resembles stibnite rather closely in phys- ical characters. Bismuth is an even more fusible metal than antimony, and the alloys which it forms are remarkable for their low melting-points; an alloy of bismuth with lead and tin fuses at a temperature below that of boiling water; an- other alloy of the same metals in different proportions is used as a kind of solder. Some bismuth alloys have the curious property of expanding instead of contracting with heat. Bismuth is also employed in medicine in the form of the subnitrate; another compound is used as a cosmetic; other uses are in calico-printing, to give luster to porcelain, etc. MOLYBDENUM. Molybdenite. Molybdenum sulphide, MoS a . MOLYBDENITE is the sulphide of the rare element mo- lybdenum. It is not a common mineral, but is found in small quantities in a good many localities, chiefly in crys- talline rocks like gneiss. Like graphite, which it much resembles, it occurs in foliated masses or in crystalline plates having a hexagonal outline; rarely in distinct hex- agonal crystals. It is also very soft (H. = 1-1.5) with a soapy feel and leaves a trace on paper. It has a bluish- black color and metallic luster. The color, however, is distinctly bluer and the specific gravity (G. = 4.7) is higher than that of graphite. DESCRIPTION OF MINERAL SPECIES. 179 The composition of Molybdenum disulphide, MoS 3 , gives : Sulphur 40.0, molybdenum 60.0 = 100. Heated in the open tube or on charcoal it gives off strong sulphur fumes and yields a deposit, which is pale yellow or white, of molybdic oxide; this coating on charcoal, if touched with an intermittent blowpipe flame (reducing flame) be- comes a bright blue (see p. 146). Molybdenum also occurs in the salts called molybdates, of which lead molybdate, the mineral wulfenite, is the most common. V GOLD. Native Gold, Au. GOLD is the most highly prized of the metals, valued because it serves as the money of all civilized people,* and because of its use for ornaments, as watches, rings, etc. It is sometimes found in isometric crystals, as in octa- hedrons, but usually in plates or scales or wirelike forms; also in larger masses sometimes very large called nug- gets (see Fig. 163). It is soft (H. = 2.5 to 3) and can be cut by the knife. It is highly malleable and ductile and especially remarkable because it can be hammered out into very thin sheets; the skillful gold-beater can make the plates so thin as to transmit a faint greenish light. Gold is very heavy and when pure has a specific gravity a little over 19. The luster is metallic and the color the familiar gold -yellow, but varying with the other metals * The gold coin of the United States and France contains gold and copper in the ratio of 9 to 1; that of England in the ratio of 11 to I. 180 MINERALS, AND HOW TO STUDY THEM. alloyed with it. The native gold practically always con- tains some silver and often a good deal, and then it has a paler color and lower density; with sixteen per cent of silver the specific gravity is only 17. The gold used for watch-cases and for ornaments, on the other hand, is often alloyed with copper and hence has a reddish color. Gold is not attacked by the ordinary acids, but is dissolved in a mixture of nitric and hydrochloric acids (called aqua regia). Gold occurs mostly in veins in the older crystalline 163. Figure of a model of a large Australian gold nugget weighing 2166 ounces and valued at about 40,000 dollars. rocks, especially associated with quartz; gold quartz is quartz often milky which either shows little particles of gold scattered through it, or from which gold can be obtained even if not visible to the eye after the rock is crushed to powder and then washed to remove the lighter material. A large part of the gold of the world has been obtained from the sands and gravels produced by the disintegration of gold-bearing rocks. These gravels in the bed of a stream may be washed by the miner in his pan; or, on a large scale, where a powerful stream of DESCRIPTION OF MINERAL SPECIES. 181 water is thrown against the gravel bank, carrying away the lighter rock and leaving the heavy gold particles be- hind, usually in the form of little flattened scales. The finest particles preserved are called " gold-dust." The chief gold-producing countries at the present time are the United States, especially in the State of California, where gold was discovered in 1848; in Australia, Russia, South Africa, where recent discoveries have proved to be very important. Gold is also produced in South America, China, British India, Canada; to a limited extent in Ger- many and Austria-Hungary and some other countries. It is remarkable that almost all the gold of the world and an amount valued at about $180,000,000 was mined in 1894 is obtained from the native metal; for minerals containing gold are very rare. The only ones known, be- sides the auriferous pyrite, arsenopyrite, etc., are a few compounds with tellurium called tellu rides. The best known of these gold tellurides is SYLVANITE, a silver-white mineral with brilliant metallic luster, soft (H. = 1.5-2) and heavy (G. = 8.0). It was long since found in Transylvania (whence it takes its name), but also occurs in Colorado. Another name for it is Graphic Tel- lurium, because of the curious forms, resembling written characters, that the crystals sometimes take on a rock surface. PLATINUM. Native Platinum, Pt. PLATINUM is reckoned among the nobler metals with gold, and like it is not attacked by any of the single acids. 182 MIN^EALS, AND HOW TO STUDY THEM. It has a rather dull gray color, and is not a beautiful metal, although now more highly valued because of its practical uses than any of the metals except gold. It is rarely found in isometric crystals, as in cubes, more commonly in scales or in larger masses (up to twenty pounds) called nuggets, washed out of the gold sand. It has a hardness of 4 to 4.5, and a specific gravity varying from 14 to 19 according to the amount of other metals alloyed with it chemically. Pure platinum, as obtained in the laboratory, has a specific gravity of 21 to 22, for native platinum is not the pure metal, but is found by the chemist to contain iron, sometimes in large amount (nearly 20 per cent), and also a number of rare metals, as palladium, rhodium, and others. Platinum is a highly useful metal. The fact that it is fused with great difficulty and is not attacked by ordinary chemical reagents makes it very valuable both to the chemist in the laboratory and in the chemical manufac- tories, where crucibles and dishes are made of it. It is also largely used by dentists. It has come into use of recent years for the attachments to the ends of the carbon wire in the incandescent electric lamp. Only a very minute quantity is required in each case, but so many lamps are called for that the demand is very great, and as only a small amount is mined chiefly in the Ural Mountains in Russia the price has risen much higher than formerly. Platinum has been used to a small extent for coins. Between the years 1828 and 1845 in Russia a considerable amount was in circulation, but the coins were recalled and the experiment has not been repeated. DESCRIPTION OF MINERAL SPECIES. 183 Platinum, like gold, does not readily combine with other elements, and in nature the only compound known is an arsenide (PtAs 2 ), called SPERRYLITE; this is found in very small quantities in a mine near Sudbury, Ontario, Canada. It is interesting to note that the name platinum is derived from plata, the Spanish word for silver, since it was re- garded in South America at the time of its discovery (1735) as an impure ore of that metal. IRIDOSMINE is a compound of the rare metals iridium and osmium resembling platinum, but of a whiter color. It is found under similar conditions in the form of flattened scales in gold-washings. It is very hard, and on this account has been used for the points of gold pens. SILVER. SILVER is one of the precious metals, useful alike as money,* for ornaments of many kinds, and for utensils. The color is a fine silver- white when perfectly fresh, but unfortunately it is very easily tarnished, and the presence of a very little sulphur or sulphur gases in the atmosphere soon turns it black. Native Silver, Ag. NATIVE SILVER is not an uncommon mineral, although the world's supply of the metal comes chiefly from its ores. It is like gold in its occurrence, sometimes, though rarely, in distinct isometric crystals, more frequently in arbores- * The silver coin of the United States and France contains silver and copper in the ratio of 9 to 1 ; that of England in the ratio of 12 tol. 184 MINERALS, AND HOW TO STUDY THEM. cent or branching groups, in plates and scales or wirelike forms (Fig. 164); sometimes in fine threads. Its hardness is 2.5 to 3; it is highly malle- able and ductile, and is the best known con- ductor for both heat and electricity. Its specific gravity is 10.6 when pure, but higher when alloyed with gold, as often in nature. Native silver occurs rather abundantly in nature, as in Mexico, Arizona, Norway, also in South America and Australia. Silver is readily dissolved by nitric acid, forming silver nitrate, and from its solution the addition of any compound containing chlorine, as hy- drochloric acid or sodium chloride, causes part to separate as a white curdy deposit of silver chloride. This is a very delicate test for silver. Argentite, or Silver Glance. Silver sulphide, Ag 2 S. Argentite is named from the Latin word of silver, argentum. It is a very valuable though not very common ore, since when pure it contains 87 per cent of metallic silver. It is found in cubic or octahedral crystals, often growing together in branching forms; more commonly it occurs simply in masses. The hardness is about 2, and the specific gravity 7.3. It is readily cut with the knife, almost like lead, and hence is said to be eminently sectile, also flattening to some extent under the hammer, while almost all other sulphides are brittle and break at once with a blow into fragments. The luster is metallic, and the color and streak grayish black. DESCRIPTION OF MINERAL SPECIES. 185 The formula is Ag 2 S, or silver sulphide, which gives: Sulphur 12.9, silver 87.1 = 100. Heated by the blowpipe flame on charcoal, the sulphur is easily roasted off and a little silver ball left behind, which can be tested chemically by dissolving in nitric acid, and adding a drop of hydro- chloric acid, as before mentioned. There are a number of other sulphur compounds of silver, but most of them are too rare to be mentioned fully here. The most interesting of these are the two beautiful minerals called red-silver ore or ruby-silver, that is, the dark red-silver ore, PYRARGYRITE, which contains sulphur, antimony, and silver, and the light red-silver ore, PROUS- TITE, which contains sulphur, arsenic, and silver. Both these minerals crystallize in hexagonal prisms with rhombohedral or scalenohedral faces, and they resemble each other closely in their characters, as hardness 2.5, specific gravity 5.8 pyrargyrite and 5.6 proustite. The color of pyrargyrite is dark red, often black, with nearly metallic luster on the surface, while proustite is bright red. Both have a red streak. Heated on charcoal, pyrargyrite gives off dense antimony fumes (Sb 2 O 3 ), while proustite yields arsenical fumes (As 2 3 ) easily recognized by their garlic odor. Both min- erals give a globule of silver if roasted with soda on charcoal (see p. 144). Cerargyrite, or Horn-silver. Silver chloride, AgCl. The name CERARGYRITE, translated into English, means horn-silver, and it is so called because of its appearance and the ease with which it is cut by a knife. 186 MINERALS, AND HOW TO STUDY THEM. It is found in cubic crystals rarely, more commonly in scales, plates or masses. The hardness is 1 to 1.5, and the specific gravity 5.5. It is remarkable for being perfectly sectile, cutting with a knife like a piece of lead or wax. The luster is adamantine and the color white or pale gray or green; it is transparent to trans- lucent. It is a rather rare but highly valuable silver ore, the per- centage composition being: Chlorine 24.7, silver 75.3 = 100. Koasted alone on charcoal the chlorine is easily driven off and a globule of silver left behind. MERCURY. MERCURY is a remarkable metal, because it is a liquid at all ordinary temperatures, only freezing, or becoming solid, at 40. It has a silver-white color and brilliant metallic luster, and is so mobile that from early times it has been called quicksilver. Its density is high, 13.6, or higher than silver (10.6) and lead (11.4), and for this reason and because of its liquid form it is of great value for scientific purposes. It is used in most thermometers and barome- ters and is employed in many experiments in the physical and chemical laboratories. It also has the property of forming a pasty mass or amalgam with some of the other metals, as gold and silver (also copper, zinc, tin, etc., but not iron), and is hence of great value in separating them from the rock in which they occur. For this purpose the rock is ground into powder, the greater part of the loose material washed off, and then the remainder is agitated with mercury. The amalgam, which forms, is collected, DESCRIPTION OF MINERAL SPECIES. 187 and by heat the mercury is driven off to be collected again in cool chambers for further use, and the gold and silver are left behind. Ordinary mirrors are made of glass backed with an amalgam of mercury and tin. The sul- phide of mercury is the valuable pigment called vermilion. Mercury in various forms is also used in medicine, but in minute doses, for it is an active poison. Corrosive sub- limate is a chloride of mercury. NATIVE MERCURY is a rare mineral in nature, though occasionally found in minute globules scattered through the rock; the common ore is cinnabar. NATIVE AMALGAM is a rather rare mineral containing mercury and silver, but in very varying amounts. Cinnabar. Mercury sulphide, HgS. CINNABAR, the sulphide of mercury, and sometimes called natural vermilion, is found in masses of a fine red color, and sometimes also in small rhombohedral or pris- matic crystals. The hardness is 2 to 2.5, and the specific gravity is about 8, or above that of metallic iron (7.8). The great weight of a specimen cannot escape the observer and is a striking character; in some cases, however, if the cinnabar is not a pure solid mass, but only scattered through a light clayey gangue, the density of the whole may be much lower than 8. The luster is adamantine and the color bright cochineal- red, sometimes becoming dull and dark; the streak is scarlet; crystals are usually perfectly transparent. The formula for mercury sulphide, HgS, gives the per- centage composition: Sulphur 13.8, mercury 86.2 = 100. 