Fig. 75.—Crystal forms cf common minerals: a, galena (Isometric); b, sphalerite (Isometric); c, beryl (Hexagonal); d, hematite (Hexagonal); e, magnetite (Isometric); f, barite (Orthorhombic); g, apatite (Hexagonal); h. sulphur (Monoclinic); i, gypsum (Monoclinic); j, chalcopyrite (Tetragonal); k, fluorite (Isometric); l, zircon (Tetragonal); m, tourmaline (Trigonal); n, corundum (Hexagonal).
Fig. 75.—Crystal forms cf common minerals: a, galena (Isometric); b, sphalerite (Isometric); c, beryl (Hexagonal); d, hematite (Hexagonal); e, magnetite (Isometric); f, barite (Orthorhombic); g, apatite (Hexagonal); h. sulphur (Monoclinic); i, gypsum (Monoclinic); j, chalcopyrite (Tetragonal); k, fluorite (Isometric); l, zircon (Tetragonal); m, tourmaline (Trigonal); n, corundum (Hexagonal).
Beryl.A silicate of aluminum and the rare chemical element beryllium. Hexagonal crystals usually of very simple six-sided prismatic habit (seeFigure 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) andaquamarine(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 (includingchalk) 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, calledtravertine, is deposited by certain hot springs, as at Mammoth Hot Springs in Yellowstone Park. A very transparent crystalline variety is calledIceland 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 (seebelow), 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 aresult 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 (seeFigure 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) andsapphire(blue)—are nearly transparent varieties of corundum, colored by certain impurities.Oriental topaz(yellow),oriental emerald(green), andoriental 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.Emeryis a fine-grainedmixture 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 areorthoclaseandmicrocline, 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 asAmazon stone. The soda-lime feldspars go by the general nameplagioclase. They are triclinic, with cleavages meeting at approximately 86 degrees. Very commonlyone 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 arealbite, a white soda feldspar, including most so-calledmoonstone;oligoclase, a usually greenish-white to reddish-gray soda-lime feldspar includingsunstone; andlabradorite, 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 aflux 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 (seebelow).
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 byFigure 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 byFigure 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 especiallyamong 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. (SeeFigure 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. (SeeFigure 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 oredeposits. 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 (seeabove).
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-calledisinglass, a potash mica which is colorless and transparent in thin sheets when pure;biotite, an iron-magnesia mica, black to dark green; andphlogopite, a brown magnesia mica.
Olivine.Often calledchrysolite. 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, calledperidot, is used as a gem stone.
Opal.An oxide of silicon, like quartz in composition except that it is combined with a varyingamount 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.” (SeeFigure 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 greenishblack 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. (SeeFigure 74k.) Composition, very similar to amphibole (seeabove). 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 isaugite, 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 ofjade.
Quartz.Next to the feldspars, quartz is probably the most common of all minerals, especially atand near the earth’s surface. (SeeFigures 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; andsmoky 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;flintandjasper, 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 formsin 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 (seeFigure 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. (SeeFigure 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 calledsteatite. 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.Soapstoneis a common variety resulting from alteration of whole rock masses. Soapstone has many practicaluses 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 byFigure 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 mineralfound 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. (SeeFigure 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 calledhyacinth, are used as gem stones. Zircon is also the source of oxide of zirconium used in making mantles for certain incandescent lights.
CHAPTER XXI
ECONOMIC GEOLOGY
I
IN this chapter it is our purpose to briefly consider geology in its direct relations to the arts and industries. When we realize that the value of strictly geologic products taken from the earth each year in the United States alone amounts to billions of dollars, we can better appreciate the practical application of geological science. Such products include coal, petroleum, natural gas, many valuable metal-bearing minerals, and many nonmetalliferous minerals and rocks. In most cases these valuable products of nature have been slowly accumulated or concentrated at many times and under widely varying conditions throughout the millions of years of known geological time. To trace the extent of, and most advantageously remove, such deposits for the use of man is always invariably impossible unless geological knowledge is brought to bear. In many cases the problems involved are intricate, and only the trained geologist is able to at all successfully cope with them. In such cases it is necessary not only to have a thorough knowledge of minerals and rocks as such, but also of their origin and structure. Much of the practical application of geology is carried out by the mining engineer who should have, above all, a thorough knowledge of the great principles of geology.
Our plan of discussion is to consider, first, coal, petroleum, and natural gas; then the most important metalliferous deposits of ores; and finally nonmetalliferous minerals and rocks of exceptional commercial importance. Underground waters have already been discussed from the practical standpoint in the chapter on “Waters Within the Earth.” Certain minerals have already been sufficiently considered from the economic standpoint in the chapter on “Mineralogy.”
