Geology: The Science of the Earth's Crust

by William J. Miller

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.