188 MINERALS, AND HOW TO STUDY THEM. If heated on charcoal, a piece of pure cinnabar is volatilized entirely; if anything is left behind, it is only the gangue. In the closed tube it is also sublimed entire, but here it collects again in the cold part of the tube above as a black ring of sulphide of mercury, which has the same compo- sition as the original mineral, for the chemist knows both a black and a red sulphide. In the open tube, if heated very slowly, so as to avoid forming a black ring in other words, so as to give the sulphur time to oxidize (go off as S0 2 ) a ring of metallic mercury is formed in the cold part of the tube (see also p. 148). Cinnabar is mined at Almaden in Spain, Idria in Car- niola, also at New Almaden and other points in California, and less abundantly elsewhere. COPPER. COPPER is one of the most useful of the metals, having been employed for utensils and in other forms, both as a metal and in different alloys, since very early times. Of recent years its use has been increased very largely be- cause of its good conductivity for electricity. It thus forms the material of the wires of the dynamo machines, those by which the electrical current is carried for the elec- tric light, the trolley, etc. Copper is also extensively used for electroplating, as in making stereotype plates. It forms further a large number of useful alloys, of which brass an alloy of copper and zinc in the ratio of about 2 : 1 is the best known. In the various kinds of bronze (bell-metal, gun-metal, antique and medal bronze, etc.) copper is also the prominent metal, alloyed with tin; in DESCRIPTION OF MINERAL SPECIES. 189 aluminium bronze it is alloyed with aluminium; in german silver it is alloyed with zinc and nickel. Copper is obtained in nature in the native state, and also from a variety of valuable ores, which are some of the most interesting and beautiful minerals. Native Copper, Cu. NATIVE COPPER is found sometimes in isometric crys- tals, but they are not often distinct, and the common forms are strings or wires which have a crystalline form but are difficult to decipher (see Figs. 127, 128, p. 61). It is also in grains, plates, and masses, sometimes very large. The hardness is 2.5 to 3, and the specific gravity 8.8. The luster is metallic and the color that peculiar reddish hue called copper-red. It is highly malleable and ductile, so that it may both be rolled out into sheets and drawn into fine wires. It is an excellent conductor of both heat and electricity. Copper is easily dissolved by nitric acid, giving a blue solution, and ammonia in excess (enough to dissolve the precipitate first formed) turns it a deep azure- blue. The most celebrated locality for native copper is in the upper peninsula of Michigan on the shores of Lake Su- perior, where it has been mined for many years. The total production has been very large. Beautiful crys- tallized specimens have been found here where it is asso- ciated with calcite, datolite, and a number of the zeolites. Sometimes it is inclosed in the crystals, as of calcite, so that they are colored bright red from the internal reflec- 190 MINERALS, AND HOW TO STUDY THEM. tions. Great masses of native copper hare also been found; one of them weighed 420 tons. Native copper is further found in Arizona, in Siberia, South America, and Australia. Chalcocite, or Copper Glance. Cuprous sulphide, Cu 2 S. CHALCOCITE is one of the most valuable ores of copper, for when pure it contains about 80 per cent of the metal. It is found in orthorhombic prisms or pyramids, occasion- ally having a hexagonal aspect; more commonly in mas- sive forms of a nearly black or bluish-black color. When fresh it has a brilliant metallic luster, which it loses easily, becoming a little dull and tarnished on the surface. The hardness is 2.5 to 3, and the specific gravity about 5.6. It is brittle when struck with the hammer, but can be cut a little with the knife. The formula for chalcocite (cuprous sulphide), Cu 2 S, gives the composition: Sulphur 20.2, copper 79.8 = 100. On charcoal it is easily reduced by the blowpipe flame alone to metallic copper. Fine specimens come from Cornwall in England (often called redruthite); it was also formerly obtained at Bristol, Conn. Bornite, or Erubescite. Sulphide of Copper and Iron, Cu 3 FeS 3 . BORNITE was named after the Austrian mineralogist von Born, but it has a variety of other names purple cop- per ore, variegated copper ore, peacock copper, erubescite all of which suggest a character by which it is easily recognized: the bright iridescent tarnish of the surface, DESCRIPTION OF MINERAL SPECIES. 191 A fresh fracture gives a color of a peculiar reddish bronze and a bright metallic luster, which has led the Cornish miners, a little fancifully, to call it horse-flesh ore. This fresh surface soon becomes slightly colored even after a day or two, and gradually the color changes and becomes more variegated, until it is indeed a peacock-copper ore. This character, with the peculiar color of the fresh frac- ture, makes it always easy to recognize. It is sometimes found in cubic crystals, but usually it is simply massive as imbedded particles or larger pieces. The hardness is 3, and the specific gravity about 5. Bornite contains both copper and iron, but not always in the same proportions; the formula Cu 3 FeS 3 gives: Sulphur 28.1, copper 55.5, iron 16.4 s 100. When heated in the open tube it gives off fumes of sulphur dioxide, which are recognized by the odor and their effect in red- dening litmus-paper. On charcoal it fuses to a brittle magnetic globule; after roasting it reacts with borax for iron and copper. It dissolves in nitric acid with separation of sulphur, giving a blue solution. Chalcopyrite, or Copper Pyrites. Sulphide of Copper and Iron, CuFeS 2 . CHALCOPYRITE, or Copper Pyrites, is the beautiful deep brass-yellow copper mineral, often called yellow copper ore. The color is so golden that it is not infrequently mistaken for gold, especially when scattered in small par- ticles through a mass of quartz; but, as we shall see, it can be easily distinguished, though the name " fool's gold," 192 MINERALS, AND HOW TO STUDY THEM. which it shares with the less golden iron pyrites, is still not inappropriate. It is generally found massive, sometimes in large speci- mens, sometimes only in specks in the inclosing rock, but it is also found in crystals which com- monly are either like octahedrons (though belonging to the tetragonal system), or in wedge-shaped forms called sphenoids (Fig. 165). The hardness of chalcopyrite is 3.5 to 4, so that, unlike pyrite, it can be easily scratched with a knife. It is brittle, and its specific gravity is a little over 4. The luster is brilliant metallic, and the color, as we have seen, deep brass-yellow; the streak is greenish black. It is often tarnished on the surface, sometimes so as to deepen the color, sometimes variegated so as to rival bor- nite, with which it might then be confounded, only that the breaking-off of a scale so as to show the color on the fresh fracture serves to distinguish them at once. It is a sulphide of both copper and iron, and the formula CuFeS, gives: Sulphur 35.0, copper 34.5, iron 30.5 = 100. Heated on charcoal, a fragment fuses to a black ball which is strongly magnetic, and this roasted with soda gives metallic copper. A fragment in nitric acid dissolves, giv- ing a blue solution which turns azure-blue when ammonia is added in excess. Chalcopyrite can be easily distinguished from pyrite (iron pyrites) because of its inferior hardness, as noted be- fore; its color, too, is deeper. It is distinguished from gold by its being brittle, breaking into fragments under DESCRIPTION OF MINERAL SPECIES. 193 the point of the knife, while the gold is cut; a particle of gold, too, is not attacked by nitric acid, while the chalco- pyrite is easily dissolved with the separation of particles of sulphur. It is a very common mineral, often occurring in veins of quartz with galena and pyrite, though sometimes only as minute specks. When present in large masses, as in Montana, it is one of the valuable ores of copper. Tetrahedrite, or Gray Copper. Sulphide of Antimony and Copper, 4Cu 2 S.Sb 2 S 3 . TETRAHEDRITE is so named because the crystals are commonly tetrahedral in habit (Figs. 166, 167), and often highly modified. Good crystals, as with many of these 166. 167. metallic minerals, are rare and the mineralogist has often to content himself with massive pieces. These he recog- nizes by the brilliant metallic luster and dark grayish-black color and streak. The hardness is 3-4.5, so that it is easily distinguished from magnetite, which is too hard to be scratched by the knife; the specific gravity varies from 4.4 to 5.1. The ordinary tetrahedrite contains sulphur, antimony, and copper, but there are a great many varieties, some of which contain arsenic in place of part of the antimony, and others silver or mercury in place of part of the copper. 194 MINERALS, AND HOW TO STUDY THEM. The typical composition is given by the formula Cu 8 Sb 2 S 7 or 4Cu 2 S.Sb 2 S 3 ; this requires: Sulphur 23.1, antimony 24.8, copper 52.1. There is also a related mineral, containing sulphur, arsenic, and copper, which is called tennantite. In the closed tube tetrahedrite gives a dark red subli- mate of antimony oxysulphide; in the open tube sulphurous fumes and a white coating of antimony trioxide. If arsenic is present, it is detected by the odor when the mineral is heated on charcoal; with soda it yields a globule of metallic copper. Cornwall, Bohemia, Hungary, also Col- orado, afford fine specimens. Cuprite, or Red Copper Ore. Cuprous oxide, Cu 2 0. CUPRITE is called red copper because of the fine red color which the clear crystals show, and because of the red color of the streak. The crystals (Figs. 168-170) are often 168. 169. 170. cubes or octahedrons or combinations of them and other forms; sometimes they are highly modified (see Fig. 29, p. 28). In one kind the cubes are spun out into long threads, forming a matted mass of bright red hairs which look very pretty in the cavities of the rock; examined closely with a glass, it is often seen that these threads cross each other at right angles as if trying to build up skeleton cubes, the threads taking the direction of the cubic edges. Common DESCRIPTION OF MINERAL SPECIES. 195 cuprite is a massive mineral, and it is in its cavities that the crystals are usually found. The hardness of cuprite is 3.5 to 4, and the specific gravity about 6. The luster is adamantine, but on some dark surfaces may look almost metallic; again it is dull and earthy. The color, as remarked above, is bright cochi- neal-red in the clear transparent crystals, but the surface is often darkened and may appear nearly black. The streak is always brownish red. The composition of cuprous oxide, Cu 2 0, gives: Oxygen 11.2, copper 88.8 = 100. A fragment on charcoal is easily robbed of its oxygen and reduced to metallic copper. Cuprite is not only a beautiful mineral, but also a valuable ore of copper, occurring usually with malachite and other ores, as in Arizona, Cornwall in England, in Australia, etc. Malachite. Green Carbonate of Copper, CuC0 3 .Cu(OH) 2 . MALACHITE, the carbonate of copper, is a bright green mineral, often found with native copper, cuprite, and other copper ores because of the readiness with which they are converted into the carbonate by the action of the carbon dioxide present in the air or dissolved in the water. It may be found in acicular crystals (monoclinic), but only rarely, and the common forms have a rounded or mammil- lary surface and a concentric fibrous structure (see Figs. 135, 136, p. 68). When close and compact it can be cut and polished and thus form a handsome ornamental stone. The malachite of Siberia is used in this way, table-tops, vases, and columns having often been veneered with it. 196 MINERALS, AND HOW TO STUDY THEM. The hardness is 3.5 to 4, and the specific gravity about 4. The color is a bright green; the streak a little paler; it is transparent only in minute crystals. The formula of malachite is CuC0 3 .Cu(OH) 2 or 2CuO, C0 2 .H 3 0, which gives: Carbon dioxide (C0 2 ) 19.9, cupric oxide (CuO) 71.9, water (H 2 0) 8.2 = 100. A fragment heated in the forceps gives a green flame characteristic of copper, and in the borax bead the reactions described on pp. 136, 137. It yields a good deal of water in the closed tube, and in nitric acid dissolves with the effervescence of carbon dioxide. Malachite is found in fine specimens at many localities, as in the Siberian mines, in Cornwall, Australia, and Arizona. Azurite, Blue Carbonate of Copper. 2CuC0 3 .Cu(OH) a . AZUEITE, or the Blue Carbonate of Copper, is not so common as malachite, but it is also a beautiful mineral, and when in large transparent crystals of a fine deep blue it forms one of the most attractive specimens in a cabinet. The crystals are oblique rhombic prisms. The hardness is 3.5 to 4, and the specific gravity is 3.8. The luster is vitreous, the color azure-blue, and the streak somewhat lighter. The composition is expressed by the formula 2CuC0 3 . Cu(OH), or 3Cu0.2C0 2 .H 2 0; this gives: Carbon dioxide (C0 2 ) 25.6, cupric oxide (CuO) 69.2, water (H 2 0) 5.2 = 100. It hence differs from malachite in containing less water; it is not uncommon to find crystals which are blue on the outside but have changed within to a fibrous mass of green malachite. The most famous localities are those DESCRIPTION OF MINERAL SPECIES. 