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Granitic Pegmatites: Storehouses of Industrial Minerals Microcline (26 cm across), Araçuaí district, Minas Gerais, Brazil. SPECIMEN AND PHOTO : DAVID L ONDON Alexander S. Glover1, William Z. Rogers2, and James E. Barton3 1811-5209/12/0008-0269$2.50 DOI: 10.2113/gselements.8.4.269 G ranitic pegmatites are mined for feldspar, quartz, mica, lithium aluminosilicate minerals, and kaolin. These industrial minerals have a myriad of uses, some as mundane as glasses, porcelains, and bulk fillers, and others that are critical to the most advanced electronic devices. The chemical fractionation that produces pegmatites refines these industrial minerals to a purity that is not achieved in other geologic environments. The high chemical purity of their constituents and the fact that they contain nearly 100% of minable rock make large granitic pegmatites some of the most valuable sources of industrial minerals. refi ne their compositions like no other magma type (see London and Morgan 2012 this issue). “CERAMIC” PEGMATITES In some classifications, the pegmatites that are most sought for their industrial minerals are referred to as “barren,” meaning that they lack the gems and rare metals that are the more typical targets for exploitation. They KEYWORDS : quartz, feldspar, mica, spodumene, kaolin, pegmatite minerals have also been termed “ceramic” pegmatites, which alludes to the value of their industrial minerals. INTRODUCTION “Ceramic” pegmatites generally are large bodies (FIG. 2). Pegmatites are known as sources of gems (see Simmons They appear as segregations within the margins of much et al. 2012 this issue) and specialty metals (see Linnen et larger granitic plutons and may also form a dense swarm of al. 2012 this issue). This paper, however, focuses on the major industrial minerals obtained from pegmatites, which dikes intruding the wall rocks of the granites. With greater include feldspar, quartz, mica, spodumene, and kaolin, the distance from their source, pegmatites evolve into those that may contain rare-metal or gem deposits. Although last being of secondary origin and derived from intense some large pegmatites may represent economic concenacidic alteration of primary feldspar (FIG. 1). trations of quartz, feldspar, and muscovite, only the rareMost pegmatites are mineralogically simple rocks, element pegmatites contain sufficient lithium to produce consisting of approximately 65% feldspar and 25% quartz, spodumene and petalite ores (see Figure 1 of Černý et al. along with 5–10% mica and about 5% of other accessory 2012 this issue) minerals (e.g. garnet, tourmaline, spodumene, oxides, etc.). Though broadly similar in their mineralogy, each pegmatite has its own distinctive attributes, such as crystal size variations, mineral distribution (homogeneous or zoned), feldspar composition, and accessory mineral assemblage (for example, see Figure 4 in London and Kontak 2012 this issue). Although quartz, feldspar, mica, and kaolin are recovered from other igneous, metamorphic, and sedimentary rocks, the high chemical purity of pegmatite minerals makes them particularly important to many industrial applications. Quartz from pegmatites is coarse grained and lacks abundant mineral inclusions that would constitute impurities in the fi nal products. Feldspars are exceedingly low in iron, an undesirable source of brown color in most glass and ceramic products, and they are low in calcium, an element that raises the fusion temperature in glass fabrication. Thus, the industrial applications of these minerals owe their unique properties to the ability of pegmatites to 1 Active Minerals International, LLC 651 Madison Road, Eatonton, GA 31024, USA E-mail: [email protected] 2 NYCO Minerals, Inc. 312 Dawson Forest Road East Dawsonville, GA 30534, USA E-mail: [email protected] FIGURE 1 Industrial minerals from pegmatites. PHOTO : DAVID LONDON 3 105 W. Copeland Circle Laurens, SC 29360, USA E-mail: [email protected] E LEMENTS , V OL . 