COAL, PETROLEUM, AND NATURAL GAS
Coal.Most valuable of all geological products is coal. Although it is not, strictly speaking, a mineral, both because of its organic origin and lack of definite chemical composition, coal is generally classed among our mineral resources. Some idea of the national importance of coal in the United States may be gained when we realize that the energy derived from a single year’s output is equivalent to that of hundreds of millions of men working full time through the year. The uses of coal are too well known to need mention here.
Coal is, beyond question, of organic (plant) origin as shown by its very composition; perfect gradations between plant deposits like peat and true coal; and the presence of microscopic plant remains and spores in the coal. An excellent summary of just what happens during the transition of ordinary vegetable matter into coal has been given by D. White as follows: "All coal was laid down in beds analogous to the peat beds of to-day. All kinds of plants, especially such species as were adapted to the particular region where the deposit was located, in whole or in part went into the deposit.
"Plants are composed chiefly of cellulose and proteins. The former, comprising by far the larger bulk, constitute the framework, whereas the latter are concerned in the vital functions. With these are associated many other substances, among which are chiefly starch, sugars, and fats and oils, constituting reserve foodstuffs; waxes, resin waxes, resins, and higher fats, performing mainly protective functions.... These components differ widely in their resistance to various agencies. Those substances involved in the life function and the support of the plant are relatively very stable under the conditions imposed upon them.
"At the death of the plants, governed by conditions imposed in the bog, a partial decomposition, maceration, elimination, and chemical reduction begins, brought about by various agencies, chiefly organic, mainly fungi at first and bacteria later. The most labile are removed first, the more resistant next, and so on, as the conditions require, leaving the most resistant behind in a residue called peat.
“The process of decomposition, elimination, and chemical reduction begun in peat, chiefly by biochemical means, is taken up and continued by dynamochemical means into and through the various successive later stages, and results in the various grades of coal, as lignite, sub-bituminous, and cannel coal, and anthracite.”
The principal chemical elements involved in the changes which take place are carbon, oxygen, and hydrogen, as shown by the following analyses of about average samples of each member of the so-called “coal series.”
From this table it is seen that the oxygen relatively diminishes while the carbon relatively increases, though, of course, all three elements actually decrease during the chemical change from cellulose to coal. These three elements disappear mainly in the form of gases, such as water vapor, marsh gas, and carbonic acid gas. The final or graphite stage is almost reached by the graphitic anthracite of Rhode Island, which is so nearly pure carbon as to be really useless as coal.
The conditions under which successive layers of vegetable matter (later turned into coal) become embedded in the earth’s crust have been outlined in the chapter on the “Evolution of Plants.” The most perfect conditions for prolific plant growth, and accumulation as great beds in the earth’s crust, were during the Pennsylvanian period of the late Paleozoic era in many parts of the world, but especially in the United States, China, Great Britain, and Germany. Most of the world’s great supply of coal comes from rocks of Pennsylvanian Age, while next in importance are Cretaceous rocks, and some comes from strata of other ages later than the Pennsylvanian, even as late as the Tertiary.
The United States not only has the greatest known coal fields, but it also produces far more coal than any other country. In 1918 the production was 678,000,000 tons, the greatest in our history,or enough, if loaded into cars of forty tons capacity, to fill a train which would reach around the earth at the equator about six times! Equally amazing is the fact that this coal was nearly all consumed by this one nation! In 1919 the production fell to 544,000,000 tons. Is there real danger that our supply of coal will soon run out? Hardly so when we consider, first, the fact that probably not more than 1 per cent of the readily available coal has thus far been removed, and, second, the high probability that rate of increase in coal production for the last twenty years will not continue. In fact, during the last two or three years the production has fallen off considerably. But even so, coal, which is our greatest natural resource, and which can never be replaced, should be scientifically conserved. In the case of the very restricted anthracite coal fields what might be called a crisis has already been reached, because a very considerable part of the available supply has been taken out.
Something like 350,000 square miles of the United States are underlain with one or more beds of workable coal (not including lignite)—in some areas five to twenty or more beds one above the other. There are also about 150,000 square miles of country underlain with the more or less imperfect coal called lignite. It has been estimated that there are more than a trillion tons of easily accessible coal, and another trillion tons accessible with some difficulty in the principal coal fields of the United States.
The greatest production of coal by far is from the Appalachian Mountain and Allegheny Plateau districts, from the western half of Pennsylvania to Alabama, where all the coal is bituminous of PennsylvanianAge. Here as well as elsewhere the coal beds are interstratified with various kinds of sedimentary rocks, most commonly with shales and sandstones. In the Appalachian field the strata including coal beds are more or less folded toward the east, while they are nearly horizontal toward the west. The famous Pittsburgh coal bed is probably the most extensive important single coal bed known. It covers an area of over 12,000 square miles and is workable, with a thickness of five to fifteen feet, over an area of 6,000 square miles of parts of western Pennsylvania, Ohio, and West Virginia.
