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.