Geology: The Science of the Earth's Crust

by William J. Miller

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.