197 of Chessy, near Lyons in France, and the Copper Queen mines and elsewhere in Arizona. DIOPTASE, or Emerald Copper, the silicate of copper, has a beautiful emerald-green color. It is a rare mineral, only known to occur at a few localities, one of which is in Asia, another in Arizona, another in the French Congo re- gion in Africa. The crystals are commonly hexagonal prisms with rhombohedral faces on the ends. The formula is H 2 O.CuO.Si0 2 , which gives : Silica 38.2, cupric oxide 50.4, water 11.4 = 100. CHRYSOCOLLA is another silicate of copper of a bluish- green or sky-blue color. It occurs in massive forms some- times earthy, also looking a little like malachite. The hard- ness is 2 to 4; the specific gravity 2.2. It contains a good deal of water, which it gives off in the closed tube. The formula is CuSi0 3 .2H 2 0. It is a not uncommon product of the alteration of other copper minerals. There are many more copper minerals, most of them too rare to be described here. They include a number of hydrous sulphates, of which the most important is CHAL- CANTHITE or blue vitriol, a common substance at the druggist's and often to be seen in clusters of large crystals, but rare in nature. The sulphate, BROCHANTITE, may also be mentioned. There are further several arsenates of copper, including OLIVENITE ; several phosphates, includ- ing LIBETHENITE ; several chlorides, as ATACAMITE ; and so on. LEAD. LEAD is one of the most important of the metals, used for many purposes familiar to all, as for pipes, to convey water, 198 MINERALS, AND HOW TO STUDY THEM. for shot and rifle-balls, etc. It has a dull blue-gray color. It is very soft and malleable, and fuses readily at a com- paratively low temperature (see p. 132.) It is often alloyed with other metals; thus with tin in common solder and pewter; with antimony in type-metal ; with arsenic in small amount for making shot. White lead (the carbonate) is largely used in making paint, also the oxide, red lead. NATIVE LEAD is a very rare mineral, though occasion- ally found in small amount, particularly in Sweden. The supply of the metal, which is used so largely in the arts, is obtained from its ores, especially the sulphide, galena. Other important lead minerals are the phosphate, pyro- morphite; the sulphate, anglesite; the carbonate, cerussite. Galena. Lead sulphide, PbS. GALENA crystallizes in the isometric system, and occurs commonly in cubes ; it is also found in octahedrons and very frequently in combinations of these two forms as, too, of other forms of this system (Figs. 171-173, also Fig. 20, 171. 172. 173. p. 25). It has very perfect cubic cleavage, and a mass often breaks up into a multitude of little rectangular blocks (see Fig. 143, p. 71). This cubic cleavage is readily seen in the common coarse-granular kinds, and is revealed also by the spangling of the surface in those which are fine-granular. DESCRIPTION OF MINERAL SPECIES. 100 The hardness is 2.5, and the specific gravity is 7.5 or nearly as high as metallic iron, for lead being a metal of high density (G. 11.4), all its compounds have the same prop- erty. The luster is metallic and usually very brilliant; the color is a bluish lead-gray, but the exposed surface of a specimen is often somewhat dull from tarnish. Galena is lead sulphide, PbS, which gives the percentage composition: Sulphur 13.4, lead 86.6 = 100. On charcoal a fragment fuses easily, yielding finally a globule of metallic lead and a yellow coating of lead oxide near it and a white coating at a distance from it (see p. 144). With soda on charcoal metallic lead is readily obtained. Galena is the most important ore of lead and one of the commonest of minerals, occurring in large deposits in many mining regions; for example, in Missouri, Illinois, Iowa, and Wisconsin; also in Colorado and abroad in Derbyshire, England; Freiberg, the Harz, and so on. It also occurs, but less abundantly, with the ores of other metals. Spha- lerite, calamine, smithsonite, also pyrite and chalcopyrite are common accompanying metallic minerals; quartz, cal- cite, barite, also fluorite, are common non-metallic minerals associated with it and then called the gangue. As resulting from its own decomposition, lead carbonate (cerussite) and also lead sulphate (anglesite) are often found with galena; less often pyromorphite and other lead minerals. Much galena carries a small amount of silver, and when this is sufficient in quantity to justify its being worked for the precious metal, it is regarded as a silver ore and called argentiferous galena. 200 MINERALS, AHD HOW TO STUDY THEM. Galena is used not only as a source of lead or sometimes of silver, but also for glazing common stoneware; it is hence called potter's ore. JAMESONITE is a rare sulphide of lead and antimony (2PbS.Sb 2 S 3 ) occurring in acicular crystals, also in fibrous or compact masses. Hardness 2 to 3; specific gravity 5.5 to 6. The luster is metallic and the color is steel-gray to dark lead -gray ; it resembles stibnite both in form and color. BOUKNONITE is another rather rare sulphide of lead and antimony containing also copper (3(Pb,Cu)S.Sb 2 S 3 ). It occurs in short prismatic or tabular crystals, often grouped in wheel-shaped forms ; also massive and compact. Hardness 2.5 to 3 ; specific gravity 5.7 to 5.9 ; luster metallic; color dark steel-gray, inclining to iron-black. Pyromorphite. Lead phosphate, 3Pb 3 P 3 O e .PbCl a . PYROMORPHITE is found in small hexagonal prisms which are sometimes cavernous in form, also often rounded into barrel-shaped forms, or even nearly spheri- cal. The crystals are frequently clustered together in a curious way, branching out from a slender stem, as shown in Fig. 174. It also occurs as a thin crust or coating, which may be drusy on the surface, or simply globular or mammillary. Pyromorphite has a hardness of 3.5 to 4, and, like all compounds of lead, a high specific gravity, viz., 6.5 to 7. The luster is resinous and the color is commonly green, varying from grass-green to both darker and lighter shades; it is also sometimes pale brown. The DESCRIPTION OF MINERAL SPECIES. 201 streak is not far from white even in the deep green varieties. It consists essentially of phosphate of lead, Pb 3 (P0 4 ) 2 , but contains also some chlorine. Hence when heated in the tube a little lead chloride is driven off and forms a white coating above. The same white coating is also de- posited on charcoal at a distance from the fragment which is being heated ; more conspicuous than this is the yellow coating of lead oxide (PbO) which is formed just about the fused fragment. Also if the fragment, after it is com- pletely fused, is examined, it will be seen that it is nearly spherical, has a brilliant luster, and sparkles on the surface from the reflection of light from a multitude of crystalline facets; it is this that gives the name to the mineral from the Greek words meaning fire and form (nvp and /iop0?/). The fused globule, if heated further on charcoal with the addition of some sodium carbonate, yields globules of metallic lead. MIMETITE and VANADINITE are two ores of lead which are closely related in form and composition to pyromor- phite. Mimetite consists essentially of lead arsenate, and vana- dinite of lead vanadate, and each, like pyromorphite, also contains a little chlorine. Certain intermediate varieties contain both arsenic and vanadium. Mimetite is usually yellow in color and has a resinous luster; it sometimes closely resembles pyromorphite in form (hence the name from //z/^;r7/, imitator), but dis- tinct crystals are more rare and rounded indistinct forms the rule. The hardness is 3.5, and the specific gravity 7 202 MINERALS, AtfD SOW TO STUDY THEM. to 7.25. It is easily recognized by yielding arsenical fumes on charcoal with their peculiar odor, while the reactions for lead are the same as for pyromorphite. Vanadinite is often of a beautiful deep red color, and when the crystals are clear and sharp it is one of the most beautiful of minerals, especially in the forms found of late years in some of the mining regions of Arizona; less brilliant yellow and light brown varieties also occur. The crystals are hexagonal prisms, often terminated by several hexagonal pyra- mids; cavernous forms also occur as with pyromorphite (Fig. 175). The hardness is about 3, the specific gravity 6.7 to 7. The luster is resin- ous. The reactions for lead on charcoal are like those of pyro- morphite; the vanadium is recognized by the yellow and emerald-green colors which it gives to the salt of phosphorus bead, the former in the oxidizing, the latter in the reduc- ing flame (see p. 139). It may be interesting to add that the rare element vana- dium, present in vanadinite and some of the vanadates (as descloizite, a vanadate of lead, zinc, and copper), has found a use in the arts in calico-printing; it is also used for vanadium black, in making ink, and to fix the colors in the manufacture of silk. Somewhat resembling vanadinite in bright color are two rather rare lead minerals, Crocoite or lead chromate, PbO0 4 , and Wulfenite or lead molybdate, PbMo0 4 . CROCOITE occurs in oblique prismatic crystals (mono- clinic) of a fine orange-red color and giving a deep yellow DESCRIPTION OF MINERAL SPECIES. 203 streak. The hardness is 2.5 to 3, and the specific gravity about 6. The luster is adamantine to vitreous. It is recognized by the lead coating which it gives on charcoal, and by the reactions for chromium, which yields a green bead with borax in both the oxidizing and reducing flames (see p. 138). WULFENITE is not so rare as crocoite, and is one of the many fine minerals which the western mining States have afforded in great variety and beauty, 176. where it often occurs with vanadin- ite. It is found in square crystals (Fig. 176) or tables frequently as thin as a knife-edge and perfectly clear; also in square pyramids, sometimes low and less often acute. The color is a bright orange-yellow to reddish yellow, or again brown or green; the luster is resinous or adamantine. The hardness is about 3, and the specific gravity 6.7 to 7. The square form and bright color make it usually easy to recognize it. With the blowpipe on charcoal it gives a lead coating, and with soda metallic lead. With salt of phosphorus it gives the reactions for molybdenum (p. 139). Cerussite, or White-lead Ore. Lead carbonate, PbC0 3 . CERUSSITE, the carbonate of lead, is the commonest ore of lead next to galena, and with this it commonly occurs for the simple reason that in nature's laboratory, where carbon dioxide (C0 2 ) is often present, the sulphide of lead (PbS) is often changed into the carbonate (PbC0 3 ). This is a good illustration of what mineralogists call the association of minerals, often a very important guide as to 204 MINERALS, AKD HOW TO STUDY THEM. their nature. It is in the same way, as has been stated, that the carbonate of copper (malachite) often occurs with the sulphides, the oxides, etc. The clear and colorless, or perhaps white or slightly yel- low, crystals of cerussite do not perhaps at first suggest to the eye that it is a mineral containing the metal lead. A 177. 178. 179. more careful examination, however, shows the adamantine luster, and this, as has been stated (p. 89), belongs to crys- tals which refract the light largely, in general either be- cause they are very hard (as the diamond), or because they contain heavy atoms, as those of lead. It is a good thing to remember that all the compounds of lead have an adaman- tine or a resinous luster, and the same is true of the kind of glass containing much lead called paste, out of which imitation jewels are made. The crystals of cerussite are sometimes thin plates (Fig. 177), with sides covered with fine horizontal lines; but rhombic prisms and pyramids also occur; Fig. 178 resem- bles a hexagonal pyramid in form and angle. This is one of the species which is frequently in twins, and six-rayed starlike forms, in which the branches cross at angles of 60 and 120, are common (Fig. 179, also Fig. 120, p. 58). The hardness is only 3 to 3.5, and the specific gravity is DESCRIPTION OF MINERAL SPECIES. 205 high, about 6.5, as must be true of a compound of lead ; this last character is one that should be noticed first if a mass approximately compact is in the hand. Sometimes, however, the mass of crystals is so open and porous that the effect of high density is lost. The composition, lead carbonate, PbC0 3 , gives: Carbon dioxide (C0 f ) 16.5, lead oxide (PbO) 83.5 = 100. Heated on charcoal carefully (for it decrepitates at first) a fragment is easily fused and soon yields a globule of metallic lead, while about it the familiar lead coating is formed. Placed in a test-tube with a little nitric acid, the carbon dioxide (CO,) is given off with effervescence. This species is com- mon in Colorado and other mining regions; it is valuable as an ore of lead and often of silver. Tt is the same as the artificial white lead used for paint. Anglesite. Lead sulphate, PbS0 4 . ANGLESITE resembles cerussite, and like it is frequently found in cavities in the sulphide, galena (PbS), from which it is formed by oxidation. It is often in crystals, which are either rhombic prisms or pyramids, and sometimes very highly modified ; the crystals resemble barite in form and angles (see p. 262). Cleavage exists parallel to the prism and the base, but it is interrupted. It is also found in closely compact forms which are not so easy to recognize, but whose high specific gravity i& suggestive. The hardness of anglesite is 2.75 to 3, and the specific gravity 6.2 to 6.4. The luster is adamantine in most cases, especially on a fracture surface, sometimes varying 206 MINERALS, AND HOW TO STUDY THEM. to resinous. The crystals are usually clear and colorless, but the masses may be brown and nearly opaque. The formula for lead sulphate, PbS0 4 , gives: Sulphur trioxide (S0 3 ) 26.4, lead protoxide (PbO) 73.6 = 100. On charcoal before the blowpipe it decrepitates and fuses readily to a clear bead, which becomes milk-white on cool- ing; in the reducing flame it yields metallic lead. With soda on charcoal lead is easily obtained, and the soda reacts for sulphur as explained on p. 146. It dissolves with dif- ficulty in nitric acid, and does not effervesce as does cerus- site, and hence the two are easily distinguished in this way. Beautiful crystals come from Anglesea (whence the name), from Sardinia, from Phoenix ville, Penn. It occurs in large quantities in Mexico and Australia. TIN, TIN, one of the most important of metals for a great variety of technical purposes, occurs, if at all, only very rarely in nature in the native metallic state. The supply is obtained almost solely from a single ore, the mineral cassiterite, or tin-stone. There is also a rare sulphide of tin, copper, and iron, called STANNITE. The use of tin that first suggests itself is for tin plate, so largely employed for vessels, roofing, etc; this is simply sheet iron coated with metallic tin. Tin enters into many alloys, as the various forms of bronze (gun -metal and bell-metal, etc.) in which it is alloyed with copper; it also forms alloys with lead in pewter and several kinds of solder; with antimony in Britannia metal; with both lead and antimony in DESCRIPTION OF MINERAL SPECIES. 