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 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. 
 

 
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30 
 
 10. MAGNETITE 
 
 Composition — iron oxide (Fe Ot) 
 
 Color— black 
 
 Luster — metallic 
 
 Hardness— 6 
 
 Streak— black 
 
 Cleavage— none 
 
 Specific gravity— 5.2 
 
 Crystal system— isometric 
 
 A4agnetite is a heavy black mineral that is easily identified because it 
 is strongly attracted by a magnet. A few other minerals are weakly 
 attracted by magnets, but they either do not look like magnetite or 
 their streaks are not black. Magnetite from some localities acts as a 
 natural magnet (that is, will attract iron filings), and is called lodestone. 
 
 Magnetite is a relatively abundant mineral, for it is a minor con- 
 stituent of many different types of rocks. Where it is sparsely dissem- 
 inated as large grains in some rocks, the characteristic octahedral crystal 
 form of magnetite is often identifiable. However, the grains are most 
 commonly irregular in shape. 
 
 Being an oxide, magnetite is highly resistant to alteration by weather- 
 ing, so grains of it tend to accumulate in sands. As a result, the easiest 
 way to find magnetite is to thrust a magnet into the sand of almost any 
 stream or beach. 
 
 Magnetite is a very important mineral to our industrial economy, for 
 it is one of the principal ore minerals of iron. 
 
31 
 
 11. MANGANESE OXIDES 
 
 Composition— largely manganese oxide (several different minerals) 
 
 Color— black, rarely brownish black 
 
 Luster— metallic to dull 
 
 Hardness— 1 to 6 
 
 Streak— dark brown to black 
 
 Cleavage— none 
 
 Specific gravity— 4 to 5 
 
 Manganese combines with oxygen in a variety of different propor- 
 tions and different lattice arrangements to yield a variety of minerals. 
 These are typically fine grained, and difficult to tell apart. They vary 
 widely in hardness, from soft, soot-like material to hard metallic-looking 
 masses. Manganese oxide is most apparent as thin black crusts or stains 
 on natural fracture surfaces of many different rock types, particularly 
 near the surface where the rocks are weathered. The most striking of 
 these crusts show branching dendritic patterns, commonly appearing 
 as if moss had been pressed into the rock. 
 
 Where manganese oxide occurs in relatively pure masses it is mined 
 as manganese ore, another very important raw material in our economy. 
 Manganese metal extracted from this ore is alloyed with iron to make 
 steel. Manganese oxide is used in the manufacture of various chemicals, 
 to decolorize glass, and in flashlight batteries. 
 
 Manganese oxide has been mined at numerous localities in California. 
 In the Coast Ranges, Sierra Nevada, and Klamath Mountains these de- 
 posits are typically found in chert. In the Mojave Desert region they are 
 associated with volcanic rocks. 
 
32 
 
 Composition— silicate of potassium and aluminum (HsKAl-iSisOiO 
 
 Color— clear, white, gray, or greenish 
 
 Luster— vitreous to pearly 
 
 Hardness— 2 
 
 Streak— white 
 
 Cleavage— very perfect in one direction 
 
 Specific gravity— 2.7 to 3.1 
 
 Crystal system— monoclinic 
 
 Mica is the family name of a group of closely related minerals that 
 have very perfect cleavage in one direction only, and can be split into 
 exceedingly thin sheets or flakes. The most abundant minerals in this 
 group are muscovite (represented by the specimen) and biotite, a black 
 or dark-brown mica. The color difference is caused by the presence of 
 iron and magnesium in biotite. These minerals are important constituents 
 of many types of igneous and metamorphic rocks. 
 
 Mica is usually easy to identify because of its perfect cleavage, and 
 the fact that thin cleavage plates or flakes are very flexible and elastic. 
 It is sufficiently soft to be scratched with the fingernail, a property that 
 helps to differentiate it from other minerals with which it is commonly 
 associated. 
 
 Various of the mica minerals are of considerable commercial value. 
 Large cleavage plates of muscovite are used as insulation in the manu- 
 facture of electrical apparatus. A variety of biotite, called vermiculite, 
 swells when heated, and is extensively used in heat and sound insulating. 
 A pink, lithium-bearing member of the mica family, called lepidolite, 
 is used as a source of lithium and in the manufacture of glass. 
 
33 
 
 13. PYRITE 
 
 Composition— iron sulfide (FeSa) 
 
 Color— pale brassy yellow 
 
 Luster— metallic 
 
 Hardness— 6 
 
 Streak— greenish black 
 
 Cleavage— none 
 
 Specific gravity— 5 
 
 Crystal system— isometric 
 
 Pyrite is a heavy, metallic-appearing mineral that is often mistaken 
 for gold because of its color. However, it is not really metallic, but 
 brittle, and has a greenish-black streak (gold is malleable, and has a gold- 
 colored streak). Where it is disseminated in rocks, pyrite commonly 
 exhibits its most frequent crystal form, the cube. These cubic crystals 
 are so diagnostic of pyrite that when they are present, the mineral can 
 be recognized even though the crystal surfaces are altered to brown 
 limonite, which is often the case. But much pyrite is fine-grained or 
 massive, and does not have crystal faces. The fact that pyrite cannot be 
 scratched with a knife blade is usually sufficient to distinguish it from 
 other similar-colored sulfide minerals, for with one exception they are 
 all softer than steel. The only other sulfide mineral equally hard is a 
 relatively rare one, marcasite, that has the same chemical formula as 
 pyrite, but a different lattice structure. 
 
