evaporation basin for the drainage waters of the west - ern San Joaquin Valley. Originally this was surface water, but by 1981 almost all the water entering the reservoir was subsurface agricultural drainage water from irrigated agricultural fields. Because of interest in saving some of Northern California’s disappearing wetlands, water that entered the reservoir was di- verted and used to preserve wetlands in the adjacent Kesterson National Wildlife Refuge in Merced County, California. By 1983 the incidence of embryo deformity and mortality among aquatic birds nesting in the Kesterson Reservoir was alarmingly high. No one im- mediately suspected, when the drainage water was used in the wetlands, that it contained almost 4.2 mil- ligrams of selenium per liter—a selenium concentra- tion one thousand times greater than in naturally oc- curring drainage in the region. As a consequence, phytoplankton in the reservoir accumulated sele- nium to levels 100 to2,600 times greater than normal. Since these plankton formed the base of the food chain in the reservoir, the levels of selenium in the fish, frogs, snakes, birds, and mammals also increased to levels 12 to 120 times greater than normal (20 to 170 milligrams of selenium per kilogram). Migratory birds that fed on plants, invertebrates, and fish in the reservoir containedup to24 times the normal level of selenium in theirtissue.Between 1983 and 1985an es- timated one thousand migratory birds diedasaconse- quence of selenium toxicity. To protect the migratory birds from future selenium exposure, the reservoir was drained in 1988 and filled with dirt, effectively burying and isolating the excess selenium. Selenium is a good demonstration of the adage “the dose is the poison.” Trace quantities of selenium are nutritionally essential, and blood concentrations of 0.1 milligram of selenium per liter are nutritionally sound. The minimum lethal concentration of sele- nium in tissue,however, isonly1.5 to 3.0 milligramsof selenium per kilogram of body weight. Symptoms of toxicity may occur when dietary intake exceeds 4 mil- ligrams per kilogram of body weight. Selenium toxic- ity leads to the syndromes known as alkali disease and blind stagger. On the other end of the scale, symp- toms of deficiency may appear if dietary intake is less than 0.04 milligram of selenium per kilogram of body weight. Selenium deficiency leads to a syndrome known as white muscle disease. In mammals, includ - ing humans, selenium is an essential component of the enzyme glutathione peroxidase, found in red blood cells.Glutathione peroxidase is an antioxidant; it protects tissues against oxidation by destroying hy- drogen peroxide or organic hydroperoxides. History In 1817, selenium was purified and identified by Jöns Jacob Berzelius. However, its environmental influ- ences, particularly its toxic effects, have been known for much longer. Marco Polo, for example, described unmistakable signs of selenium toxicity in horses, cat- tle, sheep, and humans duringhistravelsacrossChina in 1295. Selenium toxicity was described in Colombia in 1560, in South Dakota in 1857, and in Wyoming in 1908. Seleniumwas specificallyidentified as the cause of the toxicity in alkaline soils in the western United States in 1929. Its essential role in animal nutrition was identified in the 1950’s. In the mid-1980’s, the toxic effects of selenium were once more advertised when it was discovered to be the cause of widespread bird mortality at the Kesterson National Wildlife Ref- uge in Northern California. Obtaining Selenium There are no known commercially usable selenium deposits, and the concentration of selenium in soil and water is too dilute to be of economic significance. Consequently, most selenium is a by-product extracted from more abundant materialsinwhich it is acontam- inant, particularlyduringthe refining of ores contain- ing metal sulfides such as chalcopyrite. Most of the annual selenium production comes from the waste sludge produced during the electrolytic refining of copper. Uses of Selenium Selenium’s industrial uses are varied. The principal use is in the glass industry, where it is used to prevent discoloration of glassby iron oxides. Ammoniumsele- nite is alsoused as a pigmentin making red glass. Sele- nium diethyldithiocarbamate is used as a fungicide, but more important, it is used as a vulcanizing agent by the rubber industry to increase wear resistance. Selenium is also incorporated into plastics and paints because it improves resistance to heat, light, weather- ing, and chemical action. Selenium’s antioxidant properties causeit to beincluded in inks,mineral and vegetable oils, and lubricants. Cadmium selenide is found in photoelectric cells and photoconductors. In addition to its use asadietarysupplement, selenium is used in pharmaceutical remedies for eczema, fungal 1078 • Selenium Global Resources infections, and dandruff.Seleniumalso plays a nutritional role and is incorporated into di- etary supplements for animals, including hu- mans, although too much selenium in the diet can have deleterious effects. Mark S. Coyne Further Reading Adriano, Domy C. “Selenium.” In Trace Ele- ments in Terrestrial Environments: Biogeochem- istry, Bioavailability,and Risks of Metals. 