207 Queen's metal, and copper and antimony in Babbitt metal; with lead and bismuth in fusible metal (see p. 178). Cassiterite, or Tin-stone. Tin dioxide, SnO Q . CASSITERITE, or tin-stone, occurs, when crystallized, in square prisms and pyramids and other related forms; twin crystals are common (Fig. 180). The crystals have a splendent adamantine luster and a brown color, sometimes nearly black. It is also found disseminated in irregular particles in certain rocks, and there is a kind called stream-tin, which consists of rolled grains or pebbles of the mineral; the massive forms sometimes have 180. a botryoidal or reniform surface, a fibrous structure (then called loood-tin), a brownish color, and a dull luster. Cassiterite is remarkable for its hardness (6.5 to 7), and still more for its high specific gravity, about 7. The composition is tin dioxide, SnO a , which when pure contains 78. 6 per cent of metallic tin. Cassiterite occurs commonly in granite; either in veins or sprinkled through the rock, often in inconspicuous particles, which can be separated by the same process that nature has used in making stream-tin, that is, after the rock has been crushed, by washing away the lighter material by running water, the heavy tin-stone, more or less pure, being left behind. Cassiterite is rather easily recognized when in large crystals or masses by its high specific gravity, hardness, rich brown color, and brilliant luster. But confirmation is usually needed, and this can be gained by grinding some of 208 MINERALS, AND HOW TO STUDY THEM. the mineral fine in an agate mortar, mixing it with sodium carbonate, and patiently roasting it on charcoal. After a little time small globules of a white metal separate, and by the method described on pp. 145, 146 they can be shown to be malleable under the hammer, while they are harder than silver, which they resemble; in nitric acid they are oxidized to an insoluble white powder, having the same composition as the original mineral. Cassiterite is found sparingly at a number of points in the United States, but attempts to mine it, as near Harney's Peak, S. Dakota, and in San Bernardino County, Cali- fornia, have not been successful. It is obtained in Mexico rather abundantly in the state of Durango. The Cornwall mines in England have furnished it for many centuries, as also the Saxon and Bohemian mines. Borneo, Sumatra, Banca, Malacca, and other islands in the East Indies, and further, Australia, yield large quantities at the present time. TITANIUM. TITANIUM is a rare element, chemically related to tin; it is of no special economic importance at present, though it forms certain alloys which may come into use in the future. The most important minerals containing titanium are the oxides rutile, octahedrite, brookite; also the silico-titanate, called titanite or sphene; the last-men- tioned mineral is described on a later page. Rutile, octa- hedrite, and brookite have all the same composition, namely, titanium dioxide, Ti0 2 , but they differ in crystal- line form. RUTILE is tetragonal, and OCTAHEDRITE (see Figs. 43, 45, DESCRIPTION OF MINERAL SPECIES. 209 p. 32) is also, but of different form, while BROOKITE be- longs to the orthorhombic system. The last two species are so rare that they will not be particularly described, but rutile, though not common, is more important. One form of the crystals of rutile is shown in Fig. 181; others are twin crystals, sometimes quite complex, eight partial crystals occasionally going together to make one compound group. Other twins are of the knee-shaped kind, called geniculated, as shown in Fig. 182. 181. 182. The hardness is 6 to 6.5, and the specific gravity about 4.2. The color varies from reddish brown to red or yellow- ish, and also to nearly black, though even in the last variety thin splinters let through a little reddish light. The luster is usually metallic-adamantine. It is quite infusible and reacts for titanium (see p. 139), and also most varieties for iron, which is usually present (3 to 4 p. c. Fe 2 3 ). Kutile is found chiefly in gneiss or granite, also in gran- ular limestone. It is occasionally cut for mourning jewelry. When penetrating rock-crystal in very slender transparent crystals it forms specimens of great beauty, particularly when polished; these are sometimes called love's arrows or fteclies d' amour. Eutile is also used to color porcelain yellow and to give a tint to artificial teeth. 210 MINERALS, AND HOW TO STUDY THEM. URANIUM. URANIUM is another rare element, but one of some im- portance economically. Its compounds have usually a bright yellow or green color, and a little present in glass gives it a bright canary-yellow of fluorescent properties. It is employed in certain pigments; also in painting on porcelain. The most important mineral containing uranium is URANINITE, in which it is combined with oxygen ; some lead and other rare elements are also present. It occurs rarely in black octahedrons of very high specific gravity, up to 9. 7, also commonly in massive forms having a pitch- black color and luster, and hence called pitchblende. This last variety is more or less altered and yields some water in the closed tube. Two other uranium minerals are TORBERNITE, phos- phate of uranium and copper, a mineral of a bright green color, and AUTUNITE, phosphate of uranium and calcium, which is bright yellow. Both minerals occur in thin tabular crystals which have a basal cleavage, somewhat resembling mica (though the scales are brittle), and hence they are sometimes included together under the name of uranium mica. IRON. IRON may well be called the most important of all the metals. How large a place it takes in the work of the world is shown by the fact that each year some 50 million or more tons are produced from its various ores and turned DESCRIPTION" OF MINERAL SPECIES. 211 into some of the many forms needed by man in his work. Its manifold uses are too well known to need enumer- ation here. The entire supply of iron which the world uses each year is obtained from its ores, in which the iron is in combination chiefly with oxygen. These ores are the minerals hematite, magnetite, and limonite; siderite, the carbonate of iron, is also an important ore. These ores smelted, for example with limestone as a flux, in a blast-furnace with charcoal or coke, yield pig iron, an impure form containing much carbon. This is also some- times called catt iron* though now this name is chiefly given to iron, remelted in a cupola furnace and cast in any desired form, which is also rich in carbon. If purified so as to contain but little carbon, it becomes wrought iron of very different properties, while steel is in composition intermediate between the two forms mentioned. Steel is now obtained chiefly by the Bessemer process. It is remarkable because of its great strength and the varying degrees of hardness and elasticity which can at will be given to it by the process called tempering. NATIVE IRON, or iron occurring in nature in the metallic condition, is only known as a great rarity and hence is of no practical importance. The meteorites (p. 2) which occasionally fall to the earth often consist entirely of metallic iron, while others that have a stony aspect contain many particles of metallic iron distributed through the * Cast iron is hard, brittle, fusible, and not weldable; wrought iron is soft, malleable and ductile, weldable and fusible at a high temperature; steel is malleable, weldable and fusible, with a varying hardness depending upon the temper. (Gent. Did.} 212 MINERALS, AND HOW TO STUDY THEM. mass. Native iron has also been noted a few times in ter- restrial rocks, but only one occurrence is especially note- worthy that of Disko, Greenland, where it has been found in large masses imbedded in basalt. Pyrrhotite, or Magnetic Pyrites. Iron sulphide, Fe 7 S 8 . PYEEHOTITE takes its name from a Greek word meaning reddish (nvppoftis) because of its peculiar reddish bronze color; this is a very important character to remember. The common name, magnetic pyrites, refers to its still more striking character of being magnetic and hence at- tracted by a magnet. Pyrrhotite is rarely found in hexagonal crystals, but for the most part it occurs in irregular masses. The hardness is 3.5 to 4.5, and the specific gravity 4.6. The luster is metallic, and the color, as before noted, a peculiar reddish bronze quite different from the other kinds of iron pyrites; the streak is dark grayish black. It is a sulphide of iron nearly equivalent to the simple sulphide, FeS, though never having exactly this composi- tion; on the contrary the common formula is Fe 7 S g . On charcoal it fuses to a magnetic globule, and in the open tube gives sulphurous fumes. It is decomposed by hydro- chloric acid with the separation of the ill-smelling gas hydrogen sulphide. Pyrrhotite often contains nickel, and though it is not usu- ally present in large amount (rarely over 5 per cent), this species occurs so abundantly, for example at Sudbury, Out., as to constitute one of the most important ores of nickel. DESCRIPTION OF MINERAL SPECIES. 213 Pyrite, or Iron Pyrites. Iron disulphide, FeS 2 . PYRITE is one of the commonest and most striking of metallic minerals. It is often found in cubic crystals (Fig. 183), and the faces of these usually show, if carefully examined, fine lines or striations parallel in each case to one pair of edges only; further, on each face the direction is at right angles to those on the adjoining faces. These striations have been explained before (p. 52) as due to what is called oscillatory combination of the cubic faces with those of the pyrito- hedron. Octahedrons of pyrite are also common, and the twelve-sided form called from this species a pyritohedron 184. 185. 186. (Fig. 184). These last sometimes show fine striations like the cube, and often the two forms are both present and sometimes they are rounded together. Fig. 185 shows the pyritohedron and cube; Figs. 186, 187, the pyritohedron and octahedron. The angle between a and e is 153 26'; between o and e 140 46'. Pyrite is also found in massive form and sometimes in large beds which can be mined for the sake of the sulphur which the ore yields on roasting. 214 MINERALS, AKD HOW TO STUDY THEM. The hardness of pyrite is a little above 6, SQ that it scratches glass and is not scratched by an ordinary knife. It is thus unusually hard for a sulphide, and it is due to this that it strikes fire with the steel, which is the source of the name pyrites, which it shares with some other hard sulphides (see the Index). The specific gravity is 5. The luster is brilliant, metallic, and the color light brass-yellow, sometimes growing a little deeper when tarnished. The streak is dark greenish black. The composition is iron disulphide, FeS 2 , which gives: Sulphur 53.4, iron 46.6 = 100. Though consisting nearly one half of iron, it is of no value as an iron ore, but it is employed for making sulphur and sulphuric acid; some kinds (called auriferous pyrites) are mined for the small amount of gold they yield when smelted. It fuses on charcoal to a black metallic bead, giving off sulphur which burns and produces the suffocating fumes of sulphur di- oxide; in the closed tube the sulphur which is driven off collects in the cooler part of the tube in a liquid ring which is orange-red when hot and turns sulphur-yellow as it grows cool and solidifies. Pyrite is, as has been stated, a very common mineral, forming large beds, as in Spain, at Eowe, Mass., and in Virginia. In metallic veins it is almost always present, sometimes abundantly. It is also often found in crystals in slates and many kinds of rocks, and in coal, though then much diminishing its value if in large amount. Pyrite is often associated with chalcopyrite, or copper pyrites, but, as stated on p. 192, it is easy to distinguish the two minerals by the difference in hardness and color, also by blowpipe characters. DESCRIPTION OF MINERAL SPECIES. 215 Marcasite, or White Iron Pyrites. Iron disulphide, FeS 2 . MARCASITE has the same chemical composition as the more common kind of iron pyrites, called pyrite, but it differs in the form of the crystals, in specific gravity, some- what in color, and other respects. The two are conse- quently distinct minerals, and the compound FeS 2 is said to be dimorphous (p. 120). Marcasite crystallizes in orthorhombic prisms and pyra- mids, which it is generally easy to distinguish from the cubes and pyritohedrons of pyrite. The loo. crystals are often compounded together and grouped in various forms, to which some fanciful names have been given, as spear pyrites (Fig. 188), cockscomb pyrites, etc. It often forms nodules, spherical forms, or stalactites, and is also simple massive. The hardness is 6 to 6.5, or the same as pyrite, but the specific gravity is lower, only 4.8 instead of 5. The color, too, is a paler yellow when quite fresh, so that it is often called white iron pyrites. It is, however, more easily altered by the action of the weather and becomes tarnished, the metallic luster then becomes dull, and the true color is more or less obscured. Arsenopyrite, or Arsenical Pyrites. Iron sulph-arsenide, FeS 2 .FeAs 2 . ARSENOPYRITE is another member of the pyrites group, and like the others is hard enough to strike fire with a steel. It is near to pyrite and marcasite in composition, but 216 MINERALS, AKD HOW TO STUDY THEM. besides sulphur contains also arsenic, so that it is often called arsenical pyrites; mispickel is another name. It is found commonly in masses, but occurs also in orthorhom- bic crystals, which are much like those of marcasite and sometimes twinned in the same way. The angle between the front m faces is 112, and of e (over the top edge) 59|, of u 147, of q 80. The hardness is about 6, and the specific gravity also 6. The luster is metallic, and the color when fresh silver- 189. 190. 191. white, becoming a little dull and tarnished after exposure. The color is, therefore, quite different from the reddish bronze of pyrrhotite or the pale yellow of pyrite and mar- casite. The streak is grayish black. The formula is FeAsS, which, to show the relation to marcasite, may be written FeS 2 .FeAs 2 . This gives the following percentage composition: Sulphur 19.7, arsenic 46.0, iron 34. 3 = 100. Heated alone on charcoal it fuses to a black magnetic globule, giving off dense white fumes of arsenic trioxide, As 2 3 , which are so volatile that they do not condense on the coal except at a considerable distance from the flame. The touch of a flame upon this white coating drives it away. In the open tube, heated very sloivly, the sulphur is oxidized and passes out of the tube as S0 2 , and the arsenic forms As 2 3 , which condenses in the DESCRIPTION OP MINERAL SPECIES. 