 Pyrite is a relatively abundant and widespread mineral, occurring in 
 places in all types of rocks. It is perhaps best known for its occurrences 
 in quartz veins, where it is commonly associated with gold and other 
 metallic ore minerals. Thus, when pyrite is found in a quartz vein, it is 
 an indication to the geologist and prospector that valuable metals may 
 be present in the vein. Pyrite itself is of value as an ore of sulfur, and a 
 large deposit of it in Shasta County is mined for this purpose. 
 
34 
 
 14. QUARTZ 
 
 Composition— silicon dioxide (SiO) 
 
 Color— clear, white, or gray, rarely purple or yellow 
 
 Luster— vitreous 
 
 Hardness— 7 
 
 Streak— white 
 
 Cleavage— very poor (conchoidal fracture much more conspicuous) 
 
 Specific gravity— 2.65 
 
 Crystal system— hexagonal 
 
 Quartz is one of the most abundant minerals of the earth's crust, being 
 an important constituent of many types of rocks and the major con- 
 stituent in most sand. It is a hard, brittle mineral that has no apparent 
 cleavage. Like glass, it breaks with a characteristic conchoidal fracture. 
 It cannot be scratched with a knife, as can many of the minerals with 
 which it might be confused, and it will easily scratch glass. 
 
 Most people have picked up or seen quartz crystals, which are char- 
 acteristic six-sided prisms terminated by six-sided pyramids. Such crys- 
 tals usually are clear and colorless, although some are gray (smoky 
 quartz) or violet (amethyst). But crystal faces develop only in special 
 environments, such as open fissures or cavities in rock, and most quartz 
 occurs as disseminated irregular grains in many different types of rocks. 
 It also occurs as massive white aggregates of irregular grains, such as 
 milky vein quartz. Quartz is much more resistant to alteration by weath- 
 ering than are most other minerals, so it is a prominent constituent of 
 beach and stream gravels, and smaller grains accumulate as sand. 
 
 Quartz is mined for a wide variety of industrial uses. Large quantities 
 are processed in the manufacture of glass and ceramic materials, and are 
 used in the manufacture of cement and as a metallurgical flux. Carefully 
 cut sections of clear quartz crystals are used as crystal oscillators to 
 control the frequency of radio transmitters, and clear or attractively 
 colored crystals are cut for gem stones. 
 
 In many places veins of milky quartz contain disseminated gold or 
 minerals of other valuable metals. Where the content of one or more 
 valuable metals is sufficiently high, such veins are mined. 
 
35 
 
 15. QUARTZ, variety chalcedony 
 
 
 Composition — silicon dioxide (SiO-*) 
 
 Color— translucent white or gray; but with impurities may be almost any color, or 
 
 opaque; commonly banded in white, gray, and other colors 
 Luster — waxy 
 Hardness — 7 
 Streak— white 
 
 Cleavage— none (fracture conchoidal) 
 Specific gravity— 2.6 
 Crystal system— hexagonal, but always so fine-grained that individual crystals cannot 
 
 be observed except by very great magnification 
 
 Chalcedony is the name applied to microfibrous masses of quartz that 
 have a waxy luster. The individual quartz fibers are so small that they 
 can be seen only with special apparatus and a high-power microscope. 
 Minute water-filled pores between the tiny fibers result in masses of 
 chalcedony having lower specific gravities than quartz, and also are 
 probably the cause of chacedony having a waxy luster instead of being 
 vitreous like typical quartz. However, the hardness is 7, as is that of 
 coarsely-crystalline quartz. 
 
 Pure chalcedony is colorless, white, or grayish, and is translucent or 
 transparent. However, it is commonly brown, red, yellow, green, bluish, 
 or dark gray because of small amounts of finely divided colored min- 
 erals disseminated among the quartz fibers. 
 
 Chalcedony is relatively abundant in California. It is the predominant 
 constituent of chert, a common rock in the state, and occurs lining or 
 filling fissures and other cavities in various rocks, particularly volcanic 
 rocks. When deposited in wide fissures, masses of chalcedony often as- 
 sume globular or stalactitic forms. Chalcedony is also found in many 
 places where it has replaced other materials, commonly retaining the 
 form and texture of objects replaced. An example of the latter is petri- 
 fied wood. 
 
 The principal use of chalcedony is as gem material, and many gem 
 names are applied to variously colored or patterned chalcedony. When 
 banded, it is called agate, when red or yellow and opaque it is called 
 jasper, when translucent green it is chrysoprase, and when translucent 
 orange it is cariielian. Flint is dark gray or brown chalcedony. 
 
 Left. Cluster of colorless quartz crystals, illustrating characteris- 
 tic prismatic form terminated by 6-sided pyramid. 
 