2d ed. New York: Springer, 2001. Ehrlich, Henry Lutz, and Dianne K. Newman. Geomicrobiology. 5th ed. Boca Raton, Fla.: CRC Press, 2009. Frankenberger, William T., Jr., and Sally Ben- son, eds. Selenium in the Environment. New York: Marcel Dekker, 1994. Frankenberger, William T., Jr., and Richard A. Engberg, eds. EnvironmentalChemistry of Sele- nium. New York: Marcel Dekker, 1998. Greenwood, N. N., and A. Earnshaw. “Sele- nium, Tellurium, and Polonium.” In Chemis- try of the Elements.2ded.Boston:Butterworth- Heinemann, 1997. Jacobs, L. W., ed. Selenium in Agriculture and the Environment: Proceedings of a Symposium. Madison, Wis.: AmericanSociety ofAgronomy, Soil Science Society of America, 1989. Massey, A. G. “Group 16: The Chalcogens—Oxygen, Sulfur, Selenium, Tellurium, and Polonium.” In Main Group Chemistry.2d ed. New York:Wiley, 2000. Rosenfeld, Irene, and Orville A. Beath. Selenium: Geobotany, Biochemistry, Toxicity, and Nutrition. New York: Academic Press, 1964. Surai, Peter F. Selenium in Nutrition and Health. Not- tingham, England: Nottingham University Press, 2006. Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/com-eng.htm U.S. Geographical Survey Selenium and Tellurium: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/selenium See also: Food chain; Groundwater; Igneous pro- cesses, rocks, and mineral deposits; Irrigation; Leaching; Soil; Wetlands. Semiconductors Category: Products from resources Where Found Semiconductor materials are found all over the world. The most frequently used semiconductor ma- terials are composed ofcrystalline inorganicsolid ele- ments foundin nature, withsilicon the most common semiconductor material. Standardized semiconduc- tor crystals are grown in laboratories, with the global semiconductor industry dominated by Taiwan, South Korea, the United States, and Japan. Primary Uses Semiconductors form the basis for how modern tech - nology operates. Many different types of semiconduc - Global Resources Semiconductors • 1079 Glass manufacturing 35% Chemicals &pigments 20% Electronics & photocopier components 12% Other 33% Source: Historical Statistics for Mineral and Material Commodities in the United States U.S. Geological Survey, 2005, selenium statistics, in T. D. KellyandG.R.Matos,comps., ,U.S.GeologicalSurvey Data Series 140. Available online at http://pubs.usgs.gov/ds/ 2005/140/. U.S. End Uses of Selenium tor devices, including radios, diodes, microproces - sors, computer chips, cellular phones, and power grids, utilize semiconductor materials. Integrated cir- cuits comprise numerous interconnected semicon- ductors. Current “smart” technology products com- bine integrated circuits with power semiconductor technology. Technical Definition Semiconductors are special materials that conduct differently under different conditions and are fre- quently silicon-based. Semiconductors can act as a nonconductor or a conductor, depending on the po- larity of electrical charge applied to it, thus leading to the term “semiconductor.” A number of elements are classified as semiconductors, including silicon, zinc, and germanium. Other materials include gallium ar- senide and silicon carbide. Because silicon is readily obtained, it is the most widely used semiconductor material. These compounds have the ability to con- duct electrical current and can be regulated in the amount of their conductivity. Semiconductor devices operate by utilizing electronic properties of semicon- ductor materials. Description, Distribution, and Forms Semiconductor materialtakes advantage of the move- ment of electrons between materials with varied con- ductive properties.Semiconductors, special materials that are frequently silicon-based, have varying electri- cal conductivity properties depending on specific conditions. Electrical resistance properties of semi- conductor materials fall somewhere between those of a conductor and those of an insulator. Most semicon- ductor devices contain silicon chips with impurities embedded to conduct electricity under some condi- tions and not others. Silicon is the material used most frequently to create semiconductors. Applying an ex- ternal electrical field to a semiconductor material changes its resistance. The ability of a semiconductor material to conduct electricity can be changed dra- matically by adding other elements or “impurities,” a process known as “doping.” A pure semiconductor