CHAPTER XX

MINERALOGY

We are more or less familiar with the division of all materials of nature into the animal, vegetable, and mineral kingdoms. With slight exceptions minerals are the materials which make up the known part of the earth. In a very real sense, then, mineralogy is the most fundamental of the various branches of the great science of geology because the events of earth history, as interpreted by the geologist, are recorded in the mineral matter (including most rocks) of the earth. When we examine the rocky material or mineral matter of the earth in any region we find that it consists of various kinds of substances each of which may be recognized by certain characteristics. Each definite substance (barring those of organic origin) is called a mineral. Or, more specifically, a mineral is a natural, inorganic, homogeneous substance of definite chemical composition. According to this definition a mineral must be found ready made in nature, must not be a product of life, must be of the same nature throughout, and its composition must be so definite that it can be expressed by a chemical formula. All artificial substances, such as laboratory and furnace products, are excluded from the category of minerals. Coal is not a mineral because it is both organic and of indefinite composition. A few examples of very common substances which perfectly satisfy the definition of a mineral are quartz, feldspar, mica, calcite, and magnetite. Only two substances--water and mercury--are ordinarily liquid minerals. There are nearly a thousand distinct mineral species, and to them and their varieties several thousand names have been applied.

It is a surprising fact that of the eighty or more chemical elements, that is substances which cannot be subdivided into simpler ones, only eight make up more than 98 per cent of the weight of the crust of the earth, though, with one very slight exception, none of the eight exist as such in mineral form. The eight elements are oxygen (nearly 50 per cent), silicon (over 25 per cent), aluminum (over 7 per cent), iron (over 5 per cent), calcium (or "lime"), magnesium (or "magnesia"), sodium (or "soda"), and potassium (or "potash").

Certain rock formations are made up essentially of but one mineral in the form of numerous grains as, for example, limestone, which consists of calcite (carbonate of lime). Most of the ordinary rocks are, however, made up of two or more minerals mechanically bound together. Thus, in a specimen of granite on the author's desk several distinct mineral substances are distinguishable by the naked eye. These mineral grains are from one to five millimeters across. Most common among them are hard, clear, glassy grains called quartz; nearly white, hard grains, with smooth faces, called feldspar; small, silvery white plates, easily separable into very thin flakes, called mica; and small, hard, black grains, called magnetite. It is the business of the mineralogist to learn the characters of each mineral, how they may be distinguished from each other, how they may be classified, how they are found in nature, and what economic value they may have. It is an important part of the business of the geologist to learn what individual minerals combine to form the many kinds of rocks, how such rocks originate, what changes they have undergone, and what geological history they record. It is thus clear that the great science of geology is much broader in its scope than mineralogy.

One of the most remarkable facts about minerals is that most of them by far have a crystalline structure, that is they are built up of tiny particles known as molecules. Such crystalline minerals are often more or less regular solid forms bounded by plane faces and sharp angles, such forms being known as "crystals." How do crystals develop such regularity of form? Any solid is considered to be made up of many very tiny (submicroscopic) molecules held together by an attractive force called cohesion. In liquids the molecules may more or less freely roll over each other, thus altering the shape of the mass without disrupting it. In gases the molecules are considered to be relatively long distances apart and moving rapidly. During the process of change of a substance from the condition of a liquid or gas to that of a solid, due to lowering of temperature or evaporation, the cohesive force pulls the particles (molecules) together into a rigid mass. Under favorable conditions such a solid has a regular polyhedral form. "This results from the fact that the particles or molecules of the substance which, while it was liquid or gaseous, rolled about on one another, have been in some way arranged, grouped and built up. To illustrate this, suppose a quantity of small shot to be poured into a glass: the shot will represent the molecules of a substance in the liquid state, as for example a solution of alum. If, now, we suppose these same shot to be coated with varnish or glue so that they will adhere to each other, and imagine them grouped as shown in Figure 70a, they will represent the arrangement of the molecules of the alum after it has become solid or crystallized. This arranging, grouping, and piling up of molecules is called crystallization, and the solid formed in this way is called a crystal. Figures 70b and 70c show the shot arranged to reproduce two common forms of crystals (e.g., fluorite and calcite)." (Whitlock.)

A combination of certain facts regarding crystals furnish all but absolute proof of some sort of regularity of arrangement of particles within them. Among such facts are the following: (1) the wonderful regularity of arrangement of faces upon crystals is practically impossible to account for except as the outward manifestation of regularity of structure or systematic network arrangement of the interior; (2) most crystals split or cleave more or less perfectly in one or more directions presumably in accordance with certain layered structure of the constituent particles; (3) all of the many known forms of crystals can be accurately grouped in regard to their effects upon the passage of light (especially polarized light) through them, each kind or type of network structure presumably producing a different effect upon light; and (4) X-ray photographs have proved that particles, or at least groups of particles, are very systematically arranged within crystals.

It will be instructive for us to make a comparison between the growth of crystals and organisms. Both really grow, but each species of organism is rather definitely limited in size while there is no known limit to the size which may be attained by a crystal so long as material is supplied to it under proper conditions. As a matter of fact crystals vary in size from microscopic to several feet in length, those less than an inch in length being most abundant by far. Organisms mostly grow from within, while crystals grow from material externally added. It is an astonishing fact that in crystals as well as organisms growth takes most rapidly on a wound or broken place. Thus if a crystal is removed from the solution in which it is growing and put back after a corner has been broken off, the fractured surface will build up more rapidly than the rest. Finally, crystals are not necessarily limited in age like organisms. Under certain natural conditions, as, for example, weathering, crystals may decay or be broken up; but where they are protected as constituent parts of rock formations well below the earth's surface they may remain unchanged for indefinite millions of years. Thus in a ledge of the most ancient known or Archeozoic rock only recently laid bare by erosion one may see crystals which are precisely as they were when they crystallized many millions of years ago.