36 
 
 ■MM«HM| 
 
 
 16. SULFUR 
 
 
 Composition— sulfur (S) 
 Color— sulfur yellow, grayish 
 Luster— resinous 
 Hardness— 1.5 to 2.5 
 Streak — white 
 Cleavage — poor 
 Specific gravity— 2 
 Crystal system— orthorhombic 
 
 Sulfur is a soft, yellow mineral that easily burns with a bluish flame, 
 giving off the pungent odor of sulfur dioxide. These characteristics, 
 in addition to its resinous luster, allow it to be easily distinguished from 
 other minerals. 
 
 Sulfur is a relatively common mineral in active or recently active 
 volcanic areas, where it typically forms incrustations in fissures and 
 on rocks at the surface. In places it occurs in large, relatively pure de- 
 posits that can be mined. Notable deposits of sulfur in California are 
 found in Alpine County and in the Last Chance Range, Inyo County. 
 
 It is principally used to manufacture sulfuric acid, which is very im- 
 portant to industry. Sulfur is also used in fertilizers, insecticides, ex- 
 plosives, rubber, and paper. 
 
 %;«iif :; ' V 
 
 A 
 
37 
 
 ■ 
 
 17. 
 
 A\LV. 
 
 Composition— magnesium silicate (H-'MgsSiiOu) 
 
 Color— white, gray, light to dark green 
 
 Luster— pearly or greasy 
 
 Hardness — 1 
 
 Streak— white 
 
 Cleavage— perfect in one direction 
 
 Specific gravity— 2.7 
 
 Crystal system— monoclinic 
 
 Among the most distinguishing features of talc are its softness and 
 greasy feel. It is easily scratched with the fingernail, and will even 
 make a mark on cloth. None of the common minerals with which it 
 might be confused are this soft. 
 
 Most talc occurs in fine-grained, impure masses called talc schist 
 when foliated, or soapstone if they are massive and compact. In many 
 places, though, coarse flaky talc is found that is pale green or white, and 
 may resemble muscovite. Aside from the recognizable difference in the 
 hardness of these two minerals, cleavage flakes of talc are not elastic 
 (that is, they will stay bent when flexed), in contrast to mica flakes 
 that are elastic, which do not stay bent. 
 
 Talc is found only in metamorphic rocks, ordinarily formed from 
 dark minerals rich in magnesium and iron. In fact, one of the common- 
 est occurrences of the mineral in California is in talc schist derived by 
 metamorphism of serpentine, itself a metamorphic rock. 
 
 Talc is a valuable industrial mineral. It is finely ground for use in the 
 manufacture of ceramics, paint, and rubber, and in the preparation of 
 special lubricants, talcum powder, and insecticides. Slabs of soapstone 
 are used for electrical switchboards and laboratory table tops. 
 
 The southern Death Valley region of California is one of the prin- 
 cipal sources in the world for talc of high purity that is needed by man- 
 ufacturing industries. 
 
 Lefi. Cluster of sulfur crystals, the largest here about an inch 
 long, 
 
38 
 
 Characteristic outcrop appearance of 
 granite in the mountains of California. 
 Photo by C. W. Chesterman. 
 
39 
 
 18. GRANITE 
 
 : 
 
 Type of rock: igneous, coarse grained 
 
 Mineral content: feldspar, quartz, biotite, hornblende 
 
 Granite is a coarse-grained igneous rock, the mineral grains being 
 easily seen with the unaided eye and being largely uniform in size. This 
 rock is composed predominantly of feldspar and quartz, both white or 
 light-colored minerals, so that large masses of granite typically appear 
 white or light gray from a distance. Minor amounts of biotite (black 
 mica) or hornblende (also black), or both of these, are present in granite 
 and give the rock a speckled appearance at close range. 
 
 The terms "granite" and "granitic rocks" are commonly used to desig- 
 nate a number of similar-appearing plutonic igneous rocks that are 
 slightly different in their chemical composition. Such differences are 
 significant to the geologist, so for detailed study he applies different 
 names to these rocks according to the relative amount of quartz they 
 contain, and the types and relative abundances of the feldspars and dark 
 minerals present. Included among the granitic rocks are granite, grano- 
 diorite, diorite, monzonite, and syenite, all generally similar in appear- 
 ance to the untrained eye, but distinguishable on the basis of detailed 
 mineral analyses of the rocks. 
 
 Granite is formed at considerable depth by very slow cooling and 
 crystallization of magma. Yet it is one of the most abundant rock types 
 exposed at the surface in California, testifying to the great extent of 
 uplift and erosion in the mountainous areas where it is found in the state. 
 
 Recently the amount of decay of certain radioactive elements present 
 in granite specimens collected at various localities in California has been 
 studied. This work indicates that most of the granite in the state crystal- 
 lized 75,000,000 to 100,000,000 years ago. Some masses, however, are 
 much older. 
 
 Granite is of commercial value as building stone. It is also of con- 
 siderable indirect economic significance, for the formation of many 
 ore deposits, especially those of gold, copper, and tungsten, are related 
 to the igneous phenomena associated with the formation of granitic 
 rocks. 
 