without impurities is called an “intrinsic” semicon- ductor. The amount of impurity, or dopant, added to a semiconductor determines its level of conductivity. Semiconductors are used to control electricity flow- ing through a circuit, to amplify a signal, or to turn a flow of current on or off. Semiconductor devices uti - lize the electronic properties of semiconductor mate - rials and have replaced vacuum tubes in most applica - tions. They utilize conductivity of electricity in the solid state compared with the gaseous state of a vac- uum. Semiconductor devices are manufactured both as discrete devices and as integrated circuits that con- sist of numerous devices, ranging from a few to mil- lions, manufactured and interconnected on a single semiconductor substrate. Typical semiconductor cir- cuits include a combination of transistors, diodes, re- sistors, and capacitors that function in switching, reg- ulating, resisting, and storing electricity. Combining smaller circuits such as these can be used to produce integrated circuits, sensors,and microcontroller chips. These devices are important in a broad spectrum of consumer products and business equipment. Devices made from semiconductor materials are the founda- tion of modernelectronics,includingradios, comput- ers, telephones, solar cells, andmanyotherdevices.In fact, semiconductors serve as the essential compo- nent in almost every modern electronic device. Silicon, which is extracted from sand, is the most common semiconductor material. In the 1990’s, there was a tremendous growth in the semiconductor mate- rials industry. The increase in production of com- puters increased the need for semiconductors, with major industry centers emerging in South Korea, Tai- wan, Singapore, Malaysia, and Hong Kong. History Semiconductor materials were studied inlaboratories as early as 1830. Over the years, many semiconductor materials have been researched. The first materials studied were a group of elements and compounds that were generally poor conductors if heated. Shining light on some of them would generate an electrical current that could pass through them only in one direction. In the electronics field, semiconductors were used for some time before the invention of the transistor. By 1874, electricity was used not only to carry power but also to carry information. The telegraph, the tele- phone, and, later, the radio were the earliest devices in an industry that would later be called electronics. In the early part of the twentieth century, semicon- ductors became common as detectors in radios, used in a device called a “cat’s whisker.” The cat’s whisker diode was created using the galena crystal, a semicon- ductor material composed of lead sulfide, and was considered the first semiconductor device. In the late 1950’s, a process called “planar technology” enabled 1080 • Semiconductors Global Resources scientists todiffuse various layers onto the surface of a silicon wafer to make a transistor with a layer of pro- tective oxide in the junctions, making commercial production of integrated circuits possible. Obtaining Semiconductors Most semiconductor chips and transistors are created with silicon, because the material is easily obtained. Semiconductors with predictable and reliable elec- tronic properties are required for commercial pro- duction of semiconductor devices. Because the pres- ence ofeven smallamounts of impurities can result in large effects on properties of the material, an ex- tremely high level of chemical purity is necessary. High crystalline perfection is also necessary because faults in crystal structure interfere with semiconduct- ing properties. Consequently, most semiconductors are grown in laboratories as crystals. Commercial pro- duction uses crystal ingots between 10 and 30 centi- meters in diameter. These crystals are grown as cylin- ders up to 2 meters in length and weighing several hundred kilograms. They are sliced thin, into wafers of standardized dimensions. The Czochralski process is a method for growing single crystals of semiconduc- tors and results in high-purity crystals. Uses of Semiconductors Semiconductor substances, commonly composed of silicon, germanium, or compounds of gallium, are the basisof integrated circuits controlling computers, cell phones, and other electronic devices. Semicon- ductors serve as essential components in almost every electronic device in use. From outdated items such as transistor radios to continuouslyevolvingonessuch as the computer, semiconductors are responsible for current technology. Modern semiconductor devices include transistors, diodes, resistors, and capacitors. They are found in televisions, automobiles, washing machines, and computers. Automobiles use semicon- ductors to control air-conditioning, injection pro- cesses, ignition