One of the most remarkable properties of a crystal is its symmetry, by which is meant the greater or less degree of regularity in the arrangement of its faces, edges, and vertices. A given substance may, according to circumstances, crystallize in a variety of forms or combinations of forms, but, with very few exceptions, all crystals of a given substance exhibit the same kind or grade of symmetry. There are three kinds of crystal symmetry, namely, in respect to a plane, a line or axis, and a point or center. A plane of symmetry divides a crystal into halves in such a way that for every point on one side of the plane there is a corresponding point directly opposite on the other side. Crystals may be cut into halves along various surfaces which are not symmetry planes. An axis of symmetry is a line about which a complete rotation (or in a few cases rotation combined with reflection) brings the crystal into the same relative position two, three, four or six times, these being called two, three, four, and sixfold axes of symmetry--no others being possible. A crystal has a center of symmetry when any line passing through it encounters corresponding points at equal distances from it on opposite sides. There are just 32 classes or combinations of the symmetry elements among crystals and just 232 definite crystal forms. Not only is it demonstrable that no more can exist, but actual experience with crystals of hundreds of species of minerals has never revealed any more. Obviously, then, symmetry furnishes us with a very scientific basis of classification of crystals, all of the 232 crystal forms constituting the 32 symmetry classes being in turn referable to seven fundamental crystal systems. To bring out the relations of the faces of a crystal and further aid in classification, prominent, straight lines or directions passing through the center of a crystal are chosen as crystallographic axes. Such axes may or may not coincide with symmetry axes. Basing our definitions upon both symmetry axes and crystallographic axes, the seven systems are as follows:

1. Isometric. There must be at least four threefold axes of symmetry, while the highest grade symmetry class of the five in the system includes three fourfold, four threefold, and six twofold axes of symmetry; nine planes of symmetry; and a center of symmetry. There are three interchangeable crystallographic axes at right angles to each other.

2. Tetragonal. There must be one and only one fourfold symmetry axis, while the highest of its seven symmetry classes contains also four twofold axes of symmetry; five planes; and a center. Characterized by three crystallographic axes at right angles to each other, only two of them interchangeable.

3. Trigonal. Characterized by one and only one threefold symmetry axis, the highest of the five classes having also three twofold axes; four planes; and a center. Crystallographic axes as for hexagonal.

4. Hexagonal. One and only one sixfold axis of symmetry must be present, but the highest of the seven classes also has six twofold axes; seven planes; and a center. Characterized by four crystallographic axes, one vertical and three interchangeable horizontal axes making angles of 60 degrees with each other.

5. Orthorhombic. There must be no axis of symmetry higher than a twofold and three prominent directions (i.e., parallel to important faces) at right angles to each other, the highest grade of the three classes having three twofold axes; three planes; and a center. There are three noninterchangeable crystallographic axes at right angles.

6. Monoclinic. There is no axis of symmetry higher than a twofold and only two prominent directions at right angles to each other, the highest of the three classes having one twofold axis; one plane; and a center. There are three noninterchangeable crystallographic axes, only two of which are at right angles.

7. Triclinic. There is no axis of symmetry of any kind, and there are no prominent directions at right angles. One of the two classes has a center of symmetry only, and the other no symmetry at all. Characterized by three noninterchangeable crystallographic axes, none at right angles.

A fact which should be strongly emphasized is that crystals only, of all the objects of nature, can be definitely referred to the above seven systems comprising the 32 classes of symmetry, and 232 crystal forms. Since there are about 1,000 mineral species and only 232 fundamental forms, it necessarily follows that two or more species may crystallize in the same form within a class, so that it is not always possible to tell the species of mineral merely by its crystal form. It is, however, a remarkable fact that, where two or more substances crystallize in the same class (i.e., show the same grade of symmetry) each substance almost invariably exhibits "crystal habit" which is a pronounced tendency to crystallize in certain relatively few forms or combinations of forms out of the many possibilities. It is clear, then, that grade of symmetry combined with "habit" are of great practical value in determining crystallized minerals, because, on the basis of symmetry, a crystal is referred to a certain definite symmetry class in which only a limited number of substances crystallize, and then, by its characteristic "habit," the particular substance can be told.

From the above discussion it should not be presumed that crystals always develop with perfect geometric symmetry. As a matter of fact such is seldom the case because, due to variations of conditions or interference of surrounding crystals in liquids (ordinary or molten), a crystal usually grows more rapidly (by building out faces) in certain directions than in others. Under such conditions actual crystals are said to become distorted because they are not geometrically perfect.

Whether geometrically perfect or not, all crystals respond to the law of constancy of interfacial angles which means that on all crystals of the same substances the angles between similar (corresponding) faces are always equal. This is one of the most fundamental and remarkable laws of minerals. That it must be true follows from the fact that the crystal faces merely outwardly express in definite form the definite internal structure or arrangement of particles which have built up the crystal. In other words, the real structural symmetry of a crystal never varies no matter how much its geometric symmetry may vary. The practical application of the law of constancy of interfacial angles lies in the fact that in many cases a mineral may actually be identified merely by measuring the interfacial angles of its crystal form.