40 
 
 <..$ 
 
 19. GABBRO 
 
 Type of rock: igneous, coarse grained 
 Mineral content: feldspar, hornblende 
 
 Gabbro is a coarse-grained igneous rock composed essentially of white 
 feldspar and one or both of the similar-appearing black minerals, horn- 
 blende and augite. Much of the gabbro in California is similar to the 
 specimen in the collection, containing only hornblende as the black 
 mineral. 
 
 Gabbro is a plutonic igneous rock, like granite, but the two are sig- 
 nificantly different in chemical composition. Containing substantially 
 less silicon and more iron and magnesium than granite, gabbro has no 
 quartz and is rich in dark minerals. Thus the rock has an over-all dark 
 color, and typically appears coarsely mottled at close range. 
 
 Gabbro is found at numerous widely distributed localities in Cali- 
 fornia, but individual outcrop areas are normally smaller than those of 
 granite. 
 
41 
 
 
 20. RHYOLITE 
 
 Type of rock: igneous, fine-grained 
 
 Mineral content: feldspar, quartz, hornblende, biotite, volcanic glass 
 
 Rhyolite is a light-colored volcanic rock wherein most or all of the 
 individual mineral grains are too small to be identified without the aid 
 of a microscope. It originates when magma of granitic composition is 
 erupted at the surface as a lava flow, so it has a mineral content similar 
 to that of granite. Most rhyolite has a light gray, pinkish, or pale 
 purplish color. In places it exhibits prominent flow banding caused by 
 alignment of gas bubbles. 
 
 Microscopic examination of rhyolite specimens reveals that they con- 
 sist predominantly of very tiny crystals of feldspar and quartz, with 
 some interstitial volcanic glass between the crystals. Scattered large 
 crystals of one or more of the minerals feldspar, quartz, hornblende, or 
 biotite are commonly present and visible with the unaided eye, having 
 started crystallizing from the magma long before eruption, but these 
 normally constitute only a small percentage of the rock. 
 
 Rhyolite is one of the characteristic volcanic rocks of continental 
 areas, and is found at numerous localities in California. 
 
 Left. Photomicrograph of rhyolite, magnified 
 about 75 diameters. The large white grain at 
 lower left is quartz, the white tabular crystals are 
 feldspar, and the large dark-colored crystals 
 are biotite (mica). The groundmass is tiny laths 
 of feldspar in a glassy matrix. 
 
42 
 
 
 21. BASALT 
 
 Type of rock: igneous, fine-grained 
 
 Mineral content: feldspar, augite, olivine, magnetite, volcanic glass 
 
 Basalt is a dense, fine-grained volcanic rock that is typically black or 
 dark gray. It may contain scattered visible crystals of minerals that 
 formed prior to eruption. It originates by volcanic eruption of the same 
 type of magma that yields gabbro when cooled at great depth. 
 
 When examined under the microscope, basalt is seen to consist of a 
 multitude of tiny lath-shaped feldspar crystals and stubby augite crys- 
 tals, commonly with interstitial volcanic glass. Fine-divided grains of 
 magnetite (visible only with the microscope) are peppered throughout 
 the rock; although they only constitute a small percentage of the rock, 
 they are partly responsible for the black color of basalt. Small grains 
 of olivine, a green, magnesium-rich mineral, are visible in some basalt. 
 
 Spherical or ellipsoidal cavities are present in most basalt, and tend 
 to be particularly abundant in specimens taken from the tops of lava 
 flows. These represent bubbles of gas trapped in the rock when the 
 magma solidified. In many places such cavities are coated with tiny 
 crystals of light-colored minerals deposited from the gasses, and may 
 even be filled with these minerals. 
 
 Basalt is particularly characteristic of the oceanic basins; for example, 
 the Hawaiian Islands are basalt volcanoes that rise from the deep ocean 
 floor. However, large quantities also have been erupted in certain con- 
 tinental areas, and basalt is found in many parts of California. 
 
 ),"*#, <r »' "i 
 
 
 f 
 
 1 * &*& 
 
 s 
 
 y * 
 
 C 
 
 Y a ; 
 
43 
 
 
 22. OBSIDIAN 
 
 Type of rock: igneous, glassy 
 
 Mineral content: none, composed of volcanic glass 
 
 Obsidian is a glassy volcanic rock. It originates when relatively pure 
 granitic magma (essentially free of mineral crystals) flows from fissures 
 at the surface and solidifies so rapidly that crystallization of minerals 
 cannot take place. 
 
 Most obsidian is black or gray, but rarely it is reddish brown. It com- 
 monly exhibits flow banding in the form of thin layers having slightly 
 different shades of color. Like artificial glass, obsidian is quite brittle 
 and has conchoidal fracture (breaks along characteristic curved, shell- 
 like surfaces). Thin edges of obsidian are translucent. 
 
 To primitive man, obsidian was an exceedingly important raw material 
 for the manufacture of tools and weapons. The ease with which frag- 
 ments could be shaped by flaking, and the exceeding sharpness of flaked 
 edges made obsidian, where available, preferable to flint for these pur- 
 poses. (Students and teachers should take heed of this fact in handling 
 obsidian specimens, for it is quite easy to obtain a nasty cut if one is 
 careless.) 
 