8, PP. 269–273 269 A UGUS T 2012 FELDSPAR Feldspar is the most common group of minerals in the Earth’s crust, a fact that has contributed to its extensive usage by humankind. Feldspar has been used for centuries to make ceramics and glass, and has found a place in other products as new processing technologies have developed. However, glassmaking and ceramic manufacturing still account for 85–90% of the feldspar market. In the most recent mineral commodities summary by the U.S. Geological Survey (USGS), world production of feldspar was estimated at 20 million metric tons in 2010, with more than 50 countries having significant feldspar production (Tanner 2011). Another 70 countries have large and potentially economic deposits of feldspar. Italy, Turkey, and China are the top-producing countries, representing approximately 55% of the estimated world total. Feldspar production in the U.S. in 2010 is estimated at 570,000 metric tons with a value of $36 million (Tanner 2011). Feldspar was mined in seven states in the U.S. in 2010, with North Carolina’s Spruce Pine pegmatite district being the largest producer (FIG. 2). In commercial terms, feldspar falls into two categories, potash (i.e. microcline) and soda or soda-spar (i.e. albitic plagioclase), and each has specific applications related to TABLE 1 its melting temperature (SEE TABLE 1). Granular feldspar (–20 and –40 mesh) is used in glassmaking, whereas ground or milled feldspar (–170, –200, and –325 mesh) is employed in ceramics. In glassmaking, feldspar aids in batch melting of alumina- or silica-rich reagents. Feldspar makes the glass more amenable to forming, more durable, resistant to devitrification, and able to withstand pressure changes. Potash feldspar is used in specialized types of glass (TABLE 1) because of its low thermal expansion and its workability for pressing. The amount of feldspar used in these applications varies, with flat glass containing between 0 and 0.5 wt%, containers 8 wt%, and fiberglass insulation up to 18 wt%. Unlike glass, which is melted to a homogeneous mass before fabrication, ceramic bodies are mixed and then shaped before being fi red at high temperatures (1000– 1200 °C). Feldspar is added to ceramic mixtures to help create the glass binder for the high-temperature aluminosilicates that form upon fi ring. The amount of feldspar used in this process correlates with the density of the ceramic piece and is measured by the amount of water absorbed by the fi nished product. Porcelains typically can absorb less than 1% water, whereas semivitreous ceramics can absorb as much as 20%. Low absorption, hence higher feldspar content, is particularly important for high-strength ceramics (TABLE 1). Feldspar is used as a MINERAL USES Mineral Applications Albite, soda-spar NaAlSi3O8 Container glass (e.g. beer, wine, liquor bottles), cosmetics, fiberglass insulation Microcline, potash feldspar KAlSi3O8 Specialty glass, laboratory glass, dinnerware, TV picture tubes, high-voltage electrical insulators All feldspar Ceramics (plumbing, electrical insulators, tile, commercial china), high-end ceramics (implants, dentures), paint filler, PVC plastics, abrasive filler for cleansers, welding rods Quartz (normal) SiO2 Glasses (containers, flat glass, fiberglass insulation, fiberglass textiles), abrasives, ceramics (plumbing fixtures, ceramic tile glazes, dinnerware, electrical porcelain), advanced ceramics (Si-carbide and Si-nitride, fillers, oscillators, high-temperature tiles) High-purity quartz Process equipment for semiconductor chips and solar cells, high-performance lighting (automotive headlamp lighting, projector lamps, street lighting), specialized ceramics, mirrors Mica: Muscovite KAl3Si3O10(OH)2 Cosmetics, insulators, tape-joint compound, engineering resins, caulks, sealants, paints, adhesives, automotive parts, tires, noise dampeners, industrial greases and lubricants, brake pads, drilling mud (sealant), fashion industry (fabric finisher for reflecting light, moisture), welding rods, wire and cable harnesses, ornamentation Biotite K(Fe,Mg)3AlSi3O10(OH)2 Petroleum drilling fluid sealant/additive Kaolin (Kaolinite) Al2Si2O5(OH)4 Ceramics (toilets, sinks, bathtubs), paper, paint, rubber, plastics, sealants, adhesives, caulks, paper pigment, filler (crayons), cosmetics, fiberglass, Portland cement, refractories, porcelain, pottery, insecticide and herbicide carriers, inert binder in pharmaceuticals, catalytic converters and other catalysts, printed circuit boards, marine watercraft, wind turbine blades, fiberglass, golf club shafts, snow skis, bulkstorage tanks, sheeting for automobiles, aircraft, and high-speed trains, petroleum pipe, high-pressure proppants Spodumene LiAlSi2O6 Ceramics, batteries, pharmaceuticals (for gout and psychotic conditions, anti-inflammatory uses), solders, lubricants, greases, rocket propellant Petalite LiAlSi4O10 E LEMENTS 270 A UGUS T 2012 fi llers with high thermal stability. The level of quartz purity is not as high as that required for semiconductor manufacturing and can be similar to that of good, low-iron glass sand. High-performance lamps, such as halogen bulbs, are made from high-purity quartz tubing. The resultant vitreous quartz bulbs can withstand the extreme temperatures generated during use. Ultrahigh-Purity Quartz Ultrahigh-purity (UHP) quartz powders sized to –50 and +150 mesh are used to make quartz glass apparatus for the manufacture of semiconductor chips. The products include crucibles (for melting the silicon metal), tubing, and blocks (for the fabrication of quartz glass articles that are required in the production of silicon-metal wafers). The quartz glass articles must be of the highest purity in order to prevent defects during processing. Ultrahigh-purity quartz is employed in the production of Si-based solar cells, which require high-purity vitreous quartz devices (i.e. crucibles, tubing, and blocks) to turn the silicon metal into wafers for the cells. Solar cell collectors are also covered with silica glass fabricated from UHP quartz; these UHP silica glass covers can withstand heat and impact, yet remain exceedingly transparent. Aerial view of the Spruce Pine pegmatite, North Carolina (USA), where high-purity quartz, feldspar, and muscovite are mined. PHOTO COURTESY OF DANNY JARRETT (www.jarrettphotography.com) FIGURE 2 component in ceramic glazes and in manufactured materials called “frits”—premelted glaze mixtures. Processed frits are more expensive than the raw materials from which a ceramic glaze is made, but they offer a higher-quality product because of their uniform composition. For this reason, glaze frits are widely used in the ceramics industry. Finely milled feldspar (4–12 +m) is added to paint as a fi ller. The feldspar resists chemical attack and abrasion and improves the durability of the paint fi lm. Feldspar is also ideal as an inert fi ller in PVC plastics, where it improves resistance to staining, abrasion, and chemical attack. Feldspar has been used for more than 100 years as the abrasive fi ller in household cleansers. Its cleavage planes tend to align with the cleaning surface, so that it polishes rather than scratches porcelain surfaces. QUARTZ Quartz, or silica as it is termed in its industrial applications, is abundant in crustal rocks, and global reserves are vast. Crushed quartz and quartz sand are used in the manufacture of all types of glass, ceramics, abrasives, and advanced ceramics (TABLE 1). However, even though quartz is remarkably pure relative to other common minerals, only a small fraction is suitable for making high-purity (<100 ppm total impurities by weight) or ultrahigh-purity (<10 ppm total impurities by weight) quartz powder. Pegmatites are the principal sources of such high-purity quartz. Quartz pure enough for making high-purity quartz powder is produced in a relatively small number of locations throughout the world. In the U.S., pegmatites in North Carolina and South Dakota are the only significant sources, with North Carolina by far the largest. High-purity quartz powder is also produced from pegmatites in Brazil, India, Australia, Madagascar, Norway, China, and Russia. The estimated world production of high-purity quartz powder was 60,000 metric tons with a value of about US$252 million (Dolley 2011). High-Purity Quartz Pegmatitic quartz is used to make fused silica, a product used in semiconductor packaging, refractories, and other industrial applications requiring E LEMENTS Esoterica Two specialized applications of pegmatitederived quartz are noteworthy. The high-temperature insulating tiles used on NASA’s space shuttles were produced from high-purity quartz powder. Made from 99.9% -pure quartz glass fibers, the tiles can withstand the extreme temperatures encountered during reentry. The 200-inch mirror of the Hale telescope at the Mt. Palomar observatory in California was manufactured by Corning Glass Works from hand-selected natural quartz. MICA: MUSCOVITE AND BIOTITE The single, perfect cleavage of micas enables them to be separated into thin, perfectly flat, flexible, strong, and tough flakes. Muscovite