Quartz, widely distributed mineral of many varieties that consists primarily of silica, or silicon dioxide (SiO2). Minor impurities such as lithium, sodium, potassium, and titanium may be present. It is the second most abundant mineral in Earths crust after feldspar. It occurs in nearly all acid igneous, metamorphic, and sedimentary rocks. It is an essential mineral in such silica-rich felsic rocks as granites, granodiorites, and rhyolites. It is highly resistant to weatheringand tends to concentrate in sandstones and other detrital rocks. Secondary quartz serves as a cement in sedimentary rocks of this kind, forming overgrowths on detrital grains. Microcrystalline varieties of silica known as chert, flint, agate, and jasper consist of a fine network of quartz. Metamorphism of quartz-bearing igneous and sedimentary rocks typically increases the amount of quartz and its grain size.
Properties
Quartz exists in two forms: (1) Alpha or low, quartz, which is stable up to 573 C (1,063 F), and (2) Beta-, or high, quartz, stable above 573 C. The two are closely related, with only small movements of their constituent atoms during the alpha-beta transition. The structure of beta-quartz is hexagonal, with either a left- or right-handed symmetry group equally populated in crystals. The structure of alpha-quartz is trigonal, again with either a right- or left-handed symmetry group. At the transition temperature the tetrahedral framework of beta-quartz twists, resulting in the symmetry of alpha-quartz; atoms move from special space group positions to more general positions. At temperatures above 867 C (1,593 F), beta-quartz changes into tridymite, but the transformation is very slow because bond breaking takes place to form a more open structure. At very high pressures alpha-quartz transforms into coesiteand, at still higher pressures, stishovite.
Glass Making
Geological processes have occasionally deposited sands that are composed of almost 100% quartz grains. These deposits have been identified and produced as sources of high purity silica sand. These sands are used in the glass making industry. Its sand is used in the production of container glass, flat plate glass, specialty glass and fiber glass.
Abrasive
The high hardness of quartz, seven on the Mohs Scale, makes it harder than most other natural substances. As such it is an excellent abrasive material. Its sands and finely ground silica sand are used for sand blasting, scouring cleansers, grinding media, and grit for sanding and sawing.
Foundry Sand
Quartz is very resistant to both chemicals and heat. It is therefore often used as a foundry sand. With a melting temperature higher than most metals it can be used for the molds and cores of common foundry work. Refractory brick are often made of quartz sand because of its high heat resistance. Quartz sand is also used as a flux in the smelting of metals.
Petroleum Industry
Quartz sand has a high resistance to being crushed. In the petroleum industry sand slurries are forced down oil and gas wells under very high pressures in a process known as hydraulic fracturing. This high pressure fractures the reservoir rocks and the sandy slurry injects into the fractures. The durable sand grains hold the fractures open after the pressure is released. These open fractures facilitate the flow of natural gas into the well bore.
Many Other Quartz Sand Uses
Quartz sand is used as a filler in the manufacture of rubber, paint and putty. Screened and washed, carefully sized quartz grains are used as filter media and roofing granules. Quartz sands are used for traction in the railroad and mining industries. These sands are also used in recreation on golf courses, volleyball courts, baseball fields, children's sand boxes and beaches.
Uses of Quartz Crystals
High quality quartz crystals are single-crystal silica with optical or electronic properties that make them useful for specialty purposes. USGS estimates that about ten billion quartz crystals are used every year. Electronics grade crystals can be used in filters, frequency controls, timers, electronic circuits that become important components in cell phones, watches, clocks, games, television receivers, computers, navigational instruments and other products. Optical-grade crystals can be used as lenses and windows in lasers and other specialized devices. Although some natural quartz crystals are used in these applications, most of these special crystals are now manufactured.
Quartz as a Gemstone
Quartz makes an excellent gemstone. It is hard, durable and usually accepts a brilliant polish. Popular varieties of quartz that are widely used as gems include: amethyst, citrine, rose quartz, and aventurine. Agate and jasper are also varieties of quartz with a microcrystalline structure.