The relative lengths of the crystallographic axes is a very important feature of all crystals except those of the isometric system in which the axes are always of equal length so that the ratio is 1:1:1. In all the other systems, however, at least one axis differs in length from the others and, since the amount of difference is absolutely characteristic of each substance, the axial ratio of a crystal, when carefully determined by measurement of the angles between the different faces, affords a never-failing method of determining the mineral for all systems except the isometric. By way of illustration, the tetragonal crystal of the mineral zircon, with only one axis different in length, shows the very definite axial ratio 1:1:0.64, while the orthorhombic crystal of sulphur, with all three axes of different lengths, has an axial ratio 0.813:1:1.903. These ratios of course always hold true no matter what the size or particular outward form of the crystal.

As might be expected from the above discussion of the remarkable structure of crystals, experience has proved that the relative lengths of all intercepts (or distances from the center) of all faces upon any crystal can be expressed by whole numbers, definite fractions, or infinity. It necessarily follows that the ratios between the intercepts of the faces of any face on a crystal to those of any other face on the same crystal may always be expressed by rational numbers, and this is known as the law of definite mathematical ratio. It is a remarkable fact that very small whole numbers or fractions, or infinity or zero, will always express the intercepts of any crystal face.

Thus far our discussion has centered about crystals as individuals, but, in most cases by far, they form groups or aggregates. Most commonly crystal grouping is very irregular, but by no means rare is parallel grouping where whole crystals, or more usually parts of crystals, have all corresponding parts exactly parallel. But most remarkable of all are the twin crystals in which two or more crystals intergrown or in contact have all corresponding parts in exactly reverse order. The conditioning circumstances under which twin crystals develop are unknown.

In the light of the facts and principles above explained, the reader will more than likely agree with the author that crystals rank very high among nature's most wonderful objects. But there are still other characteristic features of crystals naturally resulting from their marvelous structure. Some of these will now be briefly referred to.

Many crystals and crystalline substances exhibit the important property known as cleavage which is the marked tendency to break easily in certain directions yielding more or less smooth plane surfaces. As would be expected, a cleavage surface is always parallel to an actual, or at least a possible, crystal face, and it takes place along the surfaces of weaker molecular cohesion. The degree of cleavage varies from almost perfect, as in mica, to very poor or none at all, as in quartz. The number of cleavage directions exhibited by common minerals is illustrated as follows: mica, one; feldspar, two; calcite, three; and fluorite, four.

It is a striking fact that when a crystal or cleavage piece is placed in a solvent, the action proceeds with different velocities in crystallographically different directions and little pits or cavities, called etching figures, are developed on some or all of the faces. Since the symmetry of these etching figures and their arrangement upon the faces are directly related to, and natural effects of the crystal symmetry, the figures often furnish an important method of placing a doubtful crystal or even merely a cleavage fragment in its proper symmetry class.

Another marvelous property of crystals and crystalline substances is their effect upon light. Since the study of the passage of light through crystals has really become a large separate branch of mineralogical study, we can no more than state a few fundamental facts and principles in the short space at our disposal. Light is caused by vibrations of the so-called "ether," and always travels in straight lines. The vibration directions are at right angles to the direction of transmission of the light. When a ray of light enters a crystal or crystalline mineral representing any crystal system except the isometric it is doubly refracted (i.e., broken into two rays), each of the two rays is polarized (i.e., made to vibrate in a single plane only), and one ray vibrates almost at right angles to the other. Double refraction is strikingly shown by placing a piece of clear calcite (Iceland spar) over a dot on paper when two dots instead of one are visible. The amount of double refraction varies with the substance, and in some degree according to the direction of passage of light through a crystal. Isometric crystals only are singly refracting and hence a ray of light is not affected in passing through them. Crystals of all the other six systems doubly refract and polarize light and in three systems--tetragonal, hexagonal, and trigonal--one direction (coincident with the main axis of symmetry) produces single refraction only, while in the remaining three systems--orthorhombic, monoclinic, and triclinic--there are always two directions of single refraction whose positions vary with the substance. Many crystals outside the isometric system also exhibit a remarkable tendency to absorb light differently in different crystallographic directions, thus producing two or three color tints, which vary according to the substance. After gaining a practical knowledge of the above and many other optical properties of crystals, it is possible by the aid of a specially constructed (polarizing) microscope, to recognize (with few exceptions) each one of the many mineral species. This method is of great value in determining the various minerals which are aggregated in the form of a rock, in which case a very thin slice of the rock is studied with the microscope.

An important criterion for the recognition of minerals is hardness, by which is meant the resistance of a smooth surface to abrasion or scratching. The generally adopted scale of hardness follows:

1.--Soft, greasy feel, and easily scratched by the finger nail (e.g., talc).

2.--Just scratched by the finger nail (e.g., gypsum).

3.--Just scratched by a copper coin (e.g., calcite).

4.--Easily cut by a knife, but does not cut glass (e.g., fluorite).

5.--Just scratches soft glass, and is cut by a knife (e.g., apatite).

6.--Harder than steel, and scratches glass easily (e.g., orthoclase).

7, 8, 9, and 10.--Harder than any ordinary substance and represented in order by quartz, topaz, corundum, and diamond.

Plate 18.--(_a_) Skeleton of the Largest Known Creature That Ever Flew. It was a flying reptile with spread wings of nearly twenty-five feet, and lived during the Cretaceous period several million years ago. (_Courtesy of the American Museum of Natural History._)

Plate 18.--(_b_) Skeleton of a Remarkable Swimming Reptile of the Mesozoic Era. Length about twelve feet. Parts of skeletons of unborn young are seen. (_Courtesy of the American Museum of Natural History._)

Minerals also show a great variety of colors. Many of them like quartz and calcite are colorless or white, others like galena (steel-gray) and pyrite (brass-yellow) show inherently characteristic colors, while still others like amethyst (purple) and sapphire (blue) are colored by impurities.