processes, sunroofs, mirrors, and steer- ing. Anyitem that is computerized or uses radio waves depends on semiconductors in order to function. Power semiconductor devices combineintegratedcir- cuits with power semiconductor technology, devices often referred to as “smart” power devices. Semicon- ductors serve essential roles in the control of motor systems by optimizing a wide array of manufacturing and industrial motor systems responsible for produc - tion of many diverse goods. Semiconductors are also used in light-emitting diode lighting. All items that use sensors or controllers rely on semiconductor ma- terials. Semiconductor-based power electronics are cru- cial tools in the battle for energy efficiency. Semicon- ductor technologies have enabled both performance and energyefficiency improvements in telecommuni- cation devices such as radios, televisions, emergency response networks, and networking technology, pro- cesses that require increasingly fast speeds and data- management capabilities. Semiconductors have helped increase efficiency of transportation in the United States, with automobiles increasing their fuel economy by more that 70 percent since 1980. Semi- conductor technologies are used in diverse capacities to enhance homelife, business, and personalcommu- nications. Semiconductor technologies lead to indus- trial productivity and enhanced energy efficiency and use. Although there are many modern uses of semi- conductors, their application in future devices ap- pears unlimited. C. J. Walsh Further Reading Anderson, Richard L., and Betty Lise Anderson. Fun- damentals of Semiconductor Devices. New York: McGraw-Hill, 2005. Orton, John W. The Story of Semiconductors. New York: Oxford University Press, 2009. Singh, Jasprit. Semiconductor Devices: Basic Principles. New York: Wiley, 2000. Turley, Jim. The Essential Guide to Semiconductors.Up- per Saddle River, N.J.: Pearson Education, 2003. Yacobi, B. G. Semiconductor Materials: An Introduction to Basic Principles. New York:KluwerAcademic,2003. Web Sites Nobel Prize.org Semiconductors http://nobelprize.org/educational_games/ physics/semiconductors/ U.S. Geological Survey Mineral Information: Silicon Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/silicon/ See also: Fuel cells; Gallium; Germanium; Photovol - taic cells; Silicon. Global Resources Semiconductors • 1081 Sewage disposal. See Solid waste management; Waste management and sewage disposal Shale Category: Mineral and other nonliving resources Where Found Shale is found throughout the world. It is the most common of the three principal types of sedimentary rock, that category ofrockformed by consolidationof rock fragments or by chemical precipitation. In the geologic record, for every approximate five units of shale known, threeunits of sandstone andtwo units of limestone (the remaining two common categories of sedimentary rock) are also known. Primary Uses Shale is used as a filler in numerous construction ma- terials. It is also used in everyday products such as cos- metics and toothpaste, and as an energy source. Technical Definition Shale is a fine-grained consolidated rock principally composed of silt-size (particles between 0.0039 and 0.0625 millimeter in diameter) and clay-size (lessthan 0.0039 millimeter in diameter) rock detritus. Shale is generally characterized by a tendency to break along well-defined bedding planes. Description, Distribution, and Forms The classification of shale is generally based on the presence or absenceof well-defined bedding (lamina- tion) planes. Fine-grained rock lacking thischaracter- istic is termed mudstone, while a similar rock com- posed entirely of clay-size material is known as claystone. The ubiquityofshale is explained byits rep- resenting approximately 75 percent of all sedimen- tary rock produced throughout the entirety of geo- logic time. Because of their fine-grained nature, shales cannot be conveniently examined mineralogically. Bulk chemistry and X-ray studies show, however, that the average shale iscomposed principally of thefollowing oxides: silica (approximately 58 percent), aluminum (approximately 15 percent), iron (approximately 7 percent), and calcium, potassium, and carbon (each approximately 3 percent). History Shale richin organic materialdeposited bythe Missis- sippi River over the past several tens of millions of years caused the Gulf of Mexico to be one of the rich- est hydrocarbon provinces in the world. Throughout ten of the eastern United States and three western states, the Chattanooga Shale and the Green River Shale are identified as significant oil shale resources. Obtaining Shale Shale is aprime geologic source of crude oil andnatu- ral gas (hydrocarbon). Hydrocarbon originates from organic matter that accumulates in varieties of shale generally deposited under