Physical ScLLib. QE 89 Al C32 nifttloK o£ tie tot 04 no. 33 MINERALS AND ROCKS TKiKet and (feoUftf ■-■ ■ £y Salem $. &£ee ?962 SjUcitt PMcaUoK 33 CD. LIS&fiRf Wl This publication was prepared to accompany the sets of minerals and rocks sent by the Division to California schools. One copy is supplied free with each set. Additional copies of this publication are available for 50 cents each, or 30 cents each in orders of 10 or more. This price, of course, does not include additional sets of specimens, for these are not for sale by the Division. Special Publication 33 1962 '.;* ' ■ : ■'■■ if-,;. STATE OF, CMff^NH^ fjf* EDMUND G. BROWN, Governor / THE RESOURCES AGENCY :i WILLIAM E. WARNE, Administrator DEPARTMENT OF CONSERVATION DeWITT NELSON, Dfrecfor DIVISION OF MINES AND GEOLOGY f .>. m IAN CAMPBELL, Chief Ferry Bldg. San Francisco 1 1 r\T* CONTENTS Page 5 Introduction 6 How to enlarge school collections 7 Classification and identification of minerals and rocks 7 Minerals 11 Rocks 17 Description of specimens in the set 19 1. Barite 20 2. Calcite 21 3. Chromite 22 4. Chrysotile 23 5. Colemanite 24 6. Epidote 25 7. Feldspar 26 8. Limonite 27 9. Magnesite 30 10. Magnetite 31 11 Manganese oxides 32 12. Mica 33 13. Pyrite 34 14. Quartz 35 15. Quartz, variety chalcedony 36 16. Sulfur 37 17. Talc 39 18. Granite 40 19. Gabbro 41 20. Rhyolite 42 21. Basalt 43 22. Obsidian 44 23. Pumice 44 24. Conglomerate 45 25. Sandstone 46 26. Mudstone 47 27. Limestone 48 28. Chert 49 50 29. Diatomite 30. Gypsum 51 31. Slate 51 32. Marble 52 33. Mica schist 53 34. Serpentine 54 35. Greenstone 55 Selected references ^j.UBMmr t ■■*■;' && r^$ Silky chrysotile asbestos from Lake County. ntroduction For many years the Division of Mines and Geology has furnished sets of California minerals and rocks to schools in the state. Originally these sets were assembled from specimens to be discarded from the Division's petrographic laboratory, and were sent to all schools re- questing them. The demand was so slight that some rather rare specimen material (such as gold in quartz) was included in them. The sets were informally assembled with little effort, and neither time nor money was budgeted by the Division for this service. In the decade following World War II, California's population boom, combined with an awakening of interest in the teaching of science, re- sulted in such an increase in demand for the sets that the Division's facilities to provide them were severely taxed. Some of the rarer speci- mens required for these old sets were difficult to obtain in quantity, so of necessity the sizes of the specimens were reduced and the distribu- tion of sets limited to elementary schools. The collection accompanied by this description is different from those previously distributed by the Division. None of the minerals and rocks in it are rare in California, although some of them are rare in certain areas of the state. The object of the set is to familiarize students with many of the common rocks that make up the bulk of the earth's crust * in this region. These common rocks are the host for local ac- cumulations of rarer valuable or otherwise interesting minerals, some of which are also represented by specimens in the collection. * The outer solid layer of the earth is known as the crust. It ranges in thickness from about 4 miles beneath the deep ocean floor to as much as 30 miles beneath the surfaces of the continents, and rests on a more dense layer called the mantle. Thus, the crust is thicker beneath the continents than beneath the oceans. - How to Enlarge School Collections California is one of the most complex geological areas in the world. Even a highly generalized geological map, such as figure 1, indicates something of the structural complexity of the earth's crust here, and suggests that rocks characteristic of one area in the state might be rare or lacking in another. Therefore it is impractical, in the scope of a single collection such as this one, to represent adequately the rocks of all areas. It is suggested that schools can add to this collection in order to build displays representative of their local geological settings. This can be done by having teachers or students collect appropriate specimens from the vicinity of the schools. The specimens can be identified by local geologists or by the laboratory of the Division of Mines and Geology. Such specimens should be carefully collected and numbered, and notes made regarding location and other pertinent information (such as, "layered rock from roadcut", "vein in rock", "beach pebble", etc.). If specimens are to be identified by the Division laboratory, duplicate specimens of each rock type should be collected and given the same number (small pieces of adhesive tape may be used for temporary labelling). Then one of each set of duplicate specimens can be for- warded for identification to the Laboratory, Division of Mines and Geology, Ferry Building, San Francisco 11. A laboratory report will be returned identifying each specimen submitted, but will have no i meaning to the sender if an identical specimen, identically numbered, is not retained by him. (A note of caution is appropriate here: if stu- dents are to collect the specimens, they should be assigned different number sequences so that numerous different specimens submitted will I not have the same number.) The laboratory services of the Division are free to citizens of Cali- fornia, but they are limited to identification of two specimens per: person per month. A letter of instructions stating the information de- sired and the approximate locality from which the material was col- lected should be submitted with specimens, preferably attached to the package. S "imens are not returned by the laboratory unless re- quested, in w .ch case return postage