There is also a great range in relative weights or density of minerals, commonly called the specific gravity, which range from less than one for ice to 21.5 for platinum, and even somewhat higher. The average specific gravity of all minerals of the earth is about 2.6.

In the light of the above discussion of the general properties of minerals, we shall now proceed to name and briefly describe some of the minerals which are either very common, or of special interest, or of special economic importance. Only those features are listed by which the mineral species may be recognized at sight, or by the aid of very simple nonchemical tests.

Amphibole. A number of species closely related in composition, crystal form, and properties are here included. They are silicates of lime and magnesia usually with aluminum and iron. Most common by far are those which crystallize in the monoclinic system with prismatic faces and two good prismatic cleavages meeting at about 24 degrees. Color, commonly brown to black, but sometimes green or white. Hardness varies from 5 to 6, and specific gravity from 3 to 3.4. _Hornblende_, the most common species, is a dark colored silicate of lime, magnesia, aluminum, and iron. It is one of the few most common of all mineral species, especially in igneous and metamorphic rocks. _Tremolite_ is a white to light gray silicate of lime and magnesia found especially in metamorphic limestones. _Actinolite_ is a green silicate of lime, magnesia, and iron especially common in certain metamorphic rocks. One kind of jade is an amphibole similar to tremolite and actinolite in composition, while the other kind is a pyroxene (see below). _Jade_ is and has been highly prized in the east (especially in China) where it has been carved into many objects of exceptional variety and beauty. Jade is probably the toughest (not hardest) of all minerals because of its wonderful microscopically fibrous structure. In color it is white, gray, and green.

Apatite. Crystallizes in the hexagonal system with a six-sided prism usually capped at each end by a six-sided pyramid (see Figure 75g). Composition, a phosphate of lime. Color variable, but mostly white, green, or brown. Hardness of 5, or just enough to scratch soft glass. Specific gravity, 3.2. No good cleavage. Tiny crystals are widely disseminated through many common rocks--igneous, metamorphic, and sedimentary. In certain metamorphic limestones excellent crystals a foot or more in length have been found. Apatite, mostly in uncrystallized form, is the source of most of our phosphate fertilizers.

Azurite. An azure-blue hydrous carbonate of copper which crystallizes commonly in small monoclinic crystals. Hardness, nearly 4, and specific gravity, nearly 4. Commonly occurs in veins deposited by underground water. One of the great ores of copper, especially in Arizona, Chile, and Australia.

Barite. A sulphate of barium crystallizing in orthorhombic prisms usually of tabular habit. White to light color shades. Hardness, 3.5; specific gravity, 4.5, which is notably higher than the average of light-colored minerals. Three good cleavages parallel to principal crystal faces. A common and widely distributed mineral, especially in many vein deposits associated with certain ores. Used in ground form to give weight to certain kinds of paper and cloth, and a barium compound used for refining sugar is made from it.

Beryl. A silicate of aluminum and the rare chemical element beryllium. Hexagonal crystals usually of very simple six-sided prismatic habit (see Figure 75c). Color white, green, blue, or yellow. Specific gravity, 2.8. Cleavage practically absent. It is a very exceptionally hard mineral, being 8 in the scale. Very large crystals have been found, as, for example, in New Hampshire, where single crystals several feet long weigh a ton or more. Beryl is also of special interest because two of its varieties--_emerald_ (green) and _aquamarine_ (blue)--are well-known gem stones, the emerald being one of the most highly prized gems. The colors are due to slight impurities. Beryl most commonly occurs in dikes of coarse granite called pegmatite, but also in certain metamorphic and sedimentary rocks.

Calcite. Commonly called "calc spar." A carbonate of lime. Hexagonal crystals in a great variety of forms, but all with crystal faces arranged in sixes around the principal or vertical axis forming rhombohedrons, prisms, or double-pointed pyramids. The principal axis of symmetry is sixfold by a combination of rotation and reflection. Very perfect cleavages in three directions yielding fragments whose faces make angles of 75 and 105 degrees. Color, white when pure, but variously colored when impure. Hardness, 3 (very easily scratched by a knife); specific gravity, 2.7. Calcite is a very common mineral, especially in limestone (including _chalk_) and marble which are usually largely made up of it. Also commonly found in veins, and as spring and cave deposits (stalactites). A porous, stringy variety, called _travertine_, is deposited by certain hot springs, as at Mammoth Hot Springs in Yellowstone Park. A very transparent crystalline variety is called _Iceland spar_. Calcite is a very useful mineral. Limestone and marble are widely used as a building stone, and for decorative purposes, statuary, etc. Limestone is burned for quicklime, used as a flux in smelting certain ores, in glass making, etc.

Cassiterite. The one great ore of tin whose composition is oxide of tin. Tetragonal crystallization (Figure 73c). Hardness greater than steel, being over 6 in the scale. Specific gravity 7, which is notably high. Color, brown to nearly black. Cleavage, practically absent. Fairly widespread in small amounts, and in commercial quantities in only a few localities, usually in veins in granite or metamorphic rocks near granite, as at Cornwall, England, also in the form of rounded masses in gravel deposition as in the Malay region.

Chalcocite. Crystallizes in the orthorhombic system, usually in tabular form, but crystals not common. A black sulphide of copper with metallic luster. Hardness, nearly 3; specific gravity, nearly 6. No cleavage. Chalcocite occurs in vein deposits as one of the important copper ores, especially at Butte, Montana.