marine conditions. The preserved organic matter is converted to petroleum and natural gasbyburial and related postdepositional changes through the passage of geologic time. The general lack of permeability of shale will later form a barrier to the upper subsurface migration (and thus to possible loss by surface evaporation) of generated hydrocarbon. Uses of Shale Kaolinite-rich shale supplies the basic material for a wide range of ceramic products, from pottery and fine porcelain tosewer pipe. Shale richin barite isem- ployed in thehydrocarbon industry topreventoil and natural gas blowouts during the drilling of explor- atory boreholes. Clay-rich shales are also employed in the cosmetics, insulator, printing ink, medicine, and toothpaste industries. The highly indurated form of shale known as slate is used in the construction indus- try as roofing and paving material. One major eco- nomic importance of shale is associated with the worldwide distribution of oil shale, a dark-colored rock containing 5 to more than 25 percent solid or- ganic material,from which oil can be extracted by dis- tillation. Shale rich in organic material also acts di- rectlyas a primary source of crudeoiland natural gas. Albert B. Dickas Web Sites Natural Resources Canada Stone http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/content/2006/56.pdf 1082 • Shale Global Resources U.S. Geological Survey Stone, Dimension http://minerals.usgs.gov/minerals/pubs/ commodity/stone_dimension/myb1-2007- stond.pdf See also: Limestone; Oil and natural gas formation; Oil andnatural gasreservoirs; Oilshale andtar sands; Sandstone; Sedimentary processes, rocks, and min- eral deposits. Siemens, William Category: People Born: April 4, 1823; Lenthe, Prussia (now in Germany) Died: November 19, 1883; London, England Siemens was an inventor whose work included the steam engine and the regenerative furnace. He was also a part of Siemens Brothers, a company formed with four of his brothers, which is credited for ad- vanced work on telegraph cables. Late in life, he pro- posed the use of wind and water toproduce electricity. Biographical Background Charles William Siemens was born Karl Wilhelm Sie- mens to Christian Ferdinand Siemens and Eleonore Deichmann. Scientific education was provided at an industrial school in Magdeburg, Germany, at the Uni- versity of Göttingen, and at the works of Count Stol- berg in Magdeburg. Siemens spent mostof his life workinginsuccessful collaborative relationships with four of his brothers. His work with oldest brother Werner was often in the area of electrical discovery, while collaboration with Frederick led to the regenerative furnace. The sib- lings eventually opened a company called Siemens Brothers in 1858. Siemens married Anne Gordon on July 23, 1859, becoming a naturalized British citizen that same year. The couple had no children. Siemens died in 1883 of heart disease, leaving instructions in his will that the papers pertaining to his scientific work were to be published. Though not all of his experiments had been successful, Siemens took copious notes thatpro - vide the basis of scientific research in a number of areas. Impact on Resource Use Siemens’s inventions centered on preserving and us- ing resources produced through natural or estab- lished power sources. This work progressed after Sie- mens went toEnglandin 1843 to impartknowledge of his electrical discoveries. In 1847, he settled in Man- chester and began work on the steam engine. This work suggested the harnessing of energy from heat combustion and recycling it into a working power source. In 1850, the Society of Arts awarded him a gold medal for his invention of the regenerative con- denser. He also earned the Telford Premium and medal of the Institution of Civil Engineers in 1853 for this work. In the same decade, he reaped financial rewards from the success of his water meter; it sold so well, he was able to live off the royalties. The water meter used water energy to power a screw-turned meter. Siemens received a patent for the fluid meter onApril 15,1852. The patent also allowed for an application of the water-powered screw to a meter that measured ship speed. Moving to London that year, he became an inde- Global Resources Siemens, William • 1083 William Siemens invented the regenerative furnace. (Time & Life Pictures/Getty Images) pendent civil engineer. With Frederick, he continued working on fine-tuning his steam engine. The two men developed the regenerative furnace, which Fred- erick patented in 1856. The regenerative furnace was an expansion on the regenerative condenser. In 1858, the brothers started a small factory, which eventually became known as Siemens Brothers. Here, the brothers’ work moved in a different direction. Werner’s work on insulation of telegraph wiring was so successful