18. Talc.—Hydrous silicate of magnesium: silica (SiO₂), 63 (acidic); magnesia (MgO), 32; water (H₂O), 5; = 100. Orthorhombic system, but rarely in distinct crystals. Cleavage in one direction very perfect; the cleavage lamellæ are flexible, but not elastic, as in mica. Hardness, 1; see scale. Sp. gr., 2.55-2.8. Lustre, pearly. Color, apple-green to white; and streak, white. The feel is very smooth and greasy; and, in connection with the color and foliation, affords the best means of distinguishing talc from allied minerals. Talc sometimes results from the alteration of augite, hornblende, and other minerals, but it is not always nor usually an alteration product.
19. Serpentine.—Hydrous silicate of magnesium: silica (SiO₂), 44 (basic); magnesia (MgO), 44; water (H₂O), 12; = 100. Essentially amorphous. Hardness, 2.5-4; sp. gr., 2.5-2.65. Lustre, greasy, waxy, or earthy. Color, various shades of green and usually darker than talc, but streak always white. Feel, smooth, sometimes greasy. Distinguished from talc by its hardness, compactness, and darker green. Sometimes results from the alteration of olivine and other magnesian minerals, but usually we are to regard it as an original mineral. Specimen 25.
20. Chlorite.—This is, properly, the name of a group of highly basic minerals of very variable composition, but they are all essentially hydrous silicates of aluminum, magnesium, and iron; and the average composition of the most abundant species, prochlorite, is as follows: silica (SiO₂), 30; alumina (Al₂O₃), 18; magnesia (MgO), 15; protoxide of iron (FeO), 26; and water (H₂O), 11; = 100. The chlorites crystallize in several different systems, but in all there is a highly perfect cleavage in one direction, giving, as in talc, a foliated structure with flexible but inelastic laminæ. The cleavage scales, however, are sometimes minute, and the structure massive or granular. Hardness of prochlorite, 1-2; between talc and serpentine. Sp. gr., 2.78-2.96. All the chlorites have a pearly to vitreous lustre. Color usually some shade of green; in prochlorite a dark or blackish green, darker than serpentine, as that is darker than talc. Streak, a lighter, whitish green. Less unctuous than talc, but more so than serpentine. The chlorites are produced very commonly, but not generally, by the alteration of basic anhydrous silicates, like augite and hornblende. Specimen 26.
21. Hydro-mica.—This, too, is properly the name of a group of minerals; but for geological purposes they may be regarded as one species. Taking a general view of the composition, these are simply the anhydrous or ordinary micas, which we have already studied, with from 5 to 10 per cent. of water added. In crystallization and structure they are essentially mica-like. Although not distinctly softer than the common micas, they are lighter, always more unctuous and slippery, and usually of a greenish color. The micaceous structure with elastic laminæ serves to distinguish the hydro-micas from other hydrous silicates.
22. Glauconite.—Hydrous silicate of aluminum, iron, and potassium: silica (SiO₂), 50; alumina (Al₂O₃), protoxide of iron (FeO), and potash (K₂O), together, 41; and water (H₂O), 9; = 100. Amorphous, forming rounded and generally loose grains, which often have a microscopic organic nucleus. It is dull and earthy, like chalk, and always soft, green, and light, but not particularly unctuous. Glauconite is the principal, often the sole, constituent of the rock greensand, which occurs abundantly in the newer geological formations, and is now forming in the deep water of the Gulf of Mexico and along our Atlantic sea-board. Specimen 27.
This completes our list of minerals occurring chiefly as essential constituents of rocks; and following are three of the more common and important minerals occurring chiefly as accessory, rarely as essential, rock-constituents.
23. Chrysolite (Olivine).—Silicate of magnesium and iron: silica (SiO₂), 41; magnesia (MgO), 51; protoxide of iron (Fe₂O₃), 8; = 100. Orthorhombic system; but usually in irregular glassy grains. Hardness, 6-7. Sp. gr., 3.3-3.5. Lustre, vitreous; color, usually some shade of green; and streak, white. Chrysolite sometimes closely resembles quartz, but its green color usually suffices to distinguish it. It is a common constituent of basalt and allied rocks. By absorption of water it is changed into serpentine and talc. See examples in specimen.
24. Garnet.—The composition of this mineral is extremely variable; but the most important variety is a basic silicate of aluminum and iron: silica (SiO₂), 37; alumina (Al₂O₃), 20; and protoxide of iron (FeO), 43; = 100. Isometric system, usually in distinct crystals, twelve-sided (dodecahedrons) and twenty-four-sided (trapezohedrons) forms being most common. Hardness, 6.5-7.5; average as hard as quartz. Sp. gr., 3.15-4.3; compare with the high percentage of iron. Lustre, vitreous; colors, various, usually some shade of red or brown; and streak, white. Some varieties contain iron enough to make them magnetic. Garnet is easily distinguished by its form, color, and hardness from all other minerals. It is a common but not an abundant mineral, occurring most frequently in gneiss, mica schist, and other stratified crystalline rocks. See examples in specimen.
25. Pyrite.—Sulphide of iron: sulphur, 53.3; iron, 46.7; = 100. Isometric system, occurring usually in distinct crystals, the cube and the twelve-sided form known as the pyritohedron being the most common. Hardness, 6-6.5, striking fire with steel. Sp. gr., 4.8-5.2; heavy because rich in iron. Lustre, metallic and splendent. Color, pale, brass-yellow, and streak, greenish or brownish. Pyrite is sometimes mistaken for gold, but it is not malleable; while its color, hardness, and specific gravity, combined, easily distinguish it from all common minerals. As an accessory rock-constituent, pyrite occurs usually in isolated cubes or pyritohedrons. Specimen 10.