Chalcopyrite. Known as "copper pyrites," (Figure 75j). A deep brass-yellow sulphide of iron and copper. Seldom crystallized in tetragonal forms. Hardness, 3.5; specific gravity, over 4. No cleavage. Metallic luster. Widely distributed in vein deposits associated with other metal-bearing minerals. A very important ore of copper, especially at Rio Tinto, Spain.

Chlorite. A soft, green mineral, usually in small tabular crystals, in general appearance much like mica (see below), but unlike mica, the almost perfect cleavage leaves are not elastic, though they are flexible. Composition, a silicate of aluminum and magnesia. Always of secondary origin as a result of chemical alteration of certain other minerals, such as biotite-mica, pyroxene or amphibole.

Cinnabar. A vermilion-red sulphide of mercury. An extra soft metallic mineral, only 2.5 in the scale. Specific gravity over 8, which is notably high. Completely vaporizes on being heated. Small trigonal crystals rare. Cinnabar is the one great ore of mercury, occurring in veins, especially in California and Spain.

Copper. Copper as such (so-called "native copper") is widely distributed in veins, usually in small amounts with other copper minerals, but in the great mines of northern Michigan it occurs in immense quantities as the only important ore. It is readily recognized by its color, softness (less than 3), and notable weight (specific gravity, nearly 9). Isometric crystals uncommon.

Corundum. An oxide of aluminum of hexagonal crystallization, usually in six-sided prisms, capped by very steep pyramidal faces (see Figure 75n). It is next to the hardest of all known minerals (9 in the scale), the diamond only exceeding it. Specific gravity about 4. Three good cleavages making angles of nearly 90 degrees with each other. The color of corundum is usually brown, but it varies greatly. Two of the most highly prized of all precious stones--_ruby_ (red) and _sapphire_ (blue)--are nearly transparent varieties of corundum, colored by certain impurities. _Oriental topaz_ (yellow), _oriental emerald_ (green), and _oriental amethyst_ (purple) are also clear varieties of corundum. It occurs in various igneous and metamorphic rocks, and in some stream gravels. The finest rubies, associated with some sapphires, occur in gravels in Burma, Siam, and Ceylon. _Emery_ is a fine-grained mixture of corundum and other minerals, especially magnetite.

Diamond. This mineral is remarkable not only because it is the king of precious stones, but also because it is easily the hardest known substance (10 in the scale). Specific gravity, 3.5. Very brilliant luster. Crystals of usually octahedral habit in the isometric system. Usually colorless, but often variously tinted. Composition, pure carbon. Burns completely away at high temperature. The greatest mines in the world are in South Africa, where the diamonds occur in masses of rather soft (decomposed) igneous rock, evidently having crystallized during the cooling of the molten masses. In Brazil and India diamonds are found in stream gravels.

Feldspar Group. The feldspars are by far the most abundant of all minerals in the crust of the earth. (Figures 74c, 74d.) There are several important species or varieties of feldspar with certain features in common as follows: crystal forms, either monoclinic or triclinic (closely resembling monoclinic), in prismatic forms whose faces usually meet at or near 90 or 120 degrees; two good cleavages at or near 90 degrees, hardness at or near 6; specific gravity, a little over 2.5; color, usually white, gray, or pink; and composition, silicate of aluminum with potash, soda, or lime. The two potash feldspars are _orthoclase_ and _microcline_, the former being monoclinic, with cleavages at exactly 90 degrees, and the latter triclinic, with cleavages a little less than 90 degrees. A kind of green microcline is known as _Amazon stone_. The soda-lime feldspars go by the general name _plagioclase_. They are triclinic, with cleavages meeting at approximately 86 degrees. Very commonly one of the cleavage faces exhibits characteristic, well defined striations or fine parallel lines caused by multiple twinning during crystal growth. Some of the common plagioclases are _albite_, a white soda feldspar, including most so-called _moonstone_; _oligoclase_, a usually greenish-white to reddish-gray soda-lime feldspar including _sunstone_; and _labradorite_, a lime-soda feldspar, usually gray to greenish-gray with a beautiful play of colors. The feldspars occur in all three great groups of rocks, but they have most commonly crystallized during the cooling of molten masses of igneous rocks. Where many sedimentary rocks have undergone great change (metamorphism) under conditions of heat, pressure, and moisture, feldspars have very commonly formed. Orthoclase and microcline feldspar are used in the manufacture of porcelain and chinaware. Some special varieties of feldspar are cut or polished for semiprecious stones or decorative purposes.

Fluorite. A common mineral whose composition is fluoride of lime. (Figure 73b.) Isometric crystals, usually cubes with edges modified, are common. Twinned cubes are also common. Easily scratched by a knife (hardness, 4), and specific gravity a little over 3. Clear and colorless when pure, but variously colored, especially green, blue, yellow, and brown, due to impurities in solution during crystallization. Remarkable because of its four good cleavages meeting at such angles as to permit good cleavage octahedrons to be broken out of crystals. Fluorite is widely distributed, most commonly in vein deposits, often associated with metallic ores. Occurs also as crystals in some limestones and igneous rocks. Some fissure veins of fluorite in limestone in southern Illinois are twenty to forty feet wide. Used mostly as a flux in the manufacture of certain steel, in glass making, and in making enamel ware.

Galena. Commonly as isometric crystals either as cubes or combinations of cubes and octahedrons. Composition, sulphide of lead. (Figure 75a.) Color, lead-gray with metallic luster. Hardness, 2.5; specific gravity high, 7.5. Very brittle. Three excellent cleavages at right angles and parallel to the crystal faces of the cube. Nearly all of the lead of commerce comes from the smelting of galena. It is mined in many parts of the world where it nearly always occurs in typical vein deposits often associated with sphalerite (see below).