 An interesting modern use of obsidian is in the manufacture of the 
 mirror lenses for reflecting telescopes. In some respects obsidian is 
 superior to the best artificial optical glass for this purpose. 
 
 Obsidian is found at a number of localities in California; notably at 
 Glass Mountain, Siskiyou County; Davis Creek, Modoc County; in the 
 Clear Lake area, Lake County; in the vicinity of Mono Lake, Mono 
 County, and at the south end of Salton Sea, Imperial County. 
 
 Left. Photomicrograph of basalt, magnified about 
 75 diameters. The large clear crystal of 
 feldspar and smaller gray grains of augite 
 are embedded in a groundmass of tiny 
 white feldspar crystals, augite crystals, 
 and black magnetite grains. 
 
44 
 
 23. PUMICE 
 
 
 Type of rock: igneous, highly porous 
 
 Mineral content: none, composed of volcanic glass 
 
 Pumice is volcanic glass "foam" that results from the violent erup- 
 tion of obsidian magma heavily charged with gas. An explosive eruption, 
 such as commonly precedes obsidian and rhyolite flows, allows the sud- 
 den expansion of gas that was dissolved in the magma under high pres- 
 sures. Blobs of such magma, blown into the air, cool so rapidly that 
 they solidify before these gas bubbles can escape. Thus pumice consists 
 essentially of gas cells separated by thin cell walls of volcanic glass. This 
 rock is exceedingly light-weight because of the multitude of tiny open 
 cells, and is one of the few rock types that will float on water. 
 
 Because of the sharp cutting edges of its thin glass cell walls, pumice 
 is used as an abrasive material. Large fragments are sawed to make abra- 
 sive blocks, and small fragments are ground for use as the abrasive in 
 metal polishes, rubber erasers, and other products. Pumice fragments are 
 also used an an aggregate to make lightweight concrete. 
 
 Pumice is found in many of the areas where obsidian and rhyolite 
 occur. It is mined in Mono and Siskiyou Counties. 
 
 24. CONGLOMERATE 
 
 Type of rock: sedimentary, very coarse grained 
 
 Mineral content: principally quartz, feldspar, and pebbles of various rocks and minerals 
 
 Conglomerate is a sedimentary rock consisting of rounded pebbles, 
 cobbles, or boulders in a matrix of sand. In other words, it is the most 
 coarse-grained of the sedimentary rocks formed by normal mechanical 
 transportation of debris by running water. Although conglomerate is 
 not as abundant as sandstone and mudstone, it is normally found inter- 
 bedded with these rocks in many sedimentary formations. 
 
 The pebbles, cobbles, or boulders in conglomerate are of the more 
 resistant rocks and minerals that crop out in the drainage basins from 
 which they were derived. Materials that are easily broken or altered by 
 weathering do not ordinarily survive the rigors of stream transporta- 
 tion to the final site of deposition. Thus tough, compact igneous and 
 metamorphic rocks, as well as quartz and chert, tend to be abundantly 
 represented in the coarse fraction of most conglomerates. 
 
45 
 
 z 
 
 25. SANDSTONE 
 
 Type of rock: sedimentary, medium-grained 
 Mineral content: largely quartz and feldspar 
 
 Sandstone is simply what the name implies, sand grains naturally ce- 
 mented into stone. Although quartz and feldspar are normally the 
 most abundant minerals in this type of rock, grains of numerous other 
 minerals may be present in small percentages, depending on the com- 
 position of the rocks through which the streams passed that transported 
 the sand. 
 
 After a bed of sand (or other sediments) has been deeply buried by 
 succeeding deposits, the grains normally become cemented together 
 in one of two ways. Compaction alone is sufficient to convert loose 
 sand into rock, particularly if clay is abundant between the grains. 
 More commonly, water circulating through the sand will deposit min- 
 eral material that actually cements the grains together. The most com- 
 mon of these cementing minerals are quartz, calcite, and limonite. 
 
 Sandstone can be almost any color, depending principally on the 
 color of the cementing mineral, but to a lesser extent on the color of 
 the predominant minerals present. Sand cemented simply by compaction 
 normally yields rocks that are light-to-dark gray. Most sandstones ce- 
 mented by calcite or quartz are light gray. Where limonite is the ce- 
 menting material the rock is brown or reddish, and even a trace of 
 limonite gives the rock a buff color. 
 
 The sandstone specimens in most of the sets contain fossil shells. 
 According to paleontologists, these are species that lived during the 
 Cretaceous Period, about 75,000,000 years ago. The cementing mineral 
 for these sandstone specimens is calcite. 
 
 Sandstone is used primarily as a building stone. However, the greatest 
 commercial value of this rock is an indirect one, for subsurface beds of 
 sandstone are the principal reservoir rocks for our petroleum resources. 
 Under certain specific circumstances, petroleum accumulates in the 
 pore spaces between the sand grains, and can be pumped out from wells 
 penetrating these beds. 
 
 Sandstone is very abundant in California, particularly in the areas 
 designated as "Tertiary sedimentary rocks" and "Mesozoic sedimentary 
 rocks" on the map. 
 