that the company was given responsibil- ity for laying many telegraph lines both in England and abroad. William’s major contribution during this period was his design of a cable-laying ship. Toward the end of his life, Siemens shifted his in- terest back to electricity, and in 1877 he extended his earlier work by proposing an expanded use of power transmitted through water and wind sources. As a re- sult, the family company became known for power transmission. He spent his later years studying, lectur- ing, and traveling. Theresa L. Stowell See also: Electrical power; Hydroenergy; Steam and steam turbines; Steam engine; Wind energy. Sierra Club Category: Organizations, agencies, and programs Date: Established 1892 The Sierra Club was founded in order to preserve U.S. natural habitats for future generations. Fromits incep- tion, the Sierra Club has made its goals the conserva- tion of nature, the education of the public concerning the preservation of nature, and the enjoyment of the great outdoors. Background The Sierra Club was foundedin 1892 inSan Francisco by 182 charter members led by John Muir. Muir and the othermembers incorporated the Sierra Club with the mission, as stated by Michael Cohen, “to explore, enjoy, and render accessible the mountain regions of the Pacific Coast, to publish authentic information concerning them,” and “to enlist the support and co- operation of the people and government in preserv - ing theforests and other natural features of the Sierra Nevada.” Impact on Resource Use The Sierra Club has been influential in helping to gain national parkstatus for Yosemite, Mount Rainier, and numerous other important sites. Its members have served on important governmental committees and have spurred the enactment of many pieces of legislation designed to conserve natural resources. The club also leads expeditions large and small that enable people to experience the wilderness. The Sierra Club continues to work to ensure that the legacy of clean air, water, soil, and wilderness will remain for generations to come. It publishes a num- ber of periodicals that help to educate the public con- cerning the need to preserve American natural re- sources. Judy Arlis Chesen Web Site Sierra Club http://www.sierraclub.org/ See also: Conservation; Izaak Walton League of America; Muir, John; National parks and nature re- serves; National Wildlife Federation; Nature Conser- vancy; Wilderness; Wilderness Society. Silicates Category: Mineral and other nonliving resources The two most common elementsin the Earth’s crust are silicon and oxygen. The prevalence of these two ele- ments and their ability to combine as stable complex ions accounts for the fact that silicate minerals consti- tute a major portion of the minerals in Earth’s crust. Their wide range of physical properties leads to many commercial uses. Definition Silicon and oxygen combine to form stable complex ions composed of one silicon ion surrounded by four oxygen ions. Theresulting complex ion isa four-sided figure known as a tetrahedron. Silicate tetrahedra may exist independently separated by cations or link together by sharing oxygens to form a wide range of structural groupings. Tetrahedra groupings act as skeletons in which charge neutrality is attained by the addition of cations between and within silicate 1084 • Sierra Club Global Resources tetrahedra. Silicate skeletons may exist as isolated tetrahedran; as two tetrahedra sharing one oxygen atom; as rings of three, four, or six tetrahedra; as single and doublechains;as sheets; andascontinuous three-dimensional frameworks. These skeletal arrange- ments impart many diverse physical characteristics to the various silicate minerals. Overview No generalizationdescribes allsilicates, although most are translucent to transparent, have moderate spe- cific gravity, and are chemically inert. Silicates range from extremely soft to hard. Some display excellent cleavage, but others are uniformly resistant to break- ing in all directions. Because silicate minerals exhibit such a wide varia- tion in physical properties, they have variable com- mercial uses. Talc, a magnesium silicate, is used as a filler in paint, ceramics, rubber, insecticides, roofing, and paper. Its most familiar form is as talcum powder. Clay minerals (extremely small platy grains of hy- drous aluminum silicates) are important industrial minerals. They are used in a variety of fired products, ranging from bricks to fine china and porcelain. Clay is used as a filler in many products, including paper. Montmorillonite, an expandable clay, is widely used as a sealant. Zeolites, which are hydrous silicates with open tunnels within a framework lattice, are widely used as molecular sieves and for ion exchange resins; they are valuable for oil-spill cleanup and wastewater treatment