must be supplied by the sender. - Specimens that are to be placed in the collection should be given permanent numbers. This is necessary in order to insure against con-; fusion of specimens if temporary labels are lost. For this purpose, a spot of white enamel can be put on each specimen, and the appropriate; collect; on number printed on this spot with India ink. This number.! should correspond with that on a card containing the identification and other pertinent information regarding the specimen. CLASSIFICATION AND IDENTIFICATION OF MINERALS AND ROCKS pr-:^>h : '"'- y '':'W' Si: : : ^ ■:■'-■■ Minerals To the geologist, minerals are the "building blocks" of the earth's crust, for in combinations or aggregates they make up the rocks. Each mineral has reasonably precise characteristics by which it can be dis- tinguished from other minerals. But the term "mineral" is sometimes loosely used, so its meaning in the geological sciences needs to be defined for our purposes. To a geologist a substance must have the following characteristics to be called a mineral: 1. It must occur naturally and be inorganic. 2. It must have a chemical composition and physical properties that are either fixed or that vary only slightly within definite limits. 3. It must have a charcteristic internal structure (called crystal lattice) determined by a fixed and orderly arrangement of the atoms within it. The minerals are the naturally occurring chemicals of the earth. All substances are composed of the tiny particles called atoms, of which there are some 96 different types (called elements) in our natural world. Individual atoms cannot be observed directly, so the things we see are composed of combinations of very large numbers of atoms. Only rarely do atoms of a single element make up a substance; most commonly two or more different kinds of atoms occur in precise combinations, or molecules (called chemical compounds by the chemist). Molecules have different properties than the elements ">f which they are made. For example, table salt (the mineral halite) is a brittle white substance composed of equal amounts of two ekmen' sodium and chlorine, the atoms of each being nicely arranged in an ^cernating man- ner within the cubic lattice structure of the salt molecule. If we were to break down the molecular structure of salt to separate the sodium and chlorine, we would find that in the pure state sodium is .a soft, silvery- white metal, and chlorine is a greenish gas. Whereas salt is edible, both sodium and chlorine are toxic or poisonous when pure (neither element is found pure in nature). Therefore, the things we see around us largely owe their chemical and physical characteristics to the properties of the chemical compounds that compose them, and not to the individual properties of the elements present in them. There are many hundreds of thousands of different chemical com- pounds known to us, most of them part of the organic world (the living things) or manufactured by men. Only a few thousand compounds, most of them rare, occur naturally in the inorganic crust of the earth, and these are the minerals. A chemical formula is given in the description of each mineral in the set. These formulas indicate the basic relative proportions of the ele- ments present in the minerals. For example, the formula for pyrite (FeS 2 ) indicates that this mineral is made up of iron (Fe) and sulfur (S) in the exact proportions of one iron atom for every two sulfur atoms. Minerals are classified according to their chemical compositions and lattice characteristics, but the determination of these properties requires considerable training and laboratory facilities. Fortunately, differences in these characteristics give the different minerals rather distinct prop- erties that can easily be observed, and that can be used to distinguish them one from another. The following comments discuss some of the characteristics of these properties of minerals, and methods of observing them. 1. Color— Some minerals have a single characteristic color, while others may have a considerable range in color. The true colors of many minerals, particularly the light-colored ones, can be so easily masked by stains, tarnish, or impurities that one should be very careful in observing this characteristic. For ex- ample, clay is a white mineral that is a major constituent of soils; but the white color of clay is seldom seen because of the presence of strong coloring agents such as brown iron oxide and black or- ganic material. While determining the color of a mineral, one should also ob- serve whether it is transparent, translucent, or opaque to the trans- mission of light. 2. Luster— The way a mineral reflects light, or shines, is called luster. Terms used to describe this property are mostly self-explana- tory; thus minerals may appear metallic, sub-metallic (almost metallic), vitreous (glassy), resinous, greasy, pearly, silky, or ada- mantine (gem-like). A fresh surface is desirable in most cases to observe the true luster of a mineral. 