217 tube as brilliant spangling octahedral crystals this is the poisonous "white arsenic" (or simply "arsenic") of the druggist, and it is obtained in large quantities in the pro- cess of roasting this mineral as well as arsenical ores of iron and cobalt, as in Cornwall. Heated in the closed tube, where there is no air supplied, a dark red ring of arsenic sulphide (As 2 S 3 ) is first formed; if the heating is continued, metallic arsenic now goes off, and collects as another ring, which is in black and lustrous scales. The arsenopyrite of Deloro, Canada, is auriferous and hence mined for the gold it yields. Hematite, or Red Oxide of Iron. Iron sesquioxide, Fe 2 3 . HEMATITE is named from the Greek word for blood (afya), because many kinds show a red color and all varie- ties give a red streak. It is found in a great many differ- ent forms and is a difficult mineral, consequently, for the beginner to learn thoroughly. The kinds with a brilliant metallic luster are called specular iron; beautiful speci- mens of this come from the island of Elba, where it is found in thick rhombohedral crystals with brightly pol- ished faces, often with a beautiful iridiscent tarnish on the surface. These crystals are usually rather complex (Figs. 194, 195), but occasionally the simple form resembling a cube in angle (86) is observed (Figs. 192, 193). Other crystallized kinds, perhaps more common, are in thin plates or scales (Figs. 196, 197), sometimes so thin as to be trans- parent, and blood-red in color when looked through. The specular iron is also massive, with black color and brilliant luster, and the masses have sometimes a peculiar smooth, 218 MINERALS, AND HOW TO STUDY THEM. almost conchoidal, fracture; certain forms have areniform surface. Other kinds of hematite are in scales a little like mica, sometimes black and shining, less often soft and reddish and soapy to the feel; also in minute pealike forms, called fossil ore. An earthy kind, dull in luster, is the red ocher, used for making paint. The hardness of most kinds of hematite is about 6, so 192. 193. 194. 195. 196. 197. that it is too hard to be scratched by the knife; but some of the scaly kinds are soft and unctuous to the touch. The specific gravity of the crystals is 5.2. The luster is metallic in the specular iron variety, but dull and earthy in others. The color is usually iron-black, but also red. The streak is a dull red, a little like that of dried blood ; the black micaceous kinds have to be ground quite fine to show this. Some kinds are slightly magnetic, but probably only because of a small admixture of magnetite. Hematite is the sesquioxide of iron, Fe 2 3 , and if pure DESCRIPTION OF MINERAL SPECIES. 219 contains 70 per cent of metallic iron; it is also called ferric oxide by the chemist, in distinction from the protoxide or ferrous oxide, FeO. Heated in the reducing flame of the blowpipe, a fragment is partially converted into the mag- netic oxide so that a magnet will pick it up. Hematite is the iron ore mined in much of the Lake Superior region, at Birmingham, Alabama, and elsewhere in the Southern States, and formerly at the famous Iron Mountain of Mis- souri. The most beautiful crystallized specimens have come from the island of Elba; Switzerland and France also afford fine crystals. Magnetite or Magnetic Oxide of Iron. Fe(Fe 2 )0 4 . MAGNETITE suggests in its name its most striking char- acter, that of being magnetic. All kinds are strongly at- tracted by a magnet, and one variety, called the lodestone, 198. Lodestone. found for example at Magnet Cove, Arkansas, is a powerful magnet itself. It has a north and south pole, the power of picking up particles of iron or steel, as tacks, and also, when suspended, it sets with its poles north and south like a compass-needle. 220 MINERALS, AND HOW TO STUDY THEM. Magnetite is found in octahedral or dodecahedral crys- tals (Figs. 199-201); more commonly simply massive, and then sometimes with a peculiar fracture, suggesting cleav- age, yielding octahedral fragments. The hardness is high, about 6, like that of hematite, and the specific gravity is nearly the same, 5.18. The luster is 199. 200. 201. metallic, usually very brilliant, and the color iron-black. The streak also is black, and this is a most important char- acter, for it distinguishes it at once from hematite, which, though at times iron-black in the mass, has a red streak. The composition is expressed by the formula Fe(Fe 2 )0 4 or FeO.Fe 2 3 , which is equivalent to Fe 3 4 , yielding 72.4 per cent of metallic iron. It is fused with great difficulty, but a small fragment heated carefully in the oxidizing flame loses part if not all of its magnetic property. It is soluble in hydrochloric acid. Magnetite, like hematite, is a very important ore of iron. It has been mined in the Adirondack region, as at Port Henry, in large quantities and elsewhere; also in the West Point region; at Brewster, Putnam County; in New Jersey. It also occurs in the Lake Superior region, where, however, the commoner ore is hematite. The famous Swedish iron and steel are made from magnetite. Besides these great deposits magnetite is a common mineral in many rocks, DESCRIPTION OF MINERAL SPECIES. 221 occurring in little particles distributed through the mass. It is thus prominent in the trap rocks of Connecticut, Massachusetts, and the Palisades of the Hudson. When the rocks containing magnetite are broken up by the weather and reduced to the condition of sand and gravel, the magnetic iron, being heavier, is often sorted out by the water and accumulated by itself; a stream by the side of a country road often shows a streak of the black iron sand, and at the seashore it may be found in quite large quantities. It has been mined in this way at Block Island, being separated by a large magnet from the associated sand and gravel. FRANKLINITE, so called from its sole locality, Franklin Furnace, New Jersey, is a mineral in form, color, and gen- eral appearance much resembling magnetite, but it is only feebly magnetic if at all, and has a brown, not black, streak. It is an oxide containing besides iron also zinc and man- ganese, and is hence valuable as a zinc ore and for making spiegeleisen, an alloy of iron and manganese employed in the making of steel. CHROMITE, or Chromic Iron, is another iron ore looking much like magnetite, also crystallizing in octahedrons, though the massive form is the common one. It contains chromium besides iron, and with borax yields a chrome- green bead (p. 138). It is not a particularly interesting mineral and is of limited occurrence, but valuable as a source of the element chromium, which forms a bright- colored (usually yellow or green) class of salts called cJiro- mates; these are used for pigments and in calico-printing. Chromium is also used in chrome-steel. It is often as- 222 MINERALS, AND HOW TO STUDY THEM. sociated with serpentine, as in Pennsylvania and Mary- land ; it is also mined in California and in Turkey. ILMENITE, or titanic iron, is related to hematite and magnetite, but differs from the former in having a black streak, and from the latter in not being magnetic. It con- tains titanium besides iron and oxygen, and the formula of some kinds is FeTi0 4 . Like magnetite it occurs in minute particles in certain rocks and it also forms beds of some magnitude. Part of the magnetite contains titanium (or is titaniferous) and is then of much less value as an ore because highly refractory, or hard to reduce in a furnace. Limonite, Brown Oxide of Iron. 2Fe Q 3 .3H 2 0. LIMONITE is a hydrous oxide of iron, that is, it contains some 14 per cent of water which it gives off when heated. It is often called brown hematite, because while re- sembling some kinds of hematite it has usually a brown color and always a brown streak. It is not known in crys- tallized forms, but occurs only massive, especially in stalac- titic shapes, or forms with rounded surface (see Fig. 140, p. 68). Its structure is frequently fibrous, but earthy in the brown ocher used for paint. The hardness in the compact kinds is about 5, or less than that of hematite and magnetite, and the specific gravity is a little below 4. The luster varies from sub- metallic to earthy; it is sometimes brilliant on the glossy surfaces of stalactites, but more commonly dull. It is a hydrated oxide, 2Fe 2 3 .3H 2 0, and only contains 60 p. c. of metallic iron if perfectly pure, which is rarely the case. Heated in the closed tube considerable water is DESCRIPTION OF MINERAL SPECIES. 223 given off which condenses in the colder part of the tube; the fragment after heating turns red and has a red streak; by the loss of water it has been converted into an- hydrous iron sesquioxide, Fe 2 3 , or hematite. Limonite is named from the Greek word (XeijuGJv) meaning meadow, because often found in marshy places; in fact one kind is also called bog iron ore. It is mined in many deposits in western New England and adjacent parts of New York, also in Pennsylvania, Virginia, etc. It is a low-grade ore, that is, it yields only a relatively small amount of iron because of the clay and other im- purities present. GOETHITE, named after the poet Goethe, is another oxide of iron yielding water. It occurs in brilliant pris- matic crystals and also in massive forms, often fibrous in structure. It has a yellow-brown to deep brownish-black color and a streak like that of limonite, which it also re- sembles in some of its forms. It is of limited occurrence. TURGITE is still another iron hydrate, not common; it occurs in forms like limonite, but yields only 5 per cent of water and has a red streak; it usually decrepitates when heated before the blowpipe. Siderite, or Spathic Iron. Iron carbonate, FeC0 3 . SIDERITE, the carbonate of iron, is also an important ore, although less so than the three prominent oxides. It crystallizes in rhombohedrons, often with rounded faces (Fig. 202), and has perfect rhombohedral cleavage. This cleavage is a very prominent character in the common massive kinds; the angle between two adjacent cleavage 224 MINERALS, AND HOW TO STUDY THEM. surfaces is 107 or 73; it is the same form that we shall learn with calcite; indeed, as explained on p. 119, the two species form with several others an isomor- phous group. The hardness is 3.5 to 4, and the specific gravity is rather high, 3.8; this instantly suggests to one picking up a speci- men that a heavy metal is present. The luster is vitreous and the color light yellow to brown, be- coming dark by alteration; the streak is white or nearly so. It is a carbonate of iron, FeC0 3 , and contains 48 per cent of metallic iron if pure. In acid, if slightly warmed, it dissolves with effervescence, giving off carbon dioxide gas; before the blowpipe it turns black, fuses, but not very easily, and becomes magnetic. It is largely mined in Cornwall, and is also found in Pennsylvania, Ohio, etc. COLUMBITE is a rare iron mineral, occasionally found in jet-black crystals or masses in the granite veins of New England; it resembles tourmaline somewhat, but is much denser, having a specific gravity varying from 5.4 to 6 or over. Its luster is submetallic. A figure of a twin crys- tal is given on p. 58 (Fig. 118). It is a niobate (or columbate) of iron with also some manganese, and further with the niobium there are also present varying amounts of the related element tantalum; as this increases the specific gravity runs up to about 7. TANTALITE is nearly pure iron tantalate, with G. = 7. SAMARSKITE is a velvet-black mineral a little resembling columbite and often associated with it. Besides iron it contains tantalum, niobium, the cerium metals, yttrium, DESCRIPTION OF MINERAL SPECIES. 225 and other rare elements ; it is found in the mica mines of North Carolina. WOLFRAMITE, a tungstate of iron and manganese, is a still rarer mineral than columbiter tt is iron-black in color, with fine cleavage and submetallic luster; like columbite it is very heavy, having a specific gravity of over 7. TRIPHYLITE is a rather rare phosphate of lithium and iron chiefly (LiFePOJ, but containing also manganese and hence passing into LITHIOPHILITE (LiMnPOJ. It occurs in cleavable masses of a bluish-gray color; lithiophilite is salmon color, yellow or pale brown. Both minerals have similar physical characters : hardness 4.5-5; specific grav- ity 3.5; luster resinous. In the forceps they fuse readily, giving a red flame (lithium), with bluish green on the edge (phosphorus); they give with borax reactions for iron or manganese or both. CHILDRENITE is essentially a hydrated phosphate of iron and alumina, occurring in yellow or brown orthorhombic crystals. EOSPHORITE is the closely-related manganese compound. VIVIANITE is a hydrated phosphate of iron having a blue to green color; it occurs iu crystals, also in earthy forms, the latter called blue iron earth. PHARMACOSIDERITE is a hydrated arsenate of iron com- monly occurring in small cubic crystals of a yellow to green color. SCORODITE is another arsenate of iron which is found in olive-green to brown orthorhombic crys- tals, also in aggregations. There are numerous other arsenates and phosphates of iron, but too rare to be in- cluded here. Among the sulphates of iron may be men- 226 MINERALS, AND HOW TO STUDY THEM. tioned MELANTERITE, also called iron vitriol and copperas, a mineral which has usually been derived from the decom- position of pyrite or marcasite. Copperas is employed in making ink, also much used by dyers and tanners. NICKEL. NICKEL, though formerly a little-used metal, has become of much wider application in recent years. It is exten- sively employed now to plate many articles of steel as knives, scissors, skates, etc. because unlike the steel it does not tarnish or rust rapidly in the air. It is much used also, when alloyed with copper, for small coins, as the " nickels " or five-cent pieces of this country, and similarly in Switzerland, Germany, and Belgium. Nickel steel has been found to be remarkably strong in withstanding the blow of a cannon-ball. The white alloy called " German silver " contains copper, zinc, and nickel in about the pro- portions of 5 : 3 : 2. There are not, however, many minerals which contain nickel. One of these is the sulphide millerite ; another is niccolite, or nickel arsenide. There are also some other rare compounds of nickel with sulphur, arsenic, or anti- mony. Nickel is also present in some varieties of the sulphide of iron, magnetic pyrites or pyrrhotite, and- this occurs in so large an amount as to be an important source of the metal. There are further some hydrous silicates containing nickel, which are extensively mined at the present time. It is interesting to note, finally, though a matter of no practical importance, that the iron of meteor- ites is almost always an alloy of iron and nickel, the latter DESCRIPTION OF MINERAL SPECIES. 