Geology: The Science of the Earth's Crust

by William J. Miller

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.