Fig. 76.—Map of the United States, showing the principal coal fields. Cross-lined areas represent lignitic coals. (After U. S. Geological Survey.)
Fig. 76.—Map of the United States, showing the principal coal fields. Cross-lined areas represent lignitic coals. (After U. S. Geological Survey.)
The greatest production of anthracite coal by far is from central-eastern Pennsylvania, where strata of Pennsylvania Age, including a number of anthracite beds, are mostly highly folded. Most remarkable of all in this district is the so-called “mammoth bed” of anthracite, nearly everywherepresent, with a thickness up to as much as fifty or sixty feet. Less than 500 square miles are there underlain by workable anthracite coal.
Next to the greatest production of coal in the United States is from the two large areas in the middle of the Mississippi Valley. It is all bituminous coal, associated with nearly horizontal strata of Pennsylvanian Age.
The scattering areas through the Rocky Mountains yield all types of coal—anthracite, bituminous, and lignite. In some of these areas the coal beds have been but little disturbed from their original horizontal position, but usually they are more or less folded along with the inclosing strata, the crustal disturbances affecting the coal beds having taken place late in the Mesozoic era and early in the Cenozoic era. Practically all of these coals are of Cretaceous and Tertiary Ages, the best being Cretaceous. Very little of the Rocky Mountain coal is anthracite.
On the Pacific Coast coal production is relatively very small. The coals are there bituminous to lignitic of Tertiary Age, usually folded in with the strata.
In Alaska there are widely distributed, relatively small coal fields, but they have been little developed. Alaskan coals range in age from Pennsylvanian to Tertiary, and in kind from anthracite to lignite.
Petroleum.Crude oil or petroleum is an organic substance consisting of a mixture of hydrocarbons, that is, it is made up very largely of the two chemical elements carbon and hydrogen, in rather complex and variable combinations. It is practically certain that petroleum has been derived by a sort of slow process of distillation fromorganic matter—animal or vegetable or both—in stratified rocks within the earth. Many strata, as for example carbonaceous shales, are more or less charged with dark-colored decomposing organic matter. The chemical composition itself, the kinds of rocks with which it is associated, and certain optical (microscopic) tests all point to the organic origin of petroleum. In southern California at least, certain of the oils have quite certainly been derived from the very tiny oily plants called diatoms which fill many of the strata.
Fig. 77.—Profile and structure section showing folding of strata, with included coal beds, across one of the anthracite coal fields of eastern Pennsylvania. Length of section, a little over 2 miles. (After U. S. Geological Survey.)
Fig. 77.—Profile and structure section showing folding of strata, with included coal beds, across one of the anthracite coal fields of eastern Pennsylvania. Length of section, a little over 2 miles. (After U. S. Geological Survey.)
During the last twenty years petroleum has come to be one of the most important and useful natural products. Among the many substances artificially derived from petroleum are kerosene, gasoline, naphtha, benzine, vaseline, and paraffine. The United States leads inthe production of petroleum, while southern Russia and Mexico are very important producers. In the United States the principal areas underlain with petroleum-bearing strata are the northern Appalachian field (through western Pennsylvania to central West Virginia); the Ohio-Indiana field (central Indiana to northwestern Ohio); the mid-continental field (southeastern Kansas and northeastern Oklahoma); the southeastern Texas-Louisiana field; and the southwestern California field. The total areas underlain with oil total about 10,000 square miles. In the Appalachian, Ohio-Indiana, and mid-continental fields the strata carrying oil range in age from Ordovician to Pennsylvanian, and they are mostly but little disturbed from their original horizontal position. The Texas-Louisiana oils come mainly from Cretaceous and Tertiary strata which gently downtilt under the Coastal Plain toward the Gulf. In California the oil-bearing strata are of Tertiary Age and generally considerably disturbed and folded.
Under proper conditions below the earth’s surface the derived oil accumulates in porous or fractured rocks. There must, of course, be a source from which the petroleum is derived or distilled; a porous or fractured rock formation to take it up; a cap rock or impervious layer to hold it in; and a proper geologic structure to favor accumulation. The most common porous (containing) rock is sandstone, and the most common cap rock is shale. Oil is rarely found without gas, and saline water is likewise often present. If the containing strata are horizontal, the oil and gas are usually irregularly scattered, but if tilted or folded, and the beds porous throughout, they appear to collect at the highest point possible. It was the result of observations along this line that led I. C. White to develop what is known as the “anticlinal theory.” According to this theory, in folded areas the gas collects at the summit of the fold (anticline), with the oil immediately below, on either side, followed by the water. It is, of course, necessary that the oil-bearing stratum shall be capped by a practically impervious one.