Garnet Group. The members of this very interesting mineral group very commonly occur in isometric crystallized forms, mostly twelve and twenty-four faced figures or both combined, as shown by Figure 72. All the six species of garnets are silicates, mostly of aluminum usually with either lime, magnesia, or iron. Cleavage, very imperfect or absent. Hardness great, 6.5 to 7.5, and specific gravity 3.1 to 4.3, varying according to species. Color also varies with composition, but most commonly red, brown, and more rarely yellow, black, and green. Garnets are most common as crystals embedded in metamorphic rocks, especially highly altered strata. Also occurs in many igneous rocks and in some sands. Commonly used as a semiprecious stone, and also ground for use as an abrasive, especially in making a kind of sand (or garnet) paper.

Gold. Gold as such ("native gold") is, in small amounts, really a very widely distributed mineral. It is characterized by its yellow color, softness (less than 3 in the scale), great weight (specific gravity, over 19), and extreme malleability. Most of the commercial gold occurs in river gravels (so-called "placer deposits"), and in veins associated with the very common mineral quartz.

Graphite. Commonly called "black lead," but it is not lead at all. Its composition is pure carbon--the same as that of the diamond. We here have a very remarkable example of a single substance (carbon) which, according to circumstances, crystallizes in two distinctly different systems (diamond in isometric, and graphite in hexagonal) yielding very thin, flexible flakes; greasy in feel; and easily rubs off on paper. It weighs less than the average mineral (specific gravity, a little over 2). Good crystals of hexagonal tabular form are rare. The most natural home of graphite is in the metamorphic rocks, especially certain of the highly altered strata, where it occurs in the form of more or less abundant flakes, having originated from organic matter. Some also occurs in igneous rocks and in veins. Large quantities are made at Niagara Falls from anthracite by electricity.

Gypsum. Monoclinic crystals common, usually of simple forms, as shown by Figure 75i. Sometimes twin crystals. Composition, sulphate of lime. Colorless or white when pure. Can be scratched by the finger nail (hardness, 2). Specific gravity, 2.3. Three good cleavages, especially the prismatic, yielding cleavage plates with angles of 66 and 114 degrees. Thin cleavage layers, moderately flexible. There are several varieties: (1) _selenite_, which is clear, crystalline; (2) _satin spar_, fibrous with silky luster; (3) _alabaster_, fine-grained and compact crystalline; and (4) _rock gypsum_, massive granular or earthy. Gypsum is common and widespread especially among stratified rocks often as thick beds which have mostly resulted from evaporation of bodies of water containing it in solution, and often associated with salt beds. Also occurs as scattering crystals in shales and clays, and in some veins. In greatest quantities it is burned to make plaster of Paris. Satin spar and alabaster are often cut and polished for ornaments, etc. (See Figure 73a.)

Halite. Common salt. Composition, chloride of soda. Isometric crystals, nearly always in cubes with three good cleavages at right angles, and parallel to the faces of the cube. Hardness, 2.5; specific gravity, 2.5. Colorless to white when pure. Characteristic salty taste. Abundant and widespread, often as extensive strata in rocks of nearly all ages, having resulted from evaporation of inland bodies of salt water. Also in vast quantities in solution in salt lakes and the sea. Halite has many uses, as for example, cooking and preservative purposes, indirectly in glass making and soap making, glazing pottery, and in many ore-smelting and chemical processes.

Hematite.--One of the common and important iron oxides with less iron than magnetite and no water as has limonite. Crystallizes in hexagonal forms. Color, black, with metallic luster, when crystalline, otherwise usually dull red. Hardness, about 6; specific gravity, about 5. No cleavage. Red streak when rubbed on rough porcelain. Hematite is extremely widespread in rocks of all ages, especially in metamorphic and sedimentary rocks. Some occurs as crystals in igneous rocks, and some in vein deposits. It is the greatest ore of iron in the United States, especially in Minnesota, Michigan, Wisconsin, and Alabama.

Kaolin. Commonly called "China clay." Composition, a hydrous silicate of aluminum. Crystallizes in scalelike monoclinic forms, but usually forms compact claylike masses. Hardness, a little over 2; specific gravity, 2.6. Color when pure, white. Usually feels smooth and plastic. Very abundant and widespread, especially forming the main body of clay and of much shale. Always of secondary origin, generally resulting from the decomposition of feldspar. It is the main constituent of chinaware, pottery, porcelain, tiles, bricks, etc.

Limonite. An important oxide of iron in composition like hematite except for its variable water content. Never crystallized. Hardness, about 5; specific gravity, nearly 4. Color, light to dark brown to nearly black. Leaves a characteristic yellowish-brown streak when rubbed on rough porcelain. Exceedingly common and widely distributed, always as a mineral of secondary origin as a product of weathering of various iron-bearing minerals. Where accumulated in considerable deposits it is an iron ore of some importance.

Magnetite. One of the three important oxides of iron containing no water, and richer in iron than hematite. (See Figure 75e.) Commonly crystallizes in isometric octahedral forms alone or combined with twelve-faced forms. Hardness, 6; specific gravity, 5. Color, black with metallic luster. Leaves black streaks on rough porcelain. Characteristically highly magnetic. Wide-spread as crystals in nearly all kinds of igneous rocks, and as large segregation masses in certain igneous rocks. Also very common in metamorphic rocks, in many cases forming lenses and beds as ore deposits. Occurs in some strata and sands. It is an important ore of iron.

Malachite. A light-green hydrous carbonate of copper. In almost every way, except difference in color and slight difference in composition, it is very much like azurite (see above).