46 
 
 26. MUDSTONE (Shale) 
 
 Type of rock: sedimentary, very fine-grained 
 Mineral content: predominantly clay 
 
 Mudstone and shale are closely related sedimentary rocks, both being, 
 composed of mud that has been cemented or compacted into rock. 
 Where the rock is essentially massive (that is, hand specimens do not 
 exhibit layers), the rock is called mudstone, but if the rock is thinly 
 layered it is called shale. Collectively, these are the most abundant of 
 the sedimentary rocks. 
 
 The specimens in this collection are typical mudstone. They are com- 
 posed of very fine-grained detrital material, principally clay particles, 
 but also contain finely divided fragments of quartz, feldspar, and other 
 minerals. The dark-gray color is caused by the presence of a trace of 
 charcoal-like organic debris. Where this material or other coloring 
 agents are lacking, mudstone and shale are light colored or white. In 
 places, mudstone and shale are colored buff, brown, or reddish brown ! 
 by small percentages of limonite. 
 
 Mudstone and shale are widely used in the manufacture of bricks. 
 Another use, of increasing importance, is in the manufacture of light- 
 weight aggregate for concrete. For this purpose, the rock is crushed 
 and the small fragments heated in a rotary kiln. At a temperature near 
 2000° F., near the melting point of the rock, the fragments expand like i 
 bread dough because of the formation of innumerable tiny gas bubbles. 
 The resulting expanded fragments are strong, and have only about half 
 the weight of an equal volume of standard sand and gravel aggregate. 
 
 White mudstones composed essentially of clay, without iron oxides 
 or other coloring constituents, are commercial sources of clay for mak- 
 ing pottery and other ceramic products. 
 
47 
 
 27. LIMESTONE 
 
 Type of rock: sedimentary, fine-grained 
 Mineral content: predominantly calcite 
 
 Limestone is a sedimentary rock composed predominantly of calcite. 
 Most limestone is light gray (almost white) to dark gray, but in places 
 it is brown or yellowish because of traces of iron oxide. The individual 
 calcite grains in limestone are normally too small to be seen without 
 the aid of a microscope. This characteristic helps to distinguish it from 
 most marble, a coarser-grained rock of similar composition. 
 
 Most limestone was formed by cementation of abundant shell debris 
 that accumulated in places on the sea floor. If formed from calcareous 
 ooze (largely shell debris from tiny marine organisms) it is very fine 
 grained. On the other hand, fossiliferous limestone containing shells and 
 shell fragments of larger organisms such as corals, clams, and snails may 
 appear coarse grained, although individual calcite grains cannot be seen 
 with the unaided eye. Some limestone contains no recognizable fossil 
 remains, and probably originated by inorganic precipitation of calcite 
 from water. 
 
 Limestone is a relatively soluble rock, for calcite is slowly attacked 
 by slightly acid ground-water and taken into solution. Caves are formed 
 in this manner by ground-water widening cracks in limestone, finally 
 dissolving out large chambers in places. During the last phase of a cave's 
 history, after it has been drained by uplift of the region, the process is 
 reversed and calcite is deposited within the cave by evaporation of 
 slowly dripping water. The resulting dripstone (stalactites, stalagmites, 
 and other depositional features in caves) strikingly illustrates the in- 
 organic deposition of calcite from solution. 
 
 Limestone is found in many widely distributed areas of the state, but 
 is not generally abundant except in some of the desert mountain ranges 
 in southeastern California. 
 
 Both limestone and marble (metamorphosed limestone) have a wide 
 variety of important uses in our industrial society. Immense quantities 
 of these rocks are quarried and processed each year for manufacture 
 of cement, plaster, and stucco. They also are used in the process of 
 refining sugar, as a flux in steel manufacture, as raw materials in various 
 chemical industries, and for many agricultural purposes. Limestone, or 
 products manufactured from it, are essential to the manufacture of 
 numerous products such as soap, glue, paint, and glass. 
 
48 
 
 28. CHERT 
 
 
 Type of rock: sedimentary, very fine-grained 
 Mineral content: predominantly chalcedony 
 
 Chert is a hard, brittle type of sedimentary rock that is composed 
 predominantly of chalcedony, a very fine-grained variety of quartz. It 
 is white or gray where lacking in impurities, but is commonly brown, 
 reddish brown, or pale to dark green because of iron-rich minerals dis- 
 seminated in the rock. In California most chert is thin bedded, individual 
 beds being one or two inches thick and separated from each other by 
 paper-thin layers of shale. 
 
 In general, this rock has the physical properties of chalcedony. Its 
 hardness (sufficient to easily scratch glass) is a critical characteristic j 
 in distinguishing it from other rocks that may have similar appearances. I 
 
 Most chert was formed by precipitation of silica from sea water in i| 
 the vicinity of submarine volcanic eruptions. The marine origin is com- il 
 monly indicated by the presence in the chert of abundant microscopic 
 remains of radiolaria, tiny marine organisms that have ornate skeletons 
 of opal (a non-crystalline form of silicon dioxide). 
 
 Thin-bedded chert is relatively common in California, particularly in « 
 the Coast Ranges. It is very resistant to alteration and disintegration by 
 weathering processes, so it tends to crop out more prominently than 
 most other rock types, and colorful pebbles of chert accumulate in ij 
 streams and on beaches. 
 
