Quartz

Quartz :second most common of all minerals, composed of silicon dioxide, or silica, SiO2. It is distributed all over the world as a constituent of rocks and in the form of pure deposits. It is an essential constituent of igneous rocks such as granite, rhyolite, and pegmatite, which contain an excess of silica. In metamorphic rocks, it is a major constituent of the various forms of gneiss and schist; the metamorphic rock quartzite is composed almost entirely of quartz. Quartz forms veins and nodules in sedimentary rock, principally limestone. Sandstone, a sedimentary rock, is composed mainly of quartz. Many widespread veins of quartz deposited in rock fissures form the matrix for many valuable minerals. Precious metals, such as gold, are found in sufficient quantity in quartz veins to warrant the mining of quartz to recover the precious mineral. Quartz is also the primary constituent of sand. Properties : Quartz crystallizes in the rhombohedral system. The size of the crystals varies from specimens weighing a metric ton to minute particles that sparkle in rock surfaces. Quartz is also common in massive forms, which contain particles ranging in size from coarse-

grained to cryptocrystalline (grains invisible to the naked eye but observable under a microscope). The mineral has a hardness of 7 and specific gravity of 2.65. The luster in some specimens is vitreous; in others it is greasy or splendent (shining glossily). Some specimens are transparent; others are translucent. In the pure form, the mineral is colorless, but it is commonly colored by impurities. Quartz crystals exhibit a property called the piezoelectric effect, that is, they produce an electric voltage when subjected to pressure along certain directions of the crystal. Because of this property, quartz crystal has important applications in the electronics industry for controlling the frequency of radio waves. It also has the optical property of rotating the plane of polarized light and is used in polarizing microscopes. Quartz crystals undergo structural transformations when heated. Ordinary, or low, quartz, when heated to 573° C (1063.4° F), is converted into high quartz, which has a different crystal structure and different physical properties. When cooled, however, high quartz reverts to low quartz. Between 870° and 1470° C (1598° and 2678° F), quartz exists in the form called tridymite, and

above 1470° C (2678° F), the stable form is known as cristobalite. At about 1710° C (3078° F), the mineral melts. Varieties:The coarsely crystalline varieties of quartz are, in general, transparent and lustrous. Rock crystal, a colorless form of quartz, usually occurs in distinct crystals. Rose quartz is coarsely crystalline but without distinct crystal form and is colored rose red or pink, the color often fading on exposure to light. Smoky quartz, or cairngorm stone, occurs in crystals ranging from smoky yellow to dark brown. Amethyst, a semiprecious variety of quartz, is colored purple or violet. Many other minerals form inclusions in crystalline varieties of quartz. Rutilated quartz contains fine needles of rutile that penetrate crystals of colorless quartz. Aventurine is a variety of quartz containing brilliant scales of hematite or mica. Liquids and gases also occur as inclusions in quartz. Milky quartz owes its milky-white color to the presence of numerous minute liquid or gaseous inclusions. The cryptocrystalline varieties of quartz are often divided into two general classes, fibrous and granular. The fibrous varieties, which include agate, carnelian,

heliotrope, onyx, and chrysoprase, are all forms of chalcedony. The granular varieties include chert, flint, jasper, and prase. Uses :The different forms of chalcedony and many of the crystalline varieties of quartz are used as gemstones and other ornamental materials. Pure rock crystal is used in optical and electronic equipment. In the form of sand, quartz is used extensively in the manufacture of glass and silica brick, and is also used in cement and mortar. Ground quartz is used as an abrasive in stonecutting, sandblasting, and glass grinding. Powdered quartz is used in making porcelain, scouring soaps, sandpaper, and wood fillers. Large amounts of quartz are used as a flux in smelting operations. Almost all natural high-grade quartz crystal, which is an important raw material in the electronics industry, is imported from Brazil, the only country with large deposits of the mineral in commercial quantities. Quartz crystals can also be made synthetically. For additional information, see articles on most of the minerals mentioned.