3. Hardness— The hardness of a mineral is a measure of the ease with which it can be scratched. There are all grades of hardness among minerals, from those like talc, that can be scratched with the finger nail, to diamond, the hardest natural substance known. Mineralogists have established a scale of hardness from 1 to 10, with certain minerals representing the whole-number values within the scale. These are: 1. talc 6. orthoclase (a type of feldspar) 2. gypsum 7. quartz 3. calcite 8. topaz 4. fluorite 9. corundum 5. apatite 10. diamond Each mineral on the scale will scratch any other with a lower number. Even without a set of these minerals for testing, the hardness of most minerals can be estimated with the aid of common imple- ments or materials. Thus, the finger nail has a hardness slightly more than 2, a penny is about 3, a steel knife blade is about 5, window glass is about 5 Vi , and a steel file is about 6 1 / 2 . Hardness should not be confused with properties like brittleness and toughness. For example, quartz can be shattered almost as easily as glass, but will scratch the hardest steel. On the other hand, talc forms tough masses that do not fracture easily, but they can be scratched with the finger nail. 4. Streak— The streak of a mineral is the color of its powder. It is most easily observed by rubbing the mineral on a piece of white unglazed porcelain (called a streak plate), but can also be observed by fine crushing of a specimen with a hammer. Although all white minerals have white or colorless streaks, many colored and black minerals have streaks that are entirely different from the color of the mineral. The streak is often more dependable than the apparent color of a specimen, because minor impurities that commonly change the normal color of a mineral do not affect the streak color. 5. Cleavage— Cleavage is the tendency of a mineral to break along one or more flat planes, and is controlled by the lattice structure of the mineral. Minerals may have none, one, two, or more cleavage directions, but the number characteristic of a mineral species is constant for all specimens of that mineral. Examples of these in the set are: quartz, that has no readily observable cleavage but fractures along curved surfaces; mica, that has one cleavage direction and breaks into flat sheets or flakes; hornblende (the dark mineral in the gabbro specimen), that has two cleavage directions and breaks into long, thin fragments; and calcite, that has three cleavage direc- tions to yield rhombohedral fragments when fractured. 10 6. Specific gravity— This is the ratio of the weight of a specimen to that of an equal volume of water. Thus if the weight of a min- eral specimen is three times that of the volume of water it displaces, its specific gravity is 3. With practice in hefting hand specimens of various known spe- cific gravities, one can learn to estimate specific gravities of un- known materials with useful accuracy. To illustrate this, it is sug- gested that the calcite and barite specimens in the set be hefted for comparison. (For mineralogical purposes, hefting is the process of holding a specimen in the hand and estimating its weight relative to that of another specimen of known specific gravity and of ap- proximately the same size. One often does this to judge if a letter weighs more than an ounce.) 7. Crystal forms— Crystal faces are rarely identifiable on speci- mens picked up in the field, but they are sometimes diagnostic when present. Like cleavage, they are controlled by the lattice structure of minerals. Although an adequate 'treatment of crystal- lography is beyond the scope of this brief discussion, much can be learned of the nature of crystal lattices, and the resulting pos- sible crystal faces, by constructing models and actually growing crystals. Procedures for making these experiments are outlined in the book "Crystals and Crystal Growing", by Holden and Singer,, and this as well as other books listed in the references are recom- mended for some interesting reading on the subject of crystals. Information obtained by careful observation of as many of these properties as possible can be used with mineral identification tables (see references) to identify most of the minerals commonly picked up. One should be sure, however, that he is observing a single mineral fragment or aggregate, and not a rock consisting of a fine-grained aggregate of two or more different minerals. Observation of fine-grained specimens is greatly improved by the use of a 10-power magnifying lens (the "hand lens" of the geologist and botanist). 11 Rocks Rocks are the geological units that occur in masses sufficiently large to be mapped, and it is by means of geological maps that we are able to interpret much of the history and structure of the earth's crust. The great majority of rocks are composed of aggregates of mineral grains, most commonly of two or more mineral species. A few rock types, notably coal (organic) and obsidian (non-crystalline) are com- posed of substances that are not minerals. Being essentially mixtures of materials without definite internal arrangements of these constituents, rocks are less precisely defined than minerals. The rocks we see at the surface of the earth may conveniently be classified into three broadly defined groups, called igneous, sedimentary , and metamorphic rocks. They may be further subdivided under these group headings into fine-grained, medium-grained, and coarse-grained types. For purposes of the descriptions given here, fine-grained rocks are those in which most of the individual mineral grains cannot be seen without the aid of a microscope. Medium-grained rocks are those largely made up of mineral grains visible with the unaided eye, but less than about y 1Q inch in diameter; and rocks with a larger predominant grain size are called coarse grained. 