 Chert has no commercial value as an ore, but nicely colored pieces 
 are polished for making jewelry. Of interest to the prospector is the I 
 fact that all of the manganese ore deposits of northern California are I 
 found enclosed in chert, and the search for such deposits in this region i 
 can effectively be limited to examination of chert outcrop areas. 
 
 Right. Photomicrograph of diatoms. Average 
 size of the fossils is about 1/500 inch. 
 Photo courtesy P. W. Leppla. 
 
49 
 
 
 29. DIATOMITE 
 
 Type of rock: sedimentary, fine-grained 
 Mineral content: opal 
 
 Diatomite is a soft, light-weight sedimentary rock composed predom- 
 inantly of the siliceous remains of diatoms, microscopic aquatic plants. 
 Diatoms are unusual, tiny, one-cell plants that have ornate cell walls 
 composed of opal (hydrous silicon dioxide); they live both in fresh 
 water and in the seas. Under favorable environmental conditions, par- 
 ticularly where the water becomes locally enriched in dissolved silicon 
 dioxide, diatoms reproduce so rapidly that their remains accumulate to 
 form pure layers many feet thick on the lake or sea bottom. Such 
 deposits seldom become cemented, but are merely compacted by the 
 weight of overlying sediments to yield a friable, porous rock. Diatomite 
 is much lighter than water, so fragments of it will float until they 
 become saturated. 
 
 Diatomite is found in several areas in California, being particularly 
 abundant in some of the marine sedimentary rocks of Tertiary age in 
 the southern Coast Ranges and the Transverse Ranges. Deposits that 
 were formed in fresh-water lakes are exposed in the Modoc Plateau 
 region and in the desert basins of eastern California and Nevada. 
 
 Diatomite is of commercial value for a variety of industrial purposes, 
 and is mined in California in large quantities. Being very fine-grained 
 and porous, it is used for filtering impurities from petroleum, chemicals, 
 vegetable oils, antibiotics, varnishes, polluted water, and other liquids, 
 [t is also used as a mineral filler in numerous manufactured products, 
 such as paper and plastics, to increase bulk and improve such properties 
 as toughness, elasticity, and absorptiveness. Production in California 
 comes largely from quarries in Santa Barbara County. 
 
50 
 
 
 30. GYPSUM 
 
 Type of rock: sedimentary, fine- to coarse-grained 
 Mineral content: predominantly gypsum 
 
 Gypsum is a name applied to a mineral and to a rock composed pre- 1 
 dominantly of that mineral. The mineral gypsum is a hydrous calcium 
 sulfate (CaS0 4 -2H 2 0). It is colorless or white, is so soft that it can be 
 scratched with the fingernail, and has perfect cleavage that yields pearly 
 or shiny flat surfaces. Gypsum is slightly soluble in water, and a signi- 
 ficant amount of it is dissolved in the water of the oceans. 
 
 Rock gypsum is one of the sedimentary rock types known as evap- 
 orites, so called because they were precipitated by evaporation of water I 
 rich in dissolved mineral materials. Most large deposits of gypsum are 
 thick beds deposited when bays or arms of the sea become land-locked, 
 and the water evaporated. The gypsum is normally interbedded withi: 
 limestone and rock salt that were precipitated at different times as the 1 ! 
 water evaporated. Gypsum deposited in this manner is white, gray, or 
 brown, and finely granular. 
 
 In many arid regions gypsum crystals grow in cracks in the rock and; 
 in the soil because of evaporation of ground-water rich in dissolved; 
 mineral materials. Deposits of gypsum-rich soil formed in this manner 
 are called gypsite, and are quite earthy in appearance. 
 
 Extensive beds of rock gypsum are found in several areas of south- 
 eastern California, and gypsite is abundant in some arid parts of the 
 state, particularly in the southern Coast Ranges. 
 
 Gypsum is a rock of great commercial value, and is quarried at many 
 localities in California. It is from this rock that most plaster is manu- 
 factured. Large quantities are also consumed by the cement industry, 
 for gypsum retards the naturally fast setting time of cement, allowing; 
 it to be properly mixed and placed before it hardens. 
 
 Although it is commonly abundant in poor arid soils, the chemical 
 ingredients of gypsum are lacking in many soils in moist regions. For 
 this reason, gypsum is mined and sold as a "soil conditioner" to improve 
 the agricultural quality of these otherwise-rich soils. 
 
51 
 
 v^Afc 
 
 31. SLATE 
 
 Type of rock: metamorphic, very fine-grained 
 Mineral content: mica, quartz 
 
 Slate is an exceedingly fine-grained metamorphic rock that can easily 
 be split into thin, flat slabs or sheets. It is composed predominantly of 
 tiny flakes of mica that lie parallel with each other and give the rock 
 its perfect cleavage, but individual mineral grains cannot be seen without 
 the aid of a microscope. Most slate is black, but in places it is dark 
 greenish gray. 
 
 Slate is formed by matamorphism of shale or mudstone, and the 
 nature of this transformation is discussed in the introduction on page 15. 
 
