Quartz, second most common of all minerals in Earth's crust after
feldspar. Quartz is 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.
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.
Properties of Quartz
- Quartz crystallizes in the rhombohedral system, with six faces each shaped like a rhombus. 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 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.
Uses Of Quartz
- 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 has been used in making porcelain, scouring soaps, sandpaper, and wood fillers. Large amounts of quartz have been used as a flux in smelting operations. Natural high-grade quartz crystal has been an important raw material in the electronics industry. Quartz crystals can also be made synthetically.