1. Igneous rocks are those formed by cooling and solidification of molten rock, or magma. Numerous places within the crust of the earth have been (and some are now) subjected to partial or complete melting of the rock at depths of 10 or 20 miles. In places some of the resulting magma rose along fissures to be erupted at the surface by means of the various phenomena of volcanic activity. Some gradually cooled and \ solidified near the depth of melting, a process requiring much time because of the insulating effect of the overlying rocks. According to the times required for cooling at different depths, any magma may yield rocks that differ rather widely in appearance, although they have similar chemical compositions. Magma that solidifies at great depth normally cools so slowly that the resulting rocks are coarse grained— composed of mineral grains that are I /4o to M> mcn or more in size. These are called plutonic igneous rocks, and can become exposed at the surface only by the great uplift and erosion that accompanies mountain building. In contrast, the volcanic igneous rocks result when magma solidifies rapidly at very shallow depths or is erupted at the surface. Magma erupted as a lava flow 12 cools very rapidly, so that there is little time for crystals to grow before the flow solidifies. The resulting volcanic rocks are very fine grained or glassy, but in places they contain scattered large crystals of minerals that had begun to crystallize in the magma before eruption. Many volcanic rocks contain spherical or ellipsodial cavities caused by gas bubbles that were trapped in the magma at the time of solidification. Magmas differ in chemical composition from place to place, yielding rocks composed of different mineral assemblages. As a result, the classi- fication of igneous rocks is based largely on two criteria, chemical composition (approximately revealed by the mineral assemblage), and grain size of the constitutent minerals. A magma rich in the elements silicon and potassium, but poor in sodium, iron, and magnesium, may yield granite or granodiorite (coarse- grained, light-colored rocks) where it cools at great depth; it may yield rhyolite or dacite (very fine-grained, generally light-colored rocks) where erupted at or near the surface; or it may form obsidian (gray ort] black volcanic glass) where erupted at the surface and cooled very quickly. These rocks are similar in bulk chemical composition, but quite different in appearance. Another magma, relatively poor in silicon and potassium, but richer I in sodium, iron, and magnesium, may yield gabbro (coarse-grained, dark colored rock) if it cools at depth, or basalt (very fine-grained, dark- | gray or black rocks) where it is erupted as lava flows. Magmas intermediate between these in bulk chemical composition yield plutonic rocks such as diorite and syenite, and their relatively common volcanic equivalents andesite and trachyte. Violent volcanic eruption of any of these magmas may yield light-! colored, loosely consolidated deposits of tuff, or "volcanic ash", com-i posed of finely divided volcanic glass shards and mineral grains. The predominant minerals in igneous rocks are feldspars (mostly: white or pink); quartz (white or clear); muscovite (white or clear), and the black minerals biotite, hornblende, and augite. Thus the geolo gists' field identification of an igneous rock is determined by these min erals; which of them are present, in what proportion, and in what grai size. Right. Alternating beds of black shale and buff sand stone at Point San Pedro, San Mateo County. Phot< by C W. Jennings and R. G. Strand. 13 2. Sedimentary rocks are those, composed of mineral and organic debris transported and deposited by mechanical or chemical means on the surface of the lithosphere (the solid portion of the earth). For the most part, this debris consists of the mineral grains and rock fragments loosened by weathering of the surface rocks and transported mechan- ically by streams and rivers to basins of deposition. When deposited, buried by succeeding deposits, and consolidated or cemented, the mechanically transported materials become the detrital sedimentary rocks. They are broadly classified according to the pre- dominant size of fragments within any given bed. Such rocks made up of very small particles, principally clay, are called mudstone when massive, or shale where thinly bedded. Those composed principally of sand-size grains are called sandstone, and those containing abundant pebbles are referred to as conglomerate. These terms do not imply any conditions regarding color or mineral composition of the rocks, for types of source materials are different from place to place. Thus shale 14 may be light-colored or white where composed of pure clay, dark gray where it contains carbonaceous material (charcoal-like plant de- bris), or brown where a little iron oxide is present. Ordinarily the coarse sand and pebble-size fragments in these rocks are somewhat rounded from being rolled along stream beds. If these larger fragments are angular, indicating very rapid or unusual conditions of deposition, special names are applied to the rocks by geologists. Thus sandstone with angular grains is called grayivacke or arkose, where the rocks are dark or light gray, respectively. Coarse-grained rocks equivalent to conglomerate, but with the large fragments being angular rather than rounded, are called breccia. These types, particularly graywacke, are abundant in California. In addition to detrital material carried mechanically by water, wind, glaciers, and landslides, large amounts of the soluble constituents of rocks are also carried by being dissolved in water. Although mountain streams may appear to be very pure to the taste, they always carry significant amounts of chemical material in solution, as can be deter- mined by chemical analysis. This, combined with evaporation of the water, is how interior basin lakes, such as Great Salt Lake and the Salton Sea, become so salty. Under favorable conditions, some or all of this dissolved material is precipitated by evaporation of lakes, or of bays that become isolated from the sea, to form deposits of great com- mercial value. Such deposits, known collectively as evaporites, include extensive layers of various salts, particularly rock salt and gypsum. Precipitation of material dissolved in the waters of lakes and seas is also accomplished in a more complex manner by living organisms. Marine animals such as corals and shellfish extract calcium carbonate from the