and are used in water softeners. In many applications, natural silicate minerals have been replaced by the industrial manufacture of syn- thetic and substitute materials. Muscovite mica, a sheet structure, was used largely as an electrical insu- lator in capacitors and electronic tubes. Asbestos, a term describingflexible, fibroussilicatematerials, was widely used for its heat resistance and its ability to be woven as a fabric in fire-retardant cloth, in heat- resistant sheets, in blown insulation, and in brake lin- ings. Almost all asbestos use is outlawed in the United States, as it is considereda carcinogen. Quartz (silicon dioxide) is used as anabrasive,asopticalcomponents, and as thin wafers to control the frequency of radio and radar transmission. Quartz crystals are now grown by commercial hydrothermal processes. Many semiprecious stones are silicate minerals. Microcrystalline varieties of quartz that are used in jewelry include fibrous-appearing tiger’s eye, red jas - per, multicolored agate, and the red-spotted blood - stone. Crystalline varieties ofquartzthat serve as semi - precious stones include yellow citrine and violet amethyst. Topaz, jade, garnet, opal, and peridot are silicates, as is emerald, which is gem-quality beryl. René A. De Hon See also: Asbestos; Clays; Feldspars; Mica; Minerals, structure and physical properties of; Orthosilicate minerals; Quartz; Silicon; Talc. Silicon Category: Mineral and other nonliving resources Where Found Silicon makes up 25.7 percent of theEarth’s crustand is the second most abundant element after oxygen. It is not found in itselementalform, but rather occurs in compounds such as oxides and various silicate miner- als. Silicon is a trace element participating in the metabolism of higher animals, and siliceous struc- tures are found in manybiologicalsystems in the form of cell walls, scales, and other skeletal features. Primary Uses Silicon metal and alloys, including ferrosilicon, are used mainlyby producers of aluminum, aluminum al- loys, and chemicals. Very pure silicon is an essential component of semiconductorsandhas given its name to the “silicon age,” a termthat came into prominence during the 1990’s. Technical Definition Silicon (abbreviated Si) is the fourteenth element of the periodic table, with an atomic number of 28. With carbon, germanium, and tin, it belongs to Group IVA of the periodic table and resembles germanium (Ge) most strongly in its physical, chemical, and electronic properties. Pure silicon is a hard, gray solid with a me- tallic luster and a cubic crystalline structure similar to that of carbon in diamond form. It has eight isotopes, the most abundant of which are Si 28 (92.23 percent), Si 29 (4.67 percent), and Si 30 (3.10 percent). Its density is 2.329 grams per cubic centimeter, and it has a melt- ing point of 1,410° Celsius and a boiling point of 2,355° Celsius. While the single-crystal form of silicon has been most extensively studied from both basic and practical viewpoints, the polycrystalline and Global Resources Silicon • 1085 amorphous forms of silicon have also become ex- tremely important: Polycrystalline silicon has been applied in the construction of solar panels and cen- tral processing units of computers. Amorphous sili - con has been used in thin-film transistors and solar cells. Description, Distribution, and Forms Silicon is widely available in oxides and silicates. The oxide forms include sand, quartz, rock crystal, ame- thyst, agate, flint,andopal. Granite, feldspar, clay, and mica are some of the common forms of silicates. A ba - sic requirement of silicon in all its preeminent elec - 1086 • Silicon Global Resources Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009.Source: Mineral Commodity Summaries, 2009 39,000 340,000 640,000 140,000 78,000 85,000 166,000 60,000 Metric Tons of Silicon Content 3,500,0003,000,0002,500,0002,000,0001,500,0001,000,000500,000 Venezuela Spain South Africa Russia Norway Macedonia Ukraine United States Other countries 66,000 3,300,000 160,000 74,000 39,000 68,000 Iceland France China Canada Brazil India Kazakhstan 180,000 270,000 Silicon: World Production, 2008 tronic applications is extreme purity—to levels much better than parts per billion (ppb). The single-crystal form of silicon, while essential for computer chips, has cost and size limitations for a host of other potentially high-volume applications. Hence silicon is also produced in polycrystalline and amorphous forms by techniques such as casting and thin-film deposition. Polycrystalline forms (poly-Si) contain crystalline grains separated by grain bound- aries, while amorphous silicon lacks the long-range crystalline order completely. However, both have use- ful semiconducting properties and have been widely developed for a range of uses. The interesting and extremely