Mica Group. The micas rank high in abundance among the most common minerals of the earth. All of the several species are silicates of aluminum combined with other chemical elements according to the species. All crystallize in monoclinic six-sided prisms whose angles are nearly 120 degrees. These prisms closely approach true hexagonal forms. All are characterized by one exceedingly good cleavage at right angles to the prismatic faces, yielding very thin elastic cleavage sheets. Hardness, 2 to 2.5; specific gravity, 2.7 to 3. The various species or varieties are not always sharply separated from each other. Most common are: _muscovite_, or so-called _isinglass_, a potash mica which is colorless and transparent in thin sheets when pure; _biotite_, an iron-magnesia mica, black to dark green; and _phlogopite_, a brown magnesia mica.

Olivine. Often called _chrysolite_. A silicate of iron and magnesia. Orthorhombic crystals, usually in stout prismatic form. Color, usually yellowish green. Hardness, nearly 7; specific gravity, 3.3. Transparent to translucent. No real cleavage. Its hardness, color, and crystal form generally characterize it. It is a fairly common mineral found mainly as crystalline grains in certain dark-colored igneous rocks. A clear green variety, called _peridot_, is used as a gem stone.

Opal. An oxide of silicon, like quartz in composition except that it is combined with a varying amount of water. It never crystallizes, probably because of its rather indefinite composition. Hardness 5.5 to 6.5 (softer than quartz); specific gravity, about 2. Varieties variously colored. _Common opal_, usually translucent with greasy luster. _Precious opal_, translucent with beautiful play of colors, used as a gem. _Fire opal_, with bright red to orange internal reflections. _Hyalite_, colorless and transparent in small rounded masses. _Wood opal_, wood petrified by opal. _Geyserite_, a white, porous, stringy variety deposited by certain hot springs like the Yellowstone geysers. _Tripolite_, fine-grained, chalklike in appearance, consisting of tiny siliceous shells of very simple plants called diatoms.

Platinum. This mineral occurs as an impure native metal, usually alloyed with certain other metals. Native platinum, hardness, 4.5 (exceptionally high for a metal); specific gravity as usually alloyed, 14 to 19. Pure platinum, specific gravity, over 21, or one of the very heaviest known substances. Color, light steel-gray, with metallic luster. Very malleable and ductile. A rare metal found commercially mostly in gravel or "placer" deposits mostly in the Ural Mountains, also as grains in certain dark igneous rocks. Used for many scientific instruments, in the electrical industry, as jewelry, etc.

Pyrite. Commonly called "iron pyrites." Sometimes called "fool's gold." (See Figure 74m.) A sulphide of iron which commonly crystallizes in the isometric system mostly as cubes, twelve-faced pyritohedrons, octahedrons, or combinations of these. Color, light brass-yellow, with metallic luster. Cleavage, practically absent. Hardness, greater than that of steel (over 6 in the scale); specific gravity, about 5. Leaves greenish black streak when rubbed on rough porcelain. Differs from chalcopyrite by paler color and much greater hardness. It is a common and very widely disseminated mineral in rocks of all kinds and ages, but especially in metamorphic rocks as veins, and banded or lenslike deposits. Most igneous rocks contain small scattering grains of pyrite. Many deposits of commercial value are known. Great quantities are burned for the manufacture of sulphuric acid ("oil of vitriol") which is one of the most important of all chemicals.

Pyroxene Group. Along with quartz and feldspars, the pyroxenes rank among the most common of all minerals. (See Figure 74k.) Composition, very similar to amphibole (see above). Pyroxenes crystallizing in the monoclinic system are the most important. These crystals are prismatic in habit, with prism faces making angles of nearly 45 or 90 degrees instead of about 124 degrees as in the monoclinic amphiboles which the monoclinic pyroxenes greatly resemble. Two fairly good prismatic cleavages cross at an angle of nearly 90 degrees, instead of at about 124 degrees as in the monoclinic amphiboles. Hardness, 5 to 6; specific gravity, 3.2 to 3.6. Color, variable according to species. The most common variety of pyroxene is _augite_, a dark-green to black silicate of aluminum, iron, lime, and magnesia. Certain pyroxenes also crystallize in the orthorhombic system. Pyroxene is most abundantly represented as crystals in many kinds of igneous and metamorphic rocks. It is practically useless except as one kind of _jade_.

Quartz. Next to the feldspars, quartz is probably the most common of all minerals, especially at and near the earth's surface. (See Figures 74e, 74f, and 74g.) Composition, oxide of silicon. Often crystallizes in the trigonal system almost always as six-sided prisms capped by six-sided pyramids, which are really combined three-sided forms, often with alternate corners modified by small faces. These small modifying faces, etching figures, and microscopic tests show that quartz is really trigonal in spite of the common occurrence of simple six-sided outward forms. The pyramidal faces make different angles than those of either apatite or beryl, both of which are somewhat like quartz in crystal form. Hardness, 7 (distinctly high, cannot be scratched by the knife); specific gravity, 2.6 (about average for all minerals). Cleavage, practically absent, and breaks like glass. Colorless when pure, but varieties exhibit many colors. A few only of the many varieties will be briefly described. Among the distinctly crystalline varieties are: _rock crystal_, pure colorless; _amethyst_, purple; _rose quartz_, pink; _milky quartz_, white; and _smoky quartz_, dark--due to tiny inclusions of carbon. Among the fine-grained, compact more or less indistinctly crystalline or noncrystalline varieties, usually translucent with a waxy luster, are: _chalcedony_, bluish gray, waxy looking, usually in small rounded masses; _carnelian_, red; _prase_, green; _agate_, with parallel bands, usually variously colored; _flint_ and _jasper_, opaque to translucent, dark to red.