is transparent in thin sheets, with colors described as “rum” to light brown. Biotite is darker due to its iron content. The two micas can be used interchangeably in products where color and electrical properties are not critical. World production of industrial mica in 2009 was estimated at about 348,000 tons (Hedrick 2010). Most of that mica, including all sheet and “scrap” muscovite, was from pegmatites. Muscovite has an outstanding combination of properties: good dielectric constant and electrical resistivity, low thermal conductivity, chemical resistance, high melting temperature, UV resistance, inertness, barrier properties (to sound and moisture), reinforcement properties, and a low coefficient of thermal expansion. Biotite has similar properties, but its higher iron content and oxide inclusions reduce its electrical resistivity, and the darker color eliminates it in applications such as coatings and fi llers where a whiter color is desired. Muscovite comprises 80% of the market for mica in North America. Early Uses of Muscovite Transparent sheets of muscovite, known as “isinglass,” were employed as an early form of window material, and the thermal resistance of muscovite led to its usage as windows in cooking and heating ovens. Native Americans used muscovite as an ornament for grave decoration, and they traded it extensively (Peter Margolin, personal communication 2000). In the U.S., muscovite has been called “the mineral that won World War II.” It served as the fi rst high-temperature, inert, electrically insulating material that could be 271 A UGUS T 2012 easily manufactured into thin sheets for use in electrical condensers and vacuum tubes (FIG. 3). In addition, it provided excellent insulation for tents and was a component in shields for protecting machine parts from sand and mud. When WWII began, the USGS coordinated an exhaustive exploration program for pegmatites (Cameron et al. 1949), which was motivated largely by the need for muscovite in the manufacture of vacuum tubes. Deposits in the Black Hills (South Dakota), Spruce Pine (North Carolina), and throughout New England (all in the U.S.) helped give birth to what was then a new frontier in the electronics industry. When solid-state transistors replaced vacuum tubes and high-technology ceramics made inroads into the mica market, the demand for sheet muscovite dwindled. However, new uses have been developed, and an almost endless list of applications now exists, only some of which are discussed below and summarized in TABLE 1. Although the chemistry and color of muscovite can vary from deposit to deposit, the platelet shape does not change during processing, regardless of particle size. Shape, tensile strength, and flexibility are the reasons why muscovite adds desirable performance characteristics to so many products. Contemporary Uses of Muscovite Fillers Tape-joint compound (5–7% mica) is the largest single market for muscovite. It reduces the shrinkage, during drying, of taped joints that bond the edges of wallboards, thereby reinforcing the joints and making them smooth and crack free. Mica is the optimum mineral filler for maximizing the flexural modulus of engineering resins. Whereas polypropylene is the most common resin containing mica, a loading of 10% –25% mica will enhance the flexural modulus of other resins. In automotive plastic parts, which constitute the largest market for mica-fi lled parts, mica is found in bumpers, facia, fenders, wheel covers, windshield components, selected interior parts, and underhood components. Coatings The durability of mica lends itself to widespread application in paint, stucco, and cements. Mica acts as a barrier to moisture and adds durability to all of these materials. Mica is used in many types of coatings as a performance additive. Mica’s shape is perfect for reinforcing the resin system, thus reducing shrinkage and cracking of paint fi lm, creating a barrier layer that minimizes moisture travel through the coating, and resisting UV penetration and damage, all of which add longevity and durability to the coating. Mica’s thermal stability also allows its use in foundry coatings and molds. Mica strengthens the coating and provides it with the proper permeability so that gases can escape at the correct time, thus minimizing surface defects. Lubricants Mica has been used as a coating in roofi ng