useful electronic and optoelectronic properties of silicon stem from its tetrahedral bonding and diamond cubic structure. Replacing a host silicon atom with a Group V element (such as phosphorus) or Group III element (such as boron) adds a free electron or “hole” (an electron va- cancy that behaves like a positively charged free parti- cle. Thus the electrical conductivity of silicon can be changed over several powers of ten simply by control- ling the trace quantities of phosphorus or boron. The bandgap separating the electron and hole states has a value of 1.12 electron volts for silicon, making it a nearly ideal choice for devices as varied as transistors, diodes, solar cells, and various types of sensors. Optically, silicon is transparent to infrared wave- lengths above 1.1 micrometers while it absorbs thevis- ible spectrum. Silicon is brittle, but its highly direc- tional bonds enable easy “scribing” of the silicon wafer into individual computer chips under properly chosen crystal orientations. The intricate chemical properties ofsilicon enable deployment of avariety of fabrication techniques, with individual feature sizes falling into the submicron regime. The modest ther- mal conductivity of silicon places some restraints on thermal dissipation in computer chips. History Although many chemists recognized silicon as an ele- ment by the early nineteeth century, its tight bonding with oxygen made it difficult to isolateas a separate el- ement. Jöns Jacob Berzelius achieved the isolation of silicon in 1823 using a method similar to one devel- oped by Sir Humphy Davy, who earlier had tried but failed to isolate silicon. The newly isolated element was named for the Latin wordforflint,silex,andsubse - quently was investigated by German chemist Friedrich Wöhler and others. Obtaining Silicon Semiconductor-grade silicon requires conversion of raw silicon obtained from reducing silica (SiO 2 ) into gaseous compounds such as chlorosilanes. Multiple fractional distillation of the latter leads to high-purity silicon rods. These rods are subsequently melted and grown into dislocation-free single crystals by either the Czochralski (CZ) crystal pulling process or the float zone (FZ) process. Necessary dopants such as boron (for p-type silicon) andphosphorus(for n-type silicon) are added to the melt. CZ silicon ingots are probably the largest single crystals ever produced— more than 3 meters long, with diameters as large as 300 millimeters. Wafers, about a millimeter thick, sliced from the ingots serveasthe starting materialfor the batch fabrication of microelectronic chips, each containing up to a few million transistors. Silicon by itself is inert, but a number of source gases and reagents used in manufacturing it are highly toxic, so extreme care must be exercised in waste dis- posal and protection of assembly workers. Silicon has been implicated in silicotic lung diseases and certain cancers. Uses of Silicon The principal applications of high-grade silicon are in microelectronics. The atomic structure of crystal- line silicon makes it the most important semiconduc- tor. Silicon in its highly purified form, when “doped” with elements such as boron and phosphorus, be- comes the basic element of computer chips, transis- tors, diodes, and various other electronic switching and control devices. The enormous success of the sili- con transistor, the basic electronic amplifying device, was made possible by an extremely pristine interface with silicon dioxide (an insulator readily grown on sil- icon by heating in oxygen) and by the continual scal- ing down oftransistor feature size, whichtranslates di- rectly to faster computer speed and higher memory capacity. The fieldof giant microelectronics, exemplified by portable computer displays and flat-screen television, uses silicon in its polycrystalline or amorphous forms. Another area of great impact for silicon is in terres- trial solar cells, for which extremely large volumes at low cost are necessary. Here computer-grade single crystals are not cost-effective; large-grain polycrys- talline silicon holds the key for this crucial renewable energy application. A late-twentieth century silicon technology ex - Global Resources Silicon • 1087 . concentration of sele- nium in tissue,however, isonly1.5 to 3.0 milligramsof selenium per kilogram of body weight. Symptoms of toxicity may occur when dietary intake exceeds 4 mil- ligrams per kilogram of. pro- duction of semiconductor devices. Because the pres- ence ofeven smallamounts of impurities can result in large effects on properties of the material, an ex- tremely high level of chemical. River over the past several tens of millions of years caused the Gulf of Mexico to be one of the rich- est hydrocarbon provinces in the world. Throughout ten of the eastern United States and