water in order to construct their shells of calcite; then when the animals die their shells accumulate on the ocean bottom. Where conditions are favorable, accumulations of such shell material are suffi- ciently large to form extensive thick beds that become limestone when cemented together. In a like manner, the microscopic aquatic plants called diatoms extract silica (silicon dioxide) from water to make their: decorative siliceous shells. Where silica is abundantly dissolved in water, diatoms reproduce so rapidly that pure white deposits of these tiny shells accumulate to form diatomite, a rock of commercial value. The study of sedimentary rocks is very important to our interpre- tation of geologic history. These are the rocks in which fossils are found, from which the life history of the earth is studied. In addition, the nature of sediments often yields much information regarding climate and surface relief during the time of deposition; and folding and fault- ing of originally flat strata testify to periods of mountain building (orogeny). And, of course, these rocks tell us of the former distribu-, rion of seas so that we can reconstruct the history of geographic changes of the earth's surface. 15 3. Metamorphic rocks are those that have formed by recrystallization of igneous or sedimentary rocks under the influence of heat, pressure, and chemically active fluids deep within the crust of the earth. Such changes have taken place below the melting temperatures of the rocks, otherwise the resulting rocks would be igneous. Perhaps metamorphism and the metamorphic rocks can be most easily [understood if one remembers that chemical compounds (including min- erals) are stable only within limited chemical and physical environments. For example, we know that at high temperatures and in the presence of oxygen, common organic compounds like wood and coal combine with oxygen to burn and produce new compounds, principally carbon di- oxide and water. In other words, wood and coal are not stable at high temperatures in the presence of oxygen. In a similar way, many of the minerals found at or near the surface of the earth are not stable under the high pressures and temperatures that exist at depths of several miles. When buried to such depths, these minerals react with each other, and with fluids present in the rocks, to form new minerals. Geologists call this process metamorphism. (Con- versely, when these minerals formed at high temperatures and pressures become exposed at the surface by mountain building and erosion, they jare relatively unstable and gradually react with water and the atmos- phere to form new minerals that are stable under surface conditions. The latter process is called weathering.) As an example of the metamorphism of a sedimentary rock, we might trace the transformation of mudstone, a sedimentary rock, to slate and schist, two types of metamorphic rock. When deposited on the ocean floor mudstone is composed predominantly of very fine-grained clay land quartz. As a thick sequence of sediments is deposited on our mud- istone, it is gradually buried to a depth of many thousands of feet, that is, jto an environment of higher temperatures and much higher pressures than exist at the surface. Even here, under a static load, the minerals may resist recrystallization and the rock remain a mudstone. But chemical (reaction and recrystallization within such a rock in this environment will be triggered by the introduction of shearing stresses that accompany mountain building within the earth's crust. Abundant fine-grained mica, a flaky mineral, forms at the expense of the clay, with the tiny new flakes 'all oriented in one plane parallel to the shearing forces. The resulting rock is slate, a hard, fine-grained rock with remarkably uniform platy cleavage. Quartz is stable under these conditions, so remains as tiny (grains, but both it and the mica are too fine-grained to be identified ; without a microscope. If the shearing stresses increase and the temperature rises a little, bunches of the tiny mica flakes in the slate recrystallize to form larger 16 mica flakes that are also roughly parallel to each other, while the tiny quartz grains recrystallize to form larger quartz grains. The resulting rock is a schist, a flaky, shiny rock with mineral grains sufficiently large to be seen with the unaided eye, and entirely different in appearance from the original mudstone. Other types of metamorphic rocks are the result of recrystallization of rocks under the influence of moderate heat and low or high con fining pressures, without shearing stresses playing an important role. Under such conditions there is no preferred direction of crystal growth, and the new rocks will be massive, without foliation or cleavage. The greenstone specimen in the set is an example of this type of metamorphic rock, derived from basalt. Because we are able to reproduce many of these reactions in the labor atory, both the minerals and textures of the various metamorphic rocks reveal much information to the geologist regarding the conditions under which they were formed. In addition, the exposure of metamorphic rocks at the surface in any area indicates uplift or mountain building: there at some time in the past, and removal by erosion of a considerable thickness of overlying rocks. 