227 metal being present to the amount of 5 to 10 per cent, and in rare cases much more. Millerite. Nickel sulphide, NiS. MILLERITE, the sulphide of nickel, is remarkable among minerals because of its occurrence in very fine hairlike, or capillary, forms. These sometimes resemble a wad of hair, as in the geodes in the St. Louis limestone, or they may be simply a tuft of extremely delicate radiating crystals as in cavities of hematite at Antwerp, N. Y. There are also thin crusts with fibrous structure, as those from Pennsylvania. The hardness is a little over 3, and the specific gravity 5.6. It has a metallic luster and a color like that of yellow bronze, often slightly tarnished; the streak is greenish black. The composition, NiS, gives 64.6 per cent of metallic nickel. Before the blowpipe millerite reacts for sulphur, like the sulphides, and after roasting off the sulphur a small fragment will give with borax in the oxidizing flame a characteristic violet bead (p. 138). The globule obtained on charcoal, after heating in the reducing flame, is attracted by a magnet, for nickel is a magnetic metal like iron, though of much feebler intensity. It is only rarely that millerite occurs in sufficient quan- tity to be useful as an ore of nickel. An iron-nickel sulphide called PENTLANDITE is more important; POLYDY- MITE is another nickel sulphide. Niccolite. Nickel arsenide, NiAs. NICCOLITE is often called copper-nickel, but not because it con tains copper ? b\jt only from, its conspicuous pale cop- 228 MINERALS, AND HOW TO STUDY THEM. per-red color. It is found in masses of metallic luster ; hardness 5 to 5.5, and specific gravity 7.3 to 7.6. The composition NiAs gives : Arsenic 56.1, nickel 43.9 = 100. BREITHAUPTITE is a related mineral, rarer than niccolite, though somewhat resembling it. It is an antimonide of nickel, NiSb. Nickel is also present with cobalt in the mineral smalt- ite. GENTHITE and GARNIERITE are hydrous silicates of nickel and magnesium of varying composition, having a bright green color. They are found only in massive forms and are not very interesting as minerals, but highly valu- able as ores. Garnierite is mined extensively in the French penal colony of New Caledonia. COBALT. COBALT is a metal related to nickel and often associated in nature with it in its various compounds, though of much more limited occurrence. Cobalt minerals are rather rare; they include the sul- phide, LINN^ITE; the arsenide, SMALTITE, which is the chief ore; the sulph-arsenides, COBALTITE, and GLAUCODOT. All these have a tin- white color like that of arsenopyrite, which also has a variety, called danaite, which contains cobalt. There is also a bright rose-red mineral called ERYTHRITE, or cobalt bloom, which is an arsenate of cobalt. An impure oxide of cobalt is a black earthy mineral. Cobalt, as a metal, is not used in the arts, but its salts, which are mostly brightly colored, have some applications. From the change in color that some of them undergo ou DESCRIPTION OF MINERAL SPECIES. 229 heating and losing water depends their use as sympathetic ink. Cobalt glass, called smalt, has a beautiful blue ultramarine color and ground up is used as a pigment. MANGANESE. MANGANESE is a metal which is closely allied to iron in physical characters and chemical relations. As obtained by the chemist, for it does not occur in nature, it is hard and brittle; it has a grayish- white color, and a specific gravity of about 8. Like iron it forms numerous natural compounds, but they do not find many applications in the arts. The alloys of manganese with iron, called spiegelei- sen and ferromanganese, are, however, employed in large quantities in making steel, and most of the manganese mined is used in this way. The common ores of manganese are the oxides, pyro- lusite and manganite; the silicate and carbonate are beau- tiful minerals, but relatively rare, as is still more true of the other natural compounds. Pyrolusite. Manganese dioxide, MnO a . PTROLUSITE is an oxide of manganese, Mn0 2 , and be- cause of the large amount of oxygen that it contains it is sometimes used in the laboratory as a source for that gas. The glass-maker also employs it to take out the color of glass, because the oxygen which it yields forms colorless compounds in it. On this account it takes its name, from the Greek words meaning fire (nvp) and to wash (\VGD). The French have a similar name they called it "glass- maker's soap/' It is also used as an oxidizing agent in making paints, varnishes, etc. 230 MINERALS, AND HOW TO STUDY THEM. It is a very soft mineral, soiling the fingers ; it has a grayish-black color, a black streak, and metallic luster. It usually occurs in fibrous masses, less often crystallized, also sometimes in stalactitic forms and in incrustations. The composition of pyrolusite is essentially manganese dioxide, Mn0 2 , but it also commonly contains some water. The strictly anhydrous manganese dioxide is the mineral polianite, crystallizing in tetragonal forms similar to crys- tals of rutile (TiOJ and cassiterite (SnOJ. The reactions of manganese with the fluxes are given on p. 138. Heated in the closed tube pyrolusite yields oxygen which causes a match if still glowing when inserted to start into flame; when treated with hydrochloric acid chlorine is liberated. Pyrolusite is the common ore of manganese and is mined in large quantities in Virginia, Georgia, New Brunswick, etc. It has ordinarily, perhaps always, been formed from the related mineral manganite. Manganite. Manganese hydrate, Mn 2 3 .H a O. MANGANITE is another oxide of manganese; it occurs in brilliant orthorhombic prismatic crystals and in fibrous radiated masses. The hardness is 4, and the specific grav- ity 4.2 to 4.4; the luster is metallic, and the color dark steel-gray to nearly iron-black, while the streak is dark reddish brown, thus distinguishing it from pyrolusite, the streak of which is black. The formula MnO(OH) or Mn 2 3 .H 2 gives the per- centage composition : Manganese sesquioxide 89.7 (or man- ganese 62.4), water 10.3 = 100. This is a not uncommon mineral in the Lake Superior iron region; it is mined in DESCRIPTION OF MINERAL SPECIES. 231 the Harz in Germany. By loss of water and oxidation it is converted into pyrolnsite. BRAUNITE and HAUSMANNITE are other oxides of man- ganese of rather rare occurrence. PSILOMELANE is com- moner, but not often found in a state of purity; it usually occurs in black botryoidal or stalactitic forms, often asso- ciated with pyrolusite. It consists chiefly of manganese oxide and water, with some baryta, etc. WAD, or bog manganese, is a still less definite mineral, consisting of mixtures of oxides of manganese and other metals (cobalt, lead, etc.). It is brown to black in color, dull in luster; very soft, and often extremely porous and light, sometimes sufficiently so to float on water. It is used as a paint. Rhodonite. Manganese silicate, MnSi0 3 . RHODONITE is named from the Greek word for rose (pod or), which alludes to its beautiful rose-red color. It is not a common mineral, but is found rather abundantly in some localities, as at Franklin Furnace, N. J., also in Eussia, where it is used as an ornamental stone, in the form of a veneering for table-tops, etc. The crystals are flat and usually show two cleavages; they belong to the triclinic system, and the form is not easily deciphered. The hardness is about 6, and the specific gravity 3.6. The luster is vitreous, or pearly on the cleavage faces; the color is rose-pink; the streak is white. The formula, MnSi0 3 , corresponds to the percent- age composition: Silica (Si0 3 ) 45.9, manganese protoxide (MnO) 54.1 = 100. Some varieties contain zinc, others iron, and others also lime. It fuses rather easily before 233 MINERALS, AND HOW TO STUDY THEM. the blowpipe, turning black; with borax it gives a man- ganese reaction (p. 138). It is partially dissolved by hydrochloric acid. Bhodochrosite. Manganese carbonate, MnCO,. KHODOCHEOSITE, the carbonate of manganese, is another rose-colored mineral resembling rhodonite in color as in its name. It sometimes occurs in fine clear rhombohe- drons, and in masses with rhombohedral cleavage, and then the form is found to be very near that of calcite and siderite, to which it is closely related (see p. 119); the angle between two cleavage faces is 107. It also occurs in massive forms, sometimes granular and compact; also globular or botryoidal. The hardness is about 4, and the specific gravity 3.6. The luster is vitreous, and the color rose -pink; the streak is white. The formula, MnC0 3 , requires : Carbon dioxide (C0 2 ) 38.3, manganese protoxide (MnO) 61.7 = 100. It effer- vesces with acid and reacts for manganese with the fluxes (p. 138). Beautiful clear rhombohedral crystals come from Lake County, Colorado. It is the gangue of silver and gold ores in Montana, near Butte City. It is mined in Wales and in Belgium. Of the many other manganese minerals may be men- tioned the sulphides ALABANDITE (MnS) and HAUERITE (MnS 2 ) ; further, the phosphate TRIPLITE (also triphylite and lithiophilite, p. 225); there are a number of other rare phosphates. DESCRIPTION OF MINERAL SPECIES. 233 ZINC. ZINC is one of the most common and important of the metallic elements, but it is not certainly known to occur in the form of the metal in nature. It has a crystalline structure like metallic antimony, a white color, and brill- iant luster, soon, however, tarnishing. Its specific grav- ity is 6.9 to 7.2. It is brittle at both low and high temperatures, but at 140 Centigrade it can be rolled into sheets. It fuses at a relatively low temperature, 500 0., and boils at a red heat. Its physical properties put it somewhat near the imperfect metal antimony. It is a most important metal in the arts. Iron in sheets and wire, coated by zinc, are protected from rusting, and are then said to be galvanized; a common use of the sheets is for roofing. Zinc is the negative metal in almost all forms of the chemical electric battery that is, the metal at the expense of which the electric current is obtained. With copper it forms brass and related alloys; it is also one of the constituents in german silver; an alloy of zinc is used for making raised cuts in photo-engraving. The white oxide is used for paint. Metallic zinc, as obtained from the furnace in ingots, is called spelter. The commonest ore of zinc is the sulphide, sphalerite or zinc blende, but the silicates, willemite and calamine, and the carbonate, smithsonite, are also important and valuable. Sphalerite. Zinc sulphide, ZnS. SPHALERITE is named from a Greek word which means deceiving, and the young mineralogist, after he has blun- 234 MINERALS, AND HOW TO STUDY THEM. dered over it a score of times, as he is pretty sure to do, for it is far from easy to recognize, will think it well named. It was so called because often occurring with and mistaken for the more easily recognized lead ore, galena; the miner's names, black jack, false lead, false 203. 204. galena, refer to the same fact. The common name, blende or zinc blende, will perhaps be easier to remember at first than sphalerite. It is sometimes found in tetrahedral crystals and related forms (Figs. 203, 204), but usually the crystals are indis- tinct, being not infrequently twinned, and it needs a trained and skillful eye to understand them. Usually it is found in masses or small particles, showing smooth sur- faces of cleavage, which is found on examination to be dodecahedral, since the angle between two adjacent sur- faces is 120. Sometimes it is possible to cleave out an almost perfect dodecahedron from a mass of sphalerite. Even if granular in structure the cleavage surfaces are usually prominent, though there are kinds which are closely compact and show no cleavage. The hardness is 3.5 to 4, and the specific gravity about 4. When perfectly pure, sulphide of zinc is white in the form of powder, or clear and nearly colorless in small DESCRIPTION OF MINERAL SPECIES. 235 cleavage pieces ; the latter then show an adamantine luster. Commonly it contains some iron, and often a good deal, and then the color is yellow or yellowish brown, the latter the most common, and finally dark brown and nearly or quite black; the light-colored kinds may also have a greenish tinge. The luster is usually resinous; and in all the common kinds this is so distinct that the mineralogist comes to depend upon it to enable him to identify the mineral. The streak is white, pale yellow, or brownish, becoming deeper the darker the color of the mass. The composition zinc sulphide, ZnS, gives : Sulphur 33, zinc 67 = 100. As stated above, iron is usually present, and sometimes also manganese and the rare element cadmium. Before the blowpipe it does not fuse, but if powdered and heated on charcoal (see p. 143) it gives a zinc coating, canary-yellow when hot, but white on cool- ing; this turns green when heated in the oxidizing flame after being moistened with nitrate of cobalt. When warmed in a test-tube with hydrochloric acid it effervesces, giving off bubbles of gas which a careless observer might take for carbon dioxide, only the disagreeable odor shows that it is sulphureted hydrogen (H a S). Zinc blende is one of the commonest of the metallic compounds, and where we find galena or pyrite we are likely to find it also; in one variety it is compact, alter- nating in layers with cleavable galena. It occurs in some regions, as in southwestern Missouri and the adjoining portions of Kansas, in very large deposits. Zincite, Franklinite, and Willemite are all rare minerals, 236 MINERALS, AND HOW TO STUDY THEM. but as they are important ores of zinc at that famous locality, Franklin Furnace, New Jersey, they will be men- tioned briefly; they are indeed known at only a few other places. ZINCITE, or the red oxide of zinc (ZnO), is often found in bright red grains or masses, sometimes mixed up with the other two minerals named, the black franklinite and the green willemite. It also occurs in larger masses in calcite, and then shows good cleavage. Hexagonal crystals are very rare. The hardness is 4 to 4.5, and the specific gravity 5. 4 to 5. 