Quartz is exceedingly abundant in all the great groups of rocks. It constitutes the main bulk of sandstones, is common in shales, and occurs in certain other strata. In many igneous rocks, like granite, it is a very prominent constituent. Most of the metamorphic rocks contain its crystalline forms in greater or less amounts. Quartz is the most common of all vein minerals, in many cases associated with valuable ores. Various varieties are widely used for ornamental purposes. Used in making sandpaper, glass, porcelain, mortar, concrete, and in certain ore-smelting processes. Sandstone is widely used as a building stone.

Serpentine. A hydrous silicate of magnesia never in distinct crystals as such, but shown to be monoclinic under the microscope. Hardness variable, 2.5 to 5; specific gravity, about 2.6. Mostly of variegated green or yellowish green color with waxy luster, except a fibrous variety (_asbestos_) which is light green to white. The fibrous variety of serpentine is the principal source of asbestos, an amphibole asbestos being less common. Ordinary serpentine (sometimes miscalled "green marble") is widely used as a building and decorative stone. Serpentine is common and widespread, especially in igneous and metamorphic rocks, but never as a really original mineral. It always results from alteration of certain other magnesia-bearing silicate minerals, such as pyroxene, amphibole, olivine, etc.

Silver. Native silver is not a very rare mineral and it is mined in certain parts of the world, but most of the metal is obtained from certain silver-bearing minerals, especially sulphides and a chloride. Silver crystallizes rather rarely in the isometric system. More commonly it occurs as irregular masses, plates, and wirelike forms. Characterized by its color, metallic luster, softness (less than 3 in the scale), and exceptional weight (specific gravity, 10.5). Usually occurs in vein deposits, commonly associated with other metals or metal-bearing minerals, especially copper.

Sphalerite. A sulphide of zinc commonly in crystalline form belonging in the isometric system, especially in tetrahedral combination forms (see Figure 75b). Color, usually brown, yellow or nearly black with resinous luster. Hardness, nearly 4; specific gravity, 4. Several good cleavages, yielding fragments whose faces meet at 90 and 120 degrees. Sphalerite is a fairly common and widespread mineral, occurring nearly always in veins in most kinds of rocks. It is very often associated with other ores, particularly the great ore of lead (galena). Sphalerite is by far the greatest ore of zinc.

Sulphur. Native sulphur. Crystallization, orthorhombic, usually in combination pyramidal forms. (See Figure 75h.) Characterized by yellow color, resinous luster, softness (about 2 in the scale), low specific gravity (about 2), and very poor cleavages. It has most commonly resulted from alteration of certain sulphur-bearing minerals, especially gypsum, the decomposition of which has yielded vast deposits. Some also of volcanic origin. Great quantities are used in making sulphuric acid, matches, gunpowder, fireworks, and for vulcanizing and bleaching rubber goods.

Talc. Often called _steatite_. Monoclinic crystals rare. One perfect cleavage, yielding very thin, flexible leaves. Very soft (hardness, 1). Feels greasy, and looks waxy to pearly. Color, white, gray, to light green. Specific gravity, 2.8. Composition, a hydrous silicate of magnesia, much like that of serpentine. Talc is always of secondary origin, generally derived by chemical alteration of various common minerals rich in silicate of magnesia. _Soapstone_ is a common variety resulting from alteration of whole rock masses. Soapstone has many practical uses as for washtubs, table tops, electrical switchboards, hearthstones, stove and furnace linings, blackboards, gas tips, etc. Talc proper is used as a lubricant, to weight paper, in soap, as dustless crayon, talcum powder, etc.

Topaz. A silicate of aluminum and fluorine. Orthorhombic crystals common, usually prisms capped at one end by pyramided faces and abruptly terminated at the other. Colorless when pure, but often variously colored due to impurities. Very exceptionally hard (8 in the scale); specific gravity, 3.5. One good cleavage across the prism zone; usually found as crystals in, and in cavities in, igneous rocks. Appears always to have formed from highly heated vapors or liquids given off by cooling molten rock masses. Topaz is one of the more highly prized of the gem stones.

Tourmaline. Composition, very complex, but chiefly a silicate of boron and several metals and semimetals. Commonly as crystals in the trigonal system in both long and short prismatic forms, as shown by Figure 75m, with opposite ends not unlike. Extra hard (7 in the scale); specific gravity, about 3. Color, widely various, but brown and black are most common. Practically no cleavage. Tourmaline probably always originated as a high temperature mineral, especially as crystals in granites and related rocks and in certain metamorphic rocks which have been subjected to high temperature and pressure. Certain transparent colored varieties of tourmaline rank high among the semiprecious stones.

Turquoise. A hydrous phosphate of aluminum. Massive noncrystalline, blue to green, waxy luster, mostly opaque, hardness of 6, and specific gravity of about 2.7. Turquoise is a high temperature mineral found in veins and cavities in certain igneous rocks. It is a rare mineral used as a gem stone.

Zircon. A silicate of zirconium usually crystallized in the tetragonal system as simple four-sided prisms capped by four-sided pyramids. (See Figure 75l.) Very poor cleavages. Color usually brown. Hardness, 7.5 (extra high); specific gravity, nearly 4.7. Brilliant luster. Zircon is very commonly present as scattering crystals of varying size in most igneous rocks. Also common as crystals in various metamorphosed stratified rocks, and less common in some sand and gravel deposits. Certain transparent varieties, especially the brown and pink ones called _hyacinth_, are used as gem stones. Zircon is also the source of oxide of zirconium used in making mantles for certain incandescent lights.