'DeAcnifotioHt oj Specimen* it t&e Set Characteristic outcrop of thin-bedded chert. In many places th< chert beds are highly contorted. Photo by Mary R. Hill. 19 1. BARITE Composition— barium sulfate (BaSOO Color— white, but with impurities inclining to yellowish, gray, or brown Luster — vitreous Hardness— 2.5 to 3.5 Streak— white Cleavage— three perfect cleavages Specific gravity— 4.5 Crystal system— orthorhombic Barite is a heavy white or light-colored mineral composed of barium, sulfur, and oxygen. It is a relatively soft mineral, so that it can be scratched with a copper penny, but not with the fingernail. The crystals are tabular and have three perfect cleavage directions that yield tablet- shaped fragments when crushed. However, most barite is too fine- grained for crystals or cleavage to be observed without the aid of a microscope. Fine-grained aggregates of barite may appear similar to limestone or quartz, but the high specific gravity readily distinguishes it from these and other similar-appearing rocks and minerals. This quality can easily be detected by hefting the specimen and comparing its apparent weight with that of a specimen of limestone, calcite, or quartz of similar size. Barite is commonly found in veins containing metallic minerals, such as ores of silver, lead, and copper. It also occurs as veins and masses of pure barite, especially associated with limestone. The principal commercial use of barite employs its high specific gravity. The mineral is finely ground and added to oil well drilling mud to increase the weight of the mud in order to confine gas pressures en- countered in drilling. It is also used in the paint industry and in several other industrial applications. Barite has been mined at a number of widely distributed localities in California. 20 2. CALCITE Composition—calcium carbonate (CaCOs) Color— Most commonly white, gray, or clear, rarely bluish. With impurities may be variously tinted red, green, yellow, brown, or black Luster— vitreous or earthy Hardness — 3 Streak— white Cleavage — three highly perfect cleavages at oblique angles (rhombohedral) Specific gravity— 2.7 Crystal system— hexagonal Calcite is a relatively soft mineral, and can be scratched by a copper penny. Its three perfect cleavages yield characteristics rhomb-shaped fragments bounded by flat, shiny surfaces. The mineral is easily soluble in acids, with liberation of carbon dioxide (C0 2 ) making the reaction effervescent. Since this reaction can be observed even when weak acids like vinegar (acetic acid) are put on calcite powder, it is a good test for the mineral. Calcite is often confused with quarts, but can easily be scratched with a knife whereas quartz cannot. Calcite is one of the most common minerals of the earth's crust. It is the predominant mineral comprizing such abundant rocks as limestone and marble, and also occurs as a subordinate constituent in many other rocks. White veins of pure calcite can be found penetrating various rocks in many areas of California. Calcite has numerous uses as the predominant constituent of lime- stone, so these are mentioned under the description of that rock. Large water-clear crystals or cleavage rhombs of calcite are called Iceland spar. Such crystals give a double image of things viewed through them, and are of value for the manufacture of various optical instruments, particularly in the production of Nicol prisms to produce polarized light. 21 3. CHROMITE Composition— oxide of chromium end iron (FeCn'OO Color — block to dark brownish black Luster— metallic to submetailic, sometimes pitchy Hardness— 5 Streak— brown Cleavage— none Specific gravity— 4.6 Crystal system— isometric Chromite is a heavy, black mineral that has about the same specific gravity as barite. Its hardness is close to that of steel, but it can usually be scratched with a knife blade. The brown streak (powder of the mineral) ordinarily distinguishes chromite from other similar-appearing heavy black minerals, for most of these have black streaks. Chromite occurs only in peridotite and serpentine, two closely related rock types, or in sand and gravel derived from these rocks. In its host rocks it is normally present as tiny disseminated grains, but in places is concentrated in masses sufficiently large to be mined. Chromite is the only ore mineral of chromium, an important metal that strongly resists corrosion. This metal is an important constituent of stainless steel, and is also used as thin plating to protect the surfaces of other metals that corrode more easily. Chromite is also used in the manu- facture of chromium chemicals, that find wide application in leather tanning, pigments, and other industrial uses. In addition, chromite has such a high melting point that it is made into bricks for lining steel- making furnaces. Chromite is a relatively common ore mineral in the Coast Ranges, Sierra Nevada, and Klamath Mountains, and California is the leading producer of the mineral in the United States. 22 4. CHRYSOTILE Composition— hydrous magnesium silicate (KUMgySuO) Color— pale green to white Luster— silky Hardness— about 3 Streak— white Cleavage— perfect, fibrous Specific gravity— 2.2 Crystal system— monoclinic Chrysotile is a mineral that separates into strong, flexible fibers. It is found as veins in serpentine rock, normally with the fibers ori'ented nearly perpendicular to the direction of the vein. These veins range in thickness from that of a pencil line to more than an inch, but most chrysotile veins are less than % inch thick. The color of chrysotile in the vein is green to greenish white, rarely golden, but when separated into a fluffy mass the fibers are white. Individual fibers are somewhat stronger than silk, and a bundle of fibers the diameter of a pencil lead cannot be broken by pulling between the fingers. Chrysotile is the principal asbestos mineral of industry. The term "asbestos" is a commercial one, applied to half a dozen fibrous minerals that are used primarily because of their fibrous characteristics. Of these, chrysotile is one of the strongest, and is also the most abundant; so it accounts for about 95 percent of the asbestos produced in the world. Serpentine containing chrysotile veins is mined at several localities in California for production of asbestos. The rock is crushed and passed over tilted shaking screens, allowing the small serpentine fragments and dust to fall through the screens and the fluffy fibers to be lifted from the end of the screens by air suction. Close-up of '/2-inch chrysotile veins in ser- pentine. 