7. The deep red or orange color is very characteristic; the streak is orange-yellow. It reacts for zinc on charcoal and for manganese with borax on the platinum wire. FRANKLINTTE is an oxide of zinc, manganese, and iron; it has been already mentioned on p. 221. G-AHNITE, or zinc spinel, is related to franklinite. It has often an octahedral form and a deep green color. The hardness is 7.5 to 8, and the specific gravity 4.6. The typical composition is ZnO.Al a 3 , but kinds from different localities vary widely. WILLEMITE is often found in bright yellow or apple- green masses, also in six-sided crystals usually of a flesh-red color and which are sometimes quite large and have a resinous luster; this last kind is called troostite. Barely slender prisms of a clear green are found. The hardness is 5.5, and the specific gravity 3.9 to 4.18. The luster is between vitreous and resinous, often weak, and the color varies widely, as already stated. Willemite is a silicate of zinc, Zn,Si0 4 , or 2ZnO.SiO,, DESCRIPTION OF MINERAL SPECIES. 237 and the percentage composition is: Silica (SiO a ) 27.0, zinc protoxide (ZnO) 73.0 = 100. Manganese and iron are often present, replacing part of the zinc. CALAMINE is another silicate of zinc, but different from willemite, since it contains considerable water, which it gives off when heated to a high temperature in the closed tube. It is not very often found in isolated crystals, but usually in masses with a crystalline surface, which is mam- millary or botryoidal in form. Occasionally the surface is seen to be made up of flat tabular crystals projecting from the mass. The hardness is 4.5 to 5, and the specific gravity 3. 4 to 3.5. It is usually white or slightly yellowish, but may be tinged blue from a little copper; the luster is vitreous. The composition is H a Zn 2 Si0 6 or H 3 0.2ZnO.SiO, , which gives: Silica (Si0 2 ) 25.0, zinc protoxide (ZnO) 67.5, water (H 2 0) 7.5 = 100. Before the blowpipe on charcoal it yields the characteristic zinc coating; further, a frag- ment ignited with cobalt solution assumes a fine blue. With hydrochloric acid it forms a jelly (p. 156). Smithsonite. Zinc carbonate, ZnC0 3 . SMITHSONITE is remarkable because in its common forms looking so much like calamine. Like it it may have many colors indeed, the smithsonite brought of recent years from the old zinc mines of Laurium in Greece is remarkable for the beautiful shades of blue, green, yellow, and red which it exhibits in different specimens. It is also called dry bone by the miners. It is related to calcite, siderite, and rhodo- chrosite, and crystallizes in similar rhombohedral crystals (p. 119), but they are very rare. The common form is that 238 MINERALS, AND HOW TO STUDY THEM. of mammillary or botryoidal masses, also stalactitic shapes. The hardness is 5, and the specific gravity 4.3 to 4.45. The luster is vitreous. The composition ZnC0 3 gives: Carbon dioxide (C0 2 ) 35.2, zinc protoxide (ZnO) 64.8 = 100. With acid it ef- fervesces, as do all the carbonates. It is infusible before the blowpipe, but when heated very hot in the oxidizing flame after moistening with cobalt solution it takes a green color on cooling. CADMIUM is a rare element often associated with zinc, for instance in sphalerite. GREENOCKITE is cadmium sul- phide. ALUMINIUM or ALUMINUM. ALUMINIUM is one of the most remarkable of metals, because while it has great tenacity and is in a high degree sonorous and non-oxidizable in the air, it has a specific gravity of less than calcite, or only 2.5. In other words, it is only about one third as dense as iron and one fourth as dense as silver, which it somewhat resembles. Both as the pure metal, because of its low density, and in alloys, for example with copper as aluminium bronze, because of their strength and other remarkable properties, it is highly useful. As improved methods of obtaining it are devised (e.g., by electrolysis) and the price, once very high, falls, its use is being increased, and we cannot now say to what extent in the future it may supplant other metals, especially steel. Aluminium does not occur in the native form, but it is one of the commonest of the chemical elements and is an important constituent of a great many minerals, DESCRIPTION OF MINERAL SPECIES. 239 Corundum is oxide of aluminium; gibbsite and bauxite are hydrated oxides, the latter occurring in large quantities, but more or less impure; cryolite is a fluoride of alumin- ium and sodium ; kaolin is a silicate of aluminium, and the many kinds of clay are related silicates, though usually impure; the feldspars are silicates of aluminium with po- tassium, calcium, or sodium; further, the element enters into the composition of a considerable part of the other silicates, as mica, the zeolites, etc. The supply of the metal is now chiefly obtained from bauxite, also from gibbsite and cryolite. Corundum. Alumina or Aluminium oxide, A1 2 3 . CORUNDUM is, next to diamond, the hardest of minerals and one of great interest, Its clear blue varieties make the sapphire of jewelry, and the clear red the highly-prized ruby; while the coarse and impure kinds, when pulverized, are our emery. When in distinct crystals it has a hex- agonal form, usually either that of a prism or a tapering pyramid (Figs. 205, 206). It is also found in massive forms, and these often have a frac- ture nearly like a cube in angle. The hardness is 9, so that it will scratch any other mineral except the diamond. The specific gravity is 4.0, which is high for a nonmetal- lic mineral, and remarkably high for the oxide of a metal of such low density. It is not often that the oxide of a metal is more dense than the petal itself; this great density is obviously connected with 206. 240 MINERALS, AND HOW TO STUDY THEM. the great hardness (see p. 84). The luster, like that of most very hard minerals, is brilliant and adamantine, though rather dull in some massive kinds. The color is gray to brown or nearly black in many of the common varieties, called in part adamantine spar; bright blue in the variety called the sapphire; red in the ruby; purple in the Oriental amethyst; * yellow in the Oriental topaz. Corundum is the sesquioxide of aluminium, A1 2 3 . It is infusible before the blowpipe and unattacked by acids. When heated very hot it gives with cobalt nitrate the characteristic blue of alumina. To obtain this, since the mineral is so refractory, it should be pulverized carefully, then moistened with a drop of nitrate of cobalt, so as to form a paste, and this supported in the loop of the plati- num wire and intensely heated. Common corundum occurs in Massachusetts at Chester, New Jersey, Pennsylvania, and still more in North Caro- lina and the adjacent states of South Carolina and Geor- gia; gems are rare, but when pulverized and washed from the rock it is used for emery. Beautiful sapphires have been obtained in Ceylon, and rubies in Siam and Burma and other places in the East Indies. Emery has been extensively mined near Smyrna, Asia Minor, and at Naxos and other of the Greek islands. DIASPORE is a rare oxide of aluminium (A1 2 3 .H 2 0) yielding about 15 per cent of water upon ignition. It * The word Oriental in such cases was formerly much used ; it meant originally coming from the East or Orient, and from that, as applied to gems, of great value as contrasted with stones of similar color (for example, the common amethyst) but not so highly prized. DESCRIPTION OF MINERAL SPECIES. 241 occurs in thin crystals or foliated masses with highly per- fect cleavage; the luster on the cleavage-face is pearly, elsewhere vitreous. The color is usually white or nearly so. The hardness is 6.5 to 7, and the specific gravity 3.4. This species is often associated with corundum, as at Chester, Mass., in Pennsylvania at Newlin, and elsewhere. BAUXITE is a hydrated oxide of aluminium occurring in earthy masses resembling clay; also in concretionary forms. The color varies from white to gray, yellow, also to brown or red, especially in the impurer kinds. It is not an at- tractive mineral, but is valuable as a source of aluminium. It takes its name from the principal locality at Baux (or Beaux), France; it is also found in our Southern States. GIBBSITE is a hydrated oxide of aluminium, A1(OH) 3 or A1 2 3 .3H 2 0. It occurs in opaque white stalactitic forms and incrustations, showing a radiated structure. It is often found with ores of iron and manganese, but not usually in large quantities. Spinel. Magnesium aluminate, MgAl 2 4 . SPINEL is a rather rare mineral containing in the typi- cal form magnesia and alumina. It is 207. usually found in octahedrons, often in twins, which are therefore called spinel twins (Fig. 207). The hardness is 8, or as great as that of topaz, and the specific gravity 3.5 to 4. The color is sometimes pink, as in the spinel ruby or balas ruby, which is not to be confounded with the true or Oriental ruby. It is also blue and black. The typical composition 242 MINERALS, AND HOW TO STUDY THEM. is MgAl 2 4 or MgO.Al 2 3 , but different kinds vary widely from this. CHRYSOBERYL is another rare mineral containing beryl- lium and alumina (BeO.Al 2 3 ). It is interesting because very hard (H. = 8.5), and in some of its forms used as a gem, especially a grayish-green kind (from Ceylon) with chatoyant effect, hence called cat's-eye;* also in a variety from Siberia named alexandrite, which is green as ordi- narily seen, but red by transmitted light (see Fig. 121, p. 58). The common form has a greenish -yellow color, a little resembling beryl, whence it takes its name of golden beryl. Cryolite. Fluoride of Aluminium and Sodium, Na 3 AlF 6 . CRYOLITE takes its name from two Greek words (/cpuos", which mean ice-stone, and it is so called because 208. often found in blocks which have some- thing of the appearance, as slightly clouded, of blocks of ice; it is remark- able for its easy fusibility. It is found in crystals having nearly the angles of a cube, though really monoclinic (see Fig. 208); it also has cleavages in three directions, which, unless carefully examined, could be confounded with cubic cleavage. The hardness is 2.5, and the specific gravity 3. The luster is vitreous to greasy, and the color usually white, but sometimes reddish or brownish. The composition, Na 8 AlF 8 , which may also be written 3NaF.AlF 3 , gives: Fluorine, 54.4, aluminium 12.8, sodium * The same name belongs to a less beautiful variety of quarts giving a similar effect. DESCRIPTION OF MINERAL SPECIES. 243 32.8 = 100. It fuses with great ease even in small frag- ments in the candle-flame without the blowpipe. It gives an intense yellow flame (soda), and also reacts for fluorine. Cryolite is a rare mineral, and the only locality where it occurs in quantity is near Ivigtut in Southern Greenland. Here it has been mined for many years, because useful both for making soda salts and as an ore of aluminium (it is brought to Philadelphia for this purpose). It is also found sparingly in Colorado. THOMSENOLITE and PACHNOLITE are fluorides of alu- minium, calcium, and sodium, which are related to cry- olite and occur with it. TURQUOIS, the beautiful precious stone having the color of robin Vegg blue, also bluish green in less highly prized varieties, is a hydrated phosphate of aluminium, containing also a little copper phosphate, which is proba- bly the source of the color. It occurs only in compact massive forms, filling seams and cavities in a volcanic rock. The early locality was' in Persia, but of late years a num- ber of mines have been opened in New Mexico some of them were worked hundreds of years ago by the Mexicans. WAVELLITE is another hydrated phosphate of aluminium. It is usually found in globular or hemispherical forms with radiating structure (see Fig. 133, p. 68) and a crys- talline surface. The hardness is 3 to 4, and the specific gravity 2.3. The color is white, varying to yellow or green. There are a number of other related aluminium phos- phates, but they are too rare to be included here, except perhaps the azure-blue LAZULITE found in monoclinic crystals in Georgia, also elsewhere in massive forms. 244 MINERALS, AND HOW TO STUDY THEM. AMBLYGONITE is a rare phosphate of aluminium and lithium containing fluorine (AlP0 4 .LiF). It usually oc- curs in white cleavable masses resembling albite, but easily distinguished by its fusibility. It melts before the blowpipe very readily (at 2), giving a red flame (lithium) with traces of green (phosphorus). The hardness is 6, and the specific gravity 3.05. It is found in Maine. ALUNITE is a sulphate of aluminium occurring in rhom- bohedral crystals looking a little like cubes. Another sul- phate is ALUMINITE. The ALUMS are hydrous sulphates of aluminium with potash, soda, etc. There are numerous such compounds among minerals. DAWSONITE, from near Montreal, is a rare carbonate of aluminium and sodium. CALCIUM, CALCIUM, whose oxide (CaO) is the familiar substance called limey is a white metal somewhat resembling silver or tin. It is obtained with difficulty, for example by elec- trolysis (p. 107), and is in this form of interest only to the chemist. Its compounds, however, are numerous and im- portant, and it is indeed one of the most widely distributed of all the elements. The carbonate of calcium forms the common mineral calcite and also the less common arago- nite. Other very important compounds among minerals are : the fluoride, fluorite or flu or spar; the phosphate, apatite; and the sulphates, gypsum and anhydrite. Calcium is also an essential ingredient of many of the silicates, as in some of the feldspars and zeolites, some varieties of pyroxene and garnet, and so on. DESCRIPTION OF MINERAL SPECIES. 245 Fluorite or Fluor Spar. Calcium fluoride, CaF 3 . Fluorite, or Fluor Spar, is one of the most beautiful of minerals, occurring in cubic crystals and groups of crystals (see Fig. 2, p. 15), sometimes very large and of a great variety of colors, from colorless to green,- yellow, brown, red, and purple. It is common to find the angles of one cube projecting from the faces of another, and as the posi- tion of the crystals is then such that if one of them were revolved 180 about a line joining opposite angles they would be brought into a parallel position, the group is called a twin (Fig. 211, also Fig. 116, p. 57). The edges of the cubes are often beveled by a pair of narrow planes 209. 210. 211. (Fig. 209), and one, three, or six little faces (Fig. 210) are sometimes seen on the solid angles. Octahedral crystals are also found occasionally built up of minute cubes and also other forms, but the cubic habit is so important a character that a non-metallic mineral of this form at once suggests fluorite to the careful mineral- ogist. These crystals, and the massive forms too, can often be recognized by the perfect octahedral cleavage which makes it easy to break off the angles of the cubes, and from a large cube to form by fracture a perfect octahedron. 