materials, paper, marine products, fabrics, seeds, industrial powder coatings, automotive surfaces, tires, wood fi nishes, furniture, concrete, and machinery. In these applications, the shape and smoothness of ground mica make it useful as a surface lubricant, especially in high-temperatures applications (e.g. tires). In oil well drilling, mica is used as a component of drillingmud lubricant. To prevent the escape of the drilling mud into cracks surrounding the wellbore, the mica flows into the cracks with the escaping mud and seals them, thereby maintaining the pressure of the drilling mud in the wellbore and also reducing loss of drilling fluid. E LEMENTS FIGURE 3 Muscovite grid insulators and separators in a vacuum tube. PHOTO : DAVID LONDON Esoterica One of the most expensive uses for premiumquality, wet-ground mica is in cosmetics. Muscovite adds sheen, cohesion, and moisture resistance to nail polish, eye shadow, lipstick, barrier creams, and other products of personal hygiene. The cosmetics market requires tightly controlled manufacturing conditions and higher-quality standards than most other uses of mica because of the direct application of these products to human skin. In the agricultural market, mica has been used as a dusting agent on fruit trees because the reflection of light off the mica cleavage surfaces repels aphids. Mica flakes also repel insect pests from crops by lodging in their exoskeleton, causing irritation and discomfort. KAOLIN Kaolin is derived from pegmatites (and granites) as a consequence of hydrothermal leaching or weathering of feldspars. Acid leaching removes potassium, sodium, and calcium from feldspar and produces kaolin or kaolinite, a dull, whitish, earthy material. Kaolin is a soft, white to offwhite clay mineral that generally has a very fi ne-grained particle size and the platy morphology of mica. Kaolin has been used in pottery since the Tang dynasty in China (618–907 BC). Its use in fi ne porcelain china was reported as early as 1727 in England, and in 1744, Native Americans began to trade kaolin to settlers for transport to England. Kaolin particles form wafers or platelets with a high aspect ratio (i.e. length:thickness 5 20:1). Pure kaolin is exceedingly white and cohesive. These properties render it useful as a fi ller, sealant, or lubricant in many processed materials (TABLE 1). Kaolin production in 2006 was estimated at 4,475,000 tons, the United States being the largest producer (Virta 2010). 272 A UGUS T 2012 Ceramics Kaolin is mixed with quartz, sand, and feldspar and melted at high temperature in the manufacture of ceramic products, including sanitary ware, pottery, ceramic tile, and dinnerware. Kaolin provides form (rigidity) to the ceramic as the binder vitrifies and begins structural formation. Ceramics are also used in catalyst and proppant production. Honeycomb catalyst bodies are cast from melted kaolin and are used in catalytic converters for automobiles. Proppants are small, bead-like structures used in hydraulic fracturing of rock in gas and petroleum extraction; they are used in cases where high pressures prevent the use of natural sands. The size range of proppants is generally –20 to +40 mesh. A solution containing these beads is pumped under high pressure into rock fractures. The beads act as structural “props” to hold the fractures open, thereby enhancing gas flow and extraction of the gas from the rock. Filler As a functional fi ller, kaolin is used to increase the strength of plastic, rubber, sealants, caulks, and adhesives, while reducing the amount of more expensive resins and latex in the formulations. In paper manufacturing, kaolin improves the appearance and printability of coated paper and paperboard. Kaolin particles fi ll voids in the cellulose fiber of paper, making the paper smooth, consistent, and reflective; kaolin fi ller also adds weight, or heft, to paper. Electronics High-grade, low-iron kaolin is a critical constituent of modern, printed circuit boards. Low-iron kaolin is melted in furnaces and then spun into special fiberglass. The fiberglass is then woven into fiberglass mat, which is sandwiched between resins to produce the board. Esoterica Perhaps the most esoteric use for any of the common industrial minerals is the application of kaolin as an equine poultice. It is applied in paste form to the legs of thoroughbred racehorses