 In California, slate is found primarily in the Sierra Nevada and the 
 Klamath Mountains. In the past it was extensively quarried in the Sierra 
 Nevada foothills for thin slabs that were trimmed to make slate shingles. 
 In recent years the slate quarried in California has been crushed for the 
 manufacture of roofing granules. _™„__™___ 
 
 32. MARBLE 
 
 Type of rock: metamorphic, medium to coarse grained 
 Mineral content: predominantly calcite 
 
 Unfortunately, a certain amount of confusion enters into the usage 
 of the term "marble". It is most widely known as a commercial name 
 for limestone and other rocks of similar hardness that have been pol- 
 ished for decorative purposes. 
 
 To the geologist, marble is metamorphosed limestone, and like lime- 
 stone it is composed largely or entirely of calcite. In marble, however, 
 individual grains of calcite can be seen with the unaided eye because of 
 recrystallization of the microscopic calcite particles typically present 
 in limestone. Indeed, size of the calcite grains is often the only means 
 of easily differentiating these two rock types. 
 
 Most marble is white or gray -but in places it is bluish gray, buff, or 
 other shades of color. It commonly appears streaked with parallel or 
 intersecting planes of dark gray impurities. The physical and chemical 
 tests for this rock, as well as for limestone, are the same as those for 
 calcite. 
 
 Marble has the same commercial uses as limestone, and is a very im- 
 portant raw material in our industrial economy. It also remains one of 
 the most desirable of the various rock types that are sawed and polished 
 for decorative stone, and marketed collectively as "marble". 
 
52 
 
 
 33. MICA SCHIST 
 
 Type of rock: metamorphic, medium to coarse grained, foliated 
 Mineral content: mica, quartz, and feldspar 
 
 The term schist is used for a variety of medium- to coarse-grained 
 metamorphic rocks that can easily be split along subparallel planes of i 
 weakness into thin flaky fragments. The planes of weakness are caused 
 by the presence in the schist of abundant platy or rodlike minerals that 
 are oriented roughly parallel to each other. As with slate, the parallel 
 orientation of these minerals was caused by directional stresses at right 
 angles to them while the rocks were being recrystallized. But the larger 
 grain size in schist does not permit as perfect cleavage as that of slate. In 
 naming these rocks, the word "schist" is preceded by the name of the 
 platy of rodlike mineral that makes it schistose. 
 
 Mica schist is composed essentially of mica, quartz, and feldspar. 
 Mica is the platy mineral that exhibits shiny cleavage faces when the 
 rock is viewed closely in a good light. The specimen may contain either 
 muscovite (white mica) or biotite (brown or black mica). The quartz 
 and feldspar grains are white, and about the size of sugar grains. 
 
 This rock was formed by metamorphism of a sedimentary rock such 
 as mudstone or sandstone. Other types of schist, with different mineral 
 assemblages, were formed by similar metamorphic processes from other 
 original rock types. 
 
 Schist is a relatively abundant rock in portions of those areas of Cali- 
 fornia shown on the map as "Mesozoic-Paleozoic metamorphic and 
 granitic rocks". It has no commercial value. 
 
53 
 
 1A QFDPFMTIklF 
 J4. oCKrCINIIiNC 
 
 Type of rock: metamorphic, fine-grained 
 
 Mineral content: antigorite, chrysotile, magnetite, chromite 
 
 Serpentine is one of the more colorful types of rock in California. It is 
 a fine-grained metamorphic rock that ranges in color from pale green to 
 greenish black. In many places it has been highly sheared by earth move- 
 ment, yielding a form aptly called "slickentite" because of the numerous 
 polished, slick, or slippery surfaces in the rock. Such sheared serpentine 
 is characteristically pale green, but in places the slick surfaces are pleas- 
 ingly mottled with greenish, bluish, and honey colors, and they have a 
 waxy luster. The slip surfaces commonly curve around scattered, 
 rounded blocks of unsheared serpentine that are massive and dark green 
 to greenish black where fractured. 
 
 Serpentine is composed predominantly of fine-grained magnesium 
 silicate minerals. In most places, the rock also contains abundant, finely 
 disseminated magnetite and scattered chromite grains. 
 
 Most serpentine is derived by metamorphism of peridotite, an unusual 
 igneous rock of high density that is thought to have been intruded 
 from great depths (from the mantle, below the crust of the earth). 
 
 Although structurally weak so that it is easily sheared, serpentine is 
 more resistant to weathering than most other rock types. As a result, 
 its outcrop areas are typically revealed by abundant blocks of rock 
 projecting through shallow rust-colored soil. 
 
 Serpentine is used to some extent as a decorative stone. However, the 
 most significant economic aspect of this rock is that it is the only host 
 rock for chrysotile asbestos and is the principal host rock for deposits 
 of chromite. 
 
 Left. Typical blocky outcrop of serpentine, 
 showing characteristic texture of weathered 
 surfaces. 
 
54 
 
 4:*:.;s..,.i.». -:■-'■';' : 4 
 
 -:;.;■'■■:> .-.* , 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. Note the hammer at 
 lower left for scale. 
 
 74290 10-62 5M 
 
 
 
 
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