246 MINERALS, AND HOW TO STUDY THEM. Besides the crystallized forms there are others, not so easy to recognize, which are massive. These are often fi- brous or columnar in structure, and one variety having the colors arranged in bands is used as an ornamental stone ; this includes the Derbyshire Hue-John. There are, too, granular kinds and those which are closely compact. The hardness of fluorite is 4, and its specific gravity 3.2. The variety in color, embracing many shades of green and purple, yellow and red, has already been mentioned; there are also colorless kinds. The crystals are usually trans- parent, and sometimes show on, or near, the surface a bright bluish color quite different from that observed when they are looked directly through. The blue light extends within the crystal if it is placed in the direct sun- light. This phenomenon is called, fluorescence, and having been first observed with fluorite was named accordingly from it. The name fluor spar is one of the oldest in min- eralogy, and was given because of the use of this species as a flux in smelting. Fluorite is one of a rather small group of compounds called fluorides; its formula is calcium fluoride, CaF 2 , which gives the percentage composition: Fluorine 48.9, calcium 51.1 = 100. Powdered and warmed with sulphuric acid in a lead or platinum crucible it gives off hydrofluoric acid, and a plate of glass, first covered with a layer of wax and then written on by a fine point, will have the lines thus exposed etched by the acid. This method of ornamenting glass or of making marks, for example on a thermometer- stem, is often used. Fluorite usually flies to pieces violently when heated be- DESCRIPTION" OF MINERAL SPECIES. 247 fore the blowpipe; but when pulverized, as explained on page 131, it can be fused and yields the yellowish-red flame characteristic of lime. Broken into small fragments and heated in a closed tube, not too hot, it phosphoresces, that is, becomes self-luminous, emitting sometimes a yellow light, also, as in the variety chlorophane, a beautiful green. This is best seen in the dark. Even the blow of a hammer is enough to make a mass yield a faint but beautiful phos- phorescent light for hours after. Fluorite is a common mineral in lead veins, and is then said to form the " gangue " of the ore. It occurs in this way in Derbyshire and Cumberland in England, and in the Freiberg mining region of Saxony. It is also found in cavities in limestone, as at St. Louis. A cave lined with beautiful sea-green cubes, some of them very large, was opened at Macomb, N. Y., a few years ago. Besides the use of some colored varieties for vases, etc., the massive kinds are employed as a flux in smelting ores as already stated, also in making opalescent glass. Calcite. Calcium Carbonate or Carbonate of Lime, CaC0 3 . CALCITE, next to quartz, is the most common of mineral species, remarkable for its variety of form both among the crystallized and uncrystallized varieties. It crystallizes in rhombohedrons and scalenohedrons of great variety and complexity of form, also in hexagonal prisms. The funda- mental rhombohedron, Fig. 213, has an angle between two adjacent faces of 105 (terminal edge), and each face has plane angles of 102 and 78. Parallel to the faces of this rhombohedron there is very perfect cleavage, so that a 248 MINERALS, AND HOW TO STUDY THEM. large mass breaks easily under the blow of a hammer into fragments all showing the same form (see Fig. 144, p. 72). This cleavage is the most important character of the crystallized varieties. 212. 216. m m 213. 214. 215. 217. 219. There are also other rhombohedrons, flattened or obtuse and lengthened or acute in the vertical direction, as shown in Figs. 212, 214, 215, 220. The rhombohedral angle for DESCRIPTION OF MINERAL SPECIES. 249 (Fig. 212) is 135, for/ (Fig. 214) 79, for M (Fig. 215) is G6. Fig. 216 represents a hexagonal prism, and Figs. 217, 218, 219 the same with the obtuse rhombohedron e of Fig. 212; the angle me is 116^ over the horizontal edge, and ce (Fig. 218) is 153f . Fig. 221 shows the common scaleno- hedron, the angles for whose two kinds of terminal edges are 104 40' and 144 24'; the angle for the zigzag edge is 133. Fig. 222 is a similar scalenohedron twinned, and Fig. 223 a combination of prism (w), rhombohedron (r), and scalenohedron (v). See also Figs. 81, 82, p. 41. The variety crystallizing in scalenohedral forms, or in acute rhombo- hedrons, is often called dog-tooth spar. There are also crystals with a combination of faces, or highly modified crystals as they are called, which can only be deciphered by one who has a thorough knowledge of crystallography. The remarkable experiment by which a twinning structure may be imparted to a cleavage fragment is mentioned on p. 59; as there stated, twinning lamellae, often of secondary origin, are very common in large rhombohedral crystals. A clear cleavage mass of calcite, such as that brought from Iceland, is called Iceland spar and is useful for opti- cal prisms. This is because of its remarkable double re- fraction, or power of dividing a ray of light passing through it into two separate rays, so that a line seen through it appears double. This phenomenon has been already de- scribed and illustrated (Fig. 148, p. 94). Besides the crystallized kinds there are those which have a granular structure, as statuary marble, and which sparkle in the light because of the multitude of cleavage- faces. Other kinds are fibrous with a silky luster, like 250 MINEKALS, AKD HOW TO STUDY THEM. satin spar; also close and compact, as in ordinary marble, and then of great variety of color, red, yellow, blue, black, and largely used for ornamental purposes. Some of these kinds of marble still contain shells, which come out dis- tinctly when polished. These shells are what we have to expect in such cases, for most limestone has been formed from the material of shells, crinoids, etc., left by animals whose remains have accumulated in large beds in the ocean and afterward been hardened, crystallized, and elevated into the position in which they are now found. A kind of shell marble with beautiful firelike reflections is called lumachelle. Stalactites and stalagmites are varieties of calcite which 224, are formed in caverns in limestone rocks. The water, charged more or less with the gas carbon dioxide, has the power of dissolving these rocks as it works its way through them and the calcium carbonate in solution is again slowly deposited in the forms here described. The stalactites hang like icicles (Fig. 224) from the roof of the cavern, and the stalagmites are made by the deposit from the drippings on the floor beneath. They are sometimes very large and have often great beauty and variety of shape; a cave like the Luray cavern or the Adelberg grotto at Trieste is a fairyland of strange and beautiful forms. These deposits often have a banded structure and sometimes occur on a large scale, so that the ^ DESCRIPTION OF MINERAL SPECIES. 251 rock can be quarried and used as an ornamental stone. The Mexican onyx is such a variety of calcite, a kind of stalagmite or water deposit, of great delicacy of coloring, beautifully translucent and used for ornamental purposes in a great variety of forms. Other kinds of calcite, formed by the deposit from waters containing carbonate of lime, are calc sinter or calc tufa, which often shows the impres- sion of leaves; agaric mineral or rock-milk, a soft pow- dery material; rock-meal, a light white cottonlike sub- stance. Calcite in its normal crystallized varieties has a hardness of 3, and a specific gravity of 2.7. The luster is usually vitreous, but silky in satin spar and dull in some earthy forms. It may be quite colorless, or pale yellow, pink, blue, less often dark-colored except in the marbles, -wMch are even jet-black. Calcite is calcium carbonate, or carbonate of lime, CaC0 3 ,and the percentage composition is: Carbon di- oxide (C0 2 ) 44, lime (CaO) 56 = 100. In the case of this mineral heat alone is enough to separate it into these two parts and this method is taken (as in a lime-kiln) both for obtaining the quicklime employed in making mortar, and also carbon dioxide, or carbonic-acid gas, used under pressure for charging soda-water fountains. Calcite does not fuse before the blowpipe, but a fragment after being heated (notice that it glows when ignited) and cooling turns a piece of moistened turmeric-paper brown (p. 131). In dilute hydrochloric acid it effervesces at once, giving off bubbles of carbon dioxide (carbonic-acid gas), and this is the simplest chemical test by which to identify it. 252 MINERALS, AND HOW TO STUDY THEM. Dolomite (p. 260) and siderifce (p. 223), which belong to the " Calcite Group" (see p. 119), effervesce in acid also, but only when in the state of a fine powder or on being heated. The occurrence of some of the kinds of calcite has already been spoken of. Limestone rocks are found in great quantities at many points in the world; in the United States they are especially common through the central states of Illinois, Iowa, Wisconsin, etc. In cavities in these rocks the crystallized forms occur, beautiful cavities or geodes being very common. It is also found with ores of the metals, as of lead, copper, and silver, and often in beautiful crystals wonderful for their size and complexity of crystallization. The mines of Lake Superior afford fine specimens, also those of Derbyshire and else- where in England and the Harz region of Germany. Calcite in its crystalline varieties is easily recognized by its cleavage and softness, and a drop of acid on a frag- ment in a watch-glass or test-tube effervesces at once. Calcite is useful, as already stated, for optical purposes in the form of Iceland spar; also as an ornamental stone and for building purposes in the many kinds of marble. Common limestone when burned yields the quicklime essential for mortar; further, the lime obtained from cer- tain kinds, containing some foreign substances (as silica and alumina) has the property of becoming firm and solid under water and is hence called hydraulic lime, the lime- stone being then named hydraulic limestone. Aragonite. Calcium carbonate, CaC0 3 . ARAGONITE is also calcium carbonate, having thus the DESCRIPTION OF MINERAL SPECIES. 253 same composition as the species calcite; but it crystal- lizes in the orthorhombic system, and though the forms may occasionally look like those of calcite, they are easily distinguished because they do not show its cleavage. The crystals are commonly slender needles (Fig. 225); there are also six-sided prisms 225. 226. which are really com- pound or twin crystals (Fig. 226). Besides these there also occur other kinds, as the delicate coral- like flos-ferri or flower of iron found in iron mines (Fig. 227), and further massive kinds. Aragonite is a little harder than calcite, 3.5 to 4 instead of 3, and distinctly denser, 2.9 instead of 2.7. Its luster 227. *- ^*%T^M< WPS*? is vitreous mostly, but on the cross-fracture of crystals plainly resinous, which aids the skilled eye in recognizing 254 MINERALS, AND HOW TO STUDY THEM. this species. The composition of aragonite and its be- havior with acid and before the blowpipe are the same as with calcite. Aragonite is not so common as calcite, but like it is a deposit from waters containing calcium carbonate; the question of temperature is an important element in deter- mining which of the two species is formed. Apatite. Calcium phosphate, 3Ca 3 P 2 8 .CaF 3 . APATITE can usually be recognized by its hexagonal form. A common kind is that occurring in long six-sided prisms, terminated by a low hexagonal pyramid (Figs. 228, 229); another is in short, stout hexagonal prisms, with often a large number of modifying planes (Fig. 230) ; and still another is like a low pyramid. The angle between 228. 229. 230. <^K i m m two adjacent pyramidal faces (x) is 142 16', and that of x on m is 130 18', of x on c 139 42'. There are also massive forms showing no crystallization. The hardness is 5, so that it can be scratched, but not very easily, by the knife; its specific gravity is 3.2. The luster is usually vitreous, but sometimes resinous. The common color of the long prisms is a dull green, but they are also yellow, red, brown, black, and sometimes violet; DESCRIPTION OF MINERAL SPECIES. 255 the smaller crystals are often clear and colorless; a variety in clear yellow-green crystals is called asparagus-stone. Apatite is essentially calcium phosphate, Ca 3 (P0 4 ) 2 , but it also contains a little fluorine or, less often, chlorine (or both), and the two varieties are distinguished as fluor- apatite and clilor -apatite. This distinction is not a very important one and can be made out only by a chemical analysis. Apatite is difficult to fuse before the blowpipe, and on this account it does not give the bluish-green color characteristic of phosphoric acid, unless first touched with a drop of sulphuric acid. It is easily soluble in hydro- chloric acid, also in nitric acid : and if to this last some ammonium-molybdate solution (in nitric acid) is added, a bright yellow powdery precipitate separates on gently heating; this is a delicate test for phosphoric acid. Apatite is a rather widely disseminated mineral, as in granite, also in limestone, and with ores of iron and tin; in many igneous rocks it occurs, as revealed by the micro- scope, in minute crystals. It is also found in large crys- tals, sometimes as big as a nail-keg, and in masses, associ- ciated with pyroxene, scapolite, titanite, also zircon, vesu- vianite and other species, in veins in the crystalline rocks of Canada and Norway; in both countries it has been mined extensively in recent years. By treating with sulphuric acid soluble phosphate is formed, which is em- ployed to fertilize the land. Phosphorus is also manu- factured from apatite. Eelated to apatite is the phosphate rock, which forms extensive phosphate deposits m South Carolina and Florida; this has a great economic importance. Guano, 256 MINERALS, AND HOW TO STUDY THEM. as of the West Indian islands, also consists largely of cal- cium phosphate. Gypsum. Calcium Sulphate or Sulphate of Lime, CaS0 4 + 2H 2 0. GYPSUM occurs in monoclinic crystals, and these often have the form shown in Fig. 231; twin crystals are also 231 232. also common, especially those of the "swallow-tail" type, like Fig. 232. The crystals have very per- fect cleavage parallel to the side face (b) of Figs. 231 and 233, and sometimes very large thin and perfectly transparent plates may be obtained. The variety yielding these is called selenite (from the Greek word,