following a race to prevent fast cooling that can lead to injury (Monte Trotter, personal communication 2011). SPODUMENE AND PETALITE Spodumene and petalite are the two most important hardrock sources of lithium ore (Li-rich evaporites and their brines are now the principal global sources of lithium; Kesler et al. 2012). Spodumene and petalite are found only in highly fractionated rare-element pegmatites, specifically those associated with the Li-Cs-Ta, or LCT, family of deposits, which originate mostly from aluminous (S-type) granites (see Černý et al. 2012). Lithium aluminosilicates have industrial and metallurgical applications. Linnen et al. (2012) provide details on the geology of Li-rich pegmatites, with reference to examples of world-class deposits. REFERENCES Cameron EN, Jahns RH, McNair AH, Page LR (1949) Internal Structure of Granitic Pegmatites. Economic Geology Monograph 2, 115 pp Černý P, London D, Novak M (2012) Granitic pegmatites as reflections of their sources. Elements 8: 289-294 Dolley TP (2011) Quartz crystal (industrial). In: Mineral Commodity Summaries 2011. U.S. Geological Survey, pp 126-127 E LEMENTS Ceramics Lithium aluminosilicates are utilized in the manufacture of specialty ceramics and require little or no processing beyond milling. Spodumene can contain enough Fe such that chlorine bleaching is required to remove color. Petalite is normally iron free, and spodumene derived from the replacement of petalite is similarly iron free. These minerals are the primary ingredients of Corningware™ and Visions™ cookware. In these ceramics, the addition of the Li-component of spodumene and petalite imparts high resistance to thermal shock, because Li-bearing glasses and `-structured (low-density hexagonal or tetragonal phases) synthetic ceramics have exceedingly low coefficients of thermal expansion. Electronics The growing demand for Li-based solid-state batteries in the automotive industry has prompted a review of pegmatitic sources of lithium (Kesler et al. 2012). Though pegmatite deposits are small in relation to the vast brine sources of South America, pegmatites are highly concentrated sources of Li (by comparison to brines), which are feasibly economic using artisanal mining methods. Esoterica Other uses of lithium-based compounds are as diverse as mobile phones, buoyancy devices, silver solders, lubricants and greases, rocket propellant, synthetic rubber, and alloys with aluminum (TABLE 1). Perhaps the most novel use of lithium is in pharmaceuticals. Lithium carbonate and other organic salts of lithium are the active ingredients in antidepressants, and Li-based pharmaceuticals are used in the treatment of bipolar symptoms. Because of their anti-inflammatory properties, lithium compounds are also used to treat gout in skeletal joints. CONCLUDING REMARKS Because pegmatites consist chiefly of quartz and feldspars, the ore grade of some of the most important deposits approaches 100% of minable rock, a benefit that is rare in the mining industry. Though pegmatites provide just a handful of different industrial minerals, their uses are numerous and diverse. From the mundane to the esoteric, from beer bottles to solar cells, pegmatite-derived industrial minerals play a part in the daily lives of most people living in modern societies. ACKNOWLEDGMENTS We thank two anonymous reviewers for their comments, and John Valley and Pierrette Tremblay for their editorial handling of the manuscript. Hedrick JB (2010) Mica. In: Mineral Commodity Summaries 2010. U.S. Geological Survey, pp 102-105 Kesler SE, Gruber PW, Medina PA, Keolian GA, Everson MP, Wallington TJ (2012) Global lithium resources: Relative importance of pegmatite, brine and other deposits. Ore Geology Reviews, in press Linnen RL, Van Lichtervelde M, Černý P (2012) Granitic pegmatites as sources of strategic metals. Elements 8: 275-280 London D, Kontak DJ (2012) Granitic pegmatites: Scientific wonders and economic bonanzas. Element 8: 257-261 273 London D, Morgan GB VI (2012) The pegmatite puzzle. Elements 8: 263-268 Simmons WB, Pezzotta F, Shigley JE, Beurlen H (2012) Granitic pegmatites as sources of colored gemstones. Elements 8: 281-287 Tanner AO (2011) Feldspar. In: Mineral Commodity Summaries 2011. U.S. Geological Survey, pp 54-55 Virta RL (2010) Clays. In: Mineral Commodity Summaries 2010. U.S. Geological Survey, pp 44-45 A UGUS T 2012