23 ^. W -;^ ;-* ;; ; pp-a 5. COLEMANITE Composition— hydrous calcium borate (Cai'BoOn.ShteO) Color— colorless to white Luster — vitreous Hardness — 4 Streak— white Cleavage — one perfect cleavage Specific gravity— 2.4 Crystal system — monoclinic Colemanite is similar in appearance to calcite, but the two can be differentiated by some simple tests. When a small fragment of colemanite is held with tweezers in a gas flame, the fragment decrepitates (tiny fragments are thrown off violently, with a crackling sound), and the flame is colored green. Also, colemanite does not effervesce in acids, as does calcite. Most of the colemanite specimen material for these sets is composed of aggregates of small crystals, and the flat, shiny surfaces are crystal faces rather than cleavage surfaces. Colemanite deposits originated by evaporation of boron-rich water in desert basins. Once crvstallized, however, colemanite is not easilv dis- solved in water, so it is abundant in some of the very old dry-lake sediments of the Mojave Desert region. Before the discovery of borax deposits in California, colemanite was mined and converted into borax (sodium borate) by chemical processes. Now it is mined largely for use in ceramic glazes. Cluster of large colemanite crystals, the largest about 2 inches across. 24 6. EPIDOTE Composition— silicate of calcite, aluminum, and iron (HCai'(AI,Fe)aSi30i.») Color— pistachio green or yellowish green to dark green Luster— vitreous Hardness— 6 to 7 Streak— white Cleavage— one perfect cleavage Specific gravity— 3.4 Crystal system— monoclinic Epidote is a colorful mineral of metamorphic origin that is relatively common in some parts of California. Its characteristic pistachio green color, hardness (harder than a knife blade), and relatively high specific gravity are usually sufficient for identification of this mineral. Most! commonly it is found in compact masses of tiny grains, the latter too small to be individually distinguished without the aid of a hand lens orr microscope. But in places it is found in coarsely crystalline aggregates so that the elongate, vitreous crystals can be easily distinguished. It is< most commonly associated with white quartz and calcite and reddish- brown garnet. Epidote is one of the characteristic minerals formed when limestone is metamorphosed by high temperatures and fluids emanating from granitic magma. Thus it is most abundant in areas where granitic rocks are exposed. It is also a constituent of some schist, but is not particularly, apparent in these rocks. Epidote has no commercial value, but is significant to the geologisti in that its presence near granite commonly indicates metamorphic zones in which tungsten and molybdenum ores may be found. 25 ' 7. FELDSPAR Composition— a group of minerals, alumino silicates of potassium, sodium, and calcium Color— white, pinkish, gray, clear; rarely pale yellow or green Luster— vitreous Hardness— 6 to 6.5 Streak— white Cleavage— perfect in two directions Specific gravity— 2.5 to 2.9 Crystal systems— monoclinic and triclinic Feldspar is not a single mineral, but is the name applied to a group of closely related and similar-appearing minerals that have almost iden- tical lattice structures. Collectively, they are the most abundant rock- forming minerals in the crust of the earth. Geologists apply separate names to individual minerals in the feldspar group, but they are so similar in physical properties that they are very difficult to distinguish by simple tests. The most abundant feldspar minerals are microcline and orthoclase (both of which have the chemical formula KAlSi 3 8 ), albite (NaAlSi 3 8 ), and anorthite (CaAl 2 Si 2 08). The specimen is orthoclase. Characteristics that help to distinguish feldspar from other similar- appearing minerals are its hardness, cleavage, and the nature of its oc- currence. None of the minerals of the feldspar group can be scratched by a knife blade, but they can be scratched with a steel file. They have two prominent cleavage directions at, or very nearly at, right angles to each other. As to the nature of occurrence of feldspar minerals, they comprise the bulk of the light-colored minerals in igneous rocks. They are espe- cially distinguishable in such rocks as white or light-colored crystals that are lath shaped or rectangular in outline. Grains of quartz, the other principal light-colored mineral in igneous rocks, tend to have irregular outlines and do not exhibit flat cleavage faces. Feldspar is also an important constituent of many sedimentary and metamorphic rocks. Generally it is considerably more difficult to iden- tify feldspar in these rocks than in igneous rocks because of the lack of the distinctive rectangular outline. The feldspar minerals are commercially important, being used in large quantities in the manufacture of ceramic glazes and glass. In addition, the alteration of feldspar by weathering is the principal source of clay, an extremely important constituent in most soils. 