Quartz (mineral)
Quartz is a highly abundant mineral, constituting about 12% of the Earth's crust, and is second only to feldspar in terms of prevalence. It is commonly found in various rock types, including sandstone, granite, and rhyolite. Natural quartz crystals, prized for their size and clarity, are primarily sourced from Brazil and smaller deposits in the United States, particularly in Arkansas and New York. The mineral is integral to the electronics industry due to its piezoelectric properties, which are utilized in devices like oscillators and timekeeping instruments. Additionally, quartz is featured in the gemstone market, with varieties like amethyst and citrine being particularly valued.
Quartz has a low solubility in water and is found in several natural forms, including amorphous silica in organisms like diatoms and sponges. While solid quartz is generally non-toxic, inhaling quartz dust from mining can lead to serious health issues, including silicosis, especially among smokers. Historically, quartz has been used since prehistoric times for tools and weapons and has evolved to play a crucial role in modern technology. The cultured quartz production processes developed in the 20th century now meet most industrial demands, reflecting quartz's multifaceted significance in both nature and human applications.
Quartz (mineral)
Where Found
Quartz is found to the extent of 12 percent of the Earth’s crust and is exceeded in abundance only by the feldspars. Sandstone, granite, and rhyolite are examples of widespread quartz-containing rocks. Large quartz crystals have been mined in Brazil, and smaller crystals are found in the United States—for example, in Arkansas and New York State (the “Herkimer diamond” deposits). However, Arkansas’ mining of lascas—nonelectronic-grade quartz that is used largely as feedstock to culture (synthesize) high-grade quartz crystals for use in electronic and other applications—ended in 1997, and cultured quartz crystals are now produced primarily in Asia.
Primary Uses
Cultured quartz crystals are used mainly in electronics. Lumbered (natural) quartz crystals cost twice as much per kilogram. The United States has about 7,000 kilograms of natural quartz stockpiles; the larger of these can be used to seed the culturing process. Secondary uses are found in the gemstone industry.
Technical Definition
Quartz is sparingly soluble in water (6 parts per million) and exists in solution as silicic acid (formula H4SiO4). Animals and plants take up the silicic acid, which can be incorporated in tissues.
Description, Distribution, and Forms
A striking example of quartz is the gradual petrification of trees, which occurs as the silica redeposits in the wood. Grasses such as bullrushes have considerable silica content and were chewed as primitive toothbrushes by early settlers in the United States. Cattle feeding on grass can develop silica deposits in the urinary tract that may be life-threatening. Amorphous silica is also found in sponges, sea cucumbers, rice hulls, bamboo, and palm fronds. Diatoms build their exoskeletons of silica, and after their death an adsorbent mineral called diatomaceous earth (kieselguhr) remains. Crystalline quartz occurs rarely in the biosphere, but quartz crystals (100 nanometers in size) have been found in the organism Chlorochytridion tuberculatum.
Solid quartz is not particularly toxic when swallowed, and the silica content of foods is easily tolerated, but quartz dust arising from mining activities is recognized as an inhalation hazard, the smallest particles (less than 5 microns) of which are the most harmful. Inhaled dust causes scarring and fibrosis in the lungs (silicosis), with gradual loss of function. The effects are worse in cigarette smokers. The lung lesions caused by silicosis can develop into cancer.
Silicon is an essential nutrient in a variety of species, including chickens, beets, and presumably humans. Silicon’s precise function is not known, but it appears to be involved in regulating the uptake of iron and aluminum. Until relatively recently, analytical methods involving silicon depended on wet chemical methods, which tended to give high results, so early claims of silicon should essentiality be treated with skepticism.
History
Quartz has been known since prehistoric times, when flint arrowheads and spear tips were used in hunting and fighting. The alchemist and metallurgist Georgius Agricola used the term quartzum in his sixteenth century writings in which he latinized a central European term kwardy, meaning hard. In 1813, Jean-Baptiste Biot reported the existence of left- and right-handed (chiral) quartz crystals, while, in 1880, Pierre Curie and his brother Jacques Curie described quartz’s piezoelectric property. Quartz crystals became important in radio equipment in World War II, leading to a shortage of suitable natural material and to the development of the hydrothermal process for manufacturing cultured crystals.
Obtaining Quartz
Quartz sand is abundant at the surface of the Earth and is easily collected by surface mining techniques. Large perfect crystals are rarer; they may occur underground or embedded in rock. Brazilian quartz is valued for the size and perfection of its crystals, which are mined without the use of explosives and are hand-sorted and graded. Since the 1950’s producing cultured quartz crystals has become possible, which satisfy most of the demand of the modern electronics industry. In the hydrothermal method of crystal growth, the lasca (purified silica) dissolves in an alkaline solution at elevated temperature, and layers of quartz are grown on a seed crystal in a cooler section of the apparatus. In the past, quartz crystal was considered so vital for military uses that a U.S. national defense stockpile of more than 600,000 kilograms was amassed. Toward the end of the twentieth century most of this stockpile was sold off.
Uses of Quartz
Cultured quartz finds its major use in the electronics industry, where its piezoelectric property is exploited in oscillators for controlling circuits (for example, in radio receivers). Quartz crystals are used in a wide variety of consumer electronics, from computers to cellular telephones.
Quartz also has optical uses in ultraviolet lamps and laser optical systems. Vitreous quartz is a chemically resistant high-temperature material used for laboratory ware (combustion tubes, crucibles, and so on). Silica, either amorphous or as quartz, occurs widely, not only in sand and rock but also in forms that are valued as gems. Amethyst (purple), citrine (yellow or brown), and rose quartz are semiprecious forms. Other minerals consisting largely of quartz are jasper, onyx, flint, chalcedony, and agate.
Quartz sand is made into vast quantities of glass and cement, and it serves as the source of silicon carbide, elemental silicon, and all sorts of synthetic silicates and silicones. The piezoelectric effect in quartz is exploited in crystal oscillators: An oriented quartz crystal in the form of a thin slice is clamped between electrodes and subjected to an electric field. The resulting resonant vibration of the crystal can be used as a frequency standard in radio receivers and as a time standard in watches and clocks.
Bibliography
Hall, Cally. Gemstones. 2d American ed. Photography by Harry Taylor. New York: Dorling Kindersley, 2002.
Iler, Ralph K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry. New York: Wiley, 1979.
Kogel, Jessica Elzea, et al., eds. “High Pure and Ultra-High Pure Quartz.” In Industrial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006.
O’Donaghue, Michael. Gems: Their Sources, Descriptions, and Identification. 6th ed. Oxford, England: Butterworth-Heinemann, 2006.
‗‗‗‗‗‗‗. Quartz. Boston: Butterworths, 1987.
Pellant, Chris. Rocks and Minerals. 2d American ed. New York: Dorling Kindersley, 2002.
Schumann, Walter. Gemstones of the World. 3d rev. and expanded ed. New York: Sterling, 2007.
Sofianides, Anna S., and George E. Harlow. Gems and Crystals from the American Museum of Natural History. Photographs by Erica Van Pelt and Harold Van Pelt. New York: Simon and Schuster, 1990.
Natural Resources Canada.
U.S. Geological Survey.
U.S. Geological Survey.