26 8. LIMONITE Composition— hydrous iron oxide (Fe-'O? with H2O) Color— various shades of brown, ochre yellow, rarely black, and in places with an iridescent coating Luster— dull and earthly, but submetallic in places Hardness— soft to moderately hard Streak— yellowish-brown Cleavage— none Specific gravity— about 3V2 to 4 Crystal system— none (not a crystalline substance) Limonite is not a mineral in the technical sense, for it is a mixture of molecules of iron oxide and water in various proportions, without a fixed arrangement of the constituent atoms (such natural materials are commonly called "mineraloids"). However, it is a widespread material at and near the surface of the earth, and is one of the principal inorganic "coloring agents" of our landscapes, lending yellowish, brown, and reddish-brown colors to the surface rocks and soils. Limonite is a product of weathering (oxidation) of minerals that contain iron. Typically it is disseminated in most soils, giving them brown or buff colors, and is present as thin coatings along fractures in the rock below the soil. Traces of such brown limonite coatings can be observed on some of the rock specimens in the set, masking the true color of the rock where it is present. The limonite specimen comes from the weathered surface of a mas- sive pyrite ore body in Shasta County, and may have a honeycomb structure. Although common and widely distributed, limonite does not occur in masses of high purity that are sufficiently large to be mined as iron ore. Its principal use is as a pigment in paints. 27 - 9. MAGNESITE Composition— magnesium carbonate (MgCOs) Color— white or gray; yellow or brown with impurities Luster— vitreous Hardness — 3V2 to 5 Streak— white Cleavage— three perfect cleavage directions, similar to calcite, but not visible on the specimen Specific gravity— 3.0 to 3.2 Crystal system— hexagonal Magnesite is closely related to calcite, and the two minerals are similar in most physical properties. However, magnesite is slightly heavier and harder than calcite, is less readily soluble in acids, and is not nearly as abundant. Magnesite cannot be scratched with* a copper penny but it can be scratched with a knife blade, and it will effervesce only in strong hot acids. Crystals of magnesite have perfect cleavage, but the specimen is made up of an aggregate of very tiny magnesite grains so this cleavage is not visible. Large crystals of this mineral are rare. In California, magnesite occurs principally as white veins in serpentine, a dark green rock. Such veins are characteristically an inch or so thick, but in places they are several feet thick. Magnesite has been mined from many of the larger veins, and used in the manufacture of a special type of cement (called magnesium oxychloride cement, or simply "magne- site"). The mineral also is mined for production of magnesium chemicals and magnesium metal. = ro o o> o — o — "5 Q - Z *d Si HOWl INCE o e o_ o m ll o o> E "» (0 > o O c < o o _l o R, 1938, ( eralized fr ,000 O z o ° a> O o cm 35 Q hi Jenkins c units scale o cr N a> - E o o o ^ 0.2 Zi < nces fro 0. Geol Californi tr > O M- o o o ^ S UJ q. O a. 2 z - O. E UJ o " ? e> OZ c tr :: :: : • t < < r « O o c r77r .-.* , i,v -vsfc^v ^V ^ v?:>j :':' : <: : . .V ...'>: 35. GREENSTONE Type of rock: metqmorphic, fine-grained, massive Mineral content: chlorite, feldspar Greenstone is a fine-grained rock derived by metamorphism of certain volcanic rocks, such as basalt, that are relatively rich in iron and mag- nesium. Under the influence of moderately deep burial and non- directional (hydrostatic) stresses, the black minerals in the basalt re- crystallized to form chlorite, a green micaceous mineral. The resulting metamorphic rock, greenstone, has an overall dull green or dark green color, and is massive. (In regions where directional stresses were present at the time of metamorphism, the same types of volcanic rocks become foliated because of the parallel orientation of the new chlorite flakes. This yielded a dull-green, flaky metamorphic rock known as chlorite schist). Most of the greenstone in California was derived from basalt that was erupted as submarine lava flows. Such eruptions commonly resulted in excess silica being dissolved in hot sea water, and subsequently precipi- tated as chert when the water cooled. As a result, greenstone and chert are often found together, and have about the same distribution in California. Crushed greenstone is used for roofing granules. ■ Wms : SELECTED REFERENCES i Minerals Minerals and how to study them, by E. S. Dana, revised by C. S. Hurlbut, 1949. John Wiley & Sons, Inc., New York, 323 p. This is well-illustrated book, highly recommended for the beginner interested in mineralogy. Contains well-organized determinative tables for identification of k minerals. , Rocks and minerals, by H. S. Zim, P. R. Shaffer, and Raymond Perlman, 1957. Golden Press, Inc., Rockefeller Center, New York 20, N.Y., 160 p. An exceptionally well-illustrated and authoritative treatment of minerals and rocks, suitable for use at the elementary level. Minerals and rocks, by H. W. Ball, 1959. Hanover House, Garden City, N. Y., 96 p. Very good photographs (both color and black and white) of mineral and rocks specimens. The text is a well-written and non-technical discussion of the mineral kingdom. Crystals and crystal growing, by Alan Holden and Phylis Singer, 1960. Anchor Books, Doubleday and Company, Garden City, New York. Science Study Series Number 7 (paperback). Available to students and teachers through Wesleyan University Press, Columbus 16, Ohio, 320 p. An excellent, well-illustrated discussion of crystals and their internal structures. Gives recipes and instruction for growing crystals. Although it is not limited to a discussion of minerals, this book is highly recommended as an aid in studying minerals. Rocks The rock book, by Carroll L. Fenton and Mildred A. Fenton, 1946. Doubleday and Company, Inc., Garden City, N. Y., 357 p. Principles of geology, by James Gilluly, A. C. Waters, and O. A. Woodford, 1959. W. H. Freeman and Company, San Francisco, 534 p. Left. Greenstone exposure, showing "pillow structure" that has a boulder-like appearance. This structure is characteristic of many of the greenstone outcrops in California, and indicates the lava was erupted under water. 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