Background (Encyclopedia of Global Resources)
Because the abrasives category encompasses a great variety of materials, their worldwide distributions are highly varied. Some, such as garnet and emery, are obtained from only a few localities. Others, such as sand and sandstone, are found on all continents, in all geologic settings, and in rocks representing all geologic ages.
Use of all the abrasives reflects in some manner the characteristics of hardness. That property is utilized in cutting and drilling tools, surface polishing materials, and blasting media. The largest user of abrasives is the automobile industry. Abrasives, both natural and synthetic, are used to perform one of four basic functions: the removal of foreign substances from surfaces (“dressing”), cutting, drilling, and comminution (or pulverizing) of materials. Most abrasives lie toward the upper end of the Mohs hardness scale. With respect to one another, however, they can be categorized as hard, moderate (or “siliceous”), or soft.
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Hard Abrasives (Encyclopedia of Global Resources)
The hard abrasives are diamond, corundum, emery, and garnet. Diamond, the hardest naturally occurring substance (10 on the Mohs scale), is normally used in three size categories: stone, bort, and powder. Only a small fraction of the diamond stones produced by mining are of gem quality. All others, as well as those produced synthetically (together referred to as industrial diamonds), are used in various industrial applications, including diamond saws, rock-drilling bits, and other abrasive tools. Bort consists of fragments and small, flawed stones. Most bort, as well as synthetic diamond, is crushed to powder and mixed with water or oil to form a slurry that is used to polish gems. The United States has no exploitable diamond deposits, but it is the world’s leading producer of diamond dust, easily satisfying its industrial needs.
Corundum, the second-hardest naturally occurring substance (9 on the Mohs scale), is used principally in crushed form for the polishing and finishing of optical lenses and metals. Its abrasive quality is enhanced by the fact that when broken it forms sharp edges. As it wears, it flakes, which produces new edges. Corundum occurs in contact metamorphic rocks, granite pegmatites, and placer deposits. The United States has no significant deposits of corundum.
Emery is a natural mixture of corundum and magnetite, with minor amounts of spinel, hematite, or garnet. Its value as an abrasive is...
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Siliceous Abrasives (Encyclopedia of Global Resources)
The term “silica sand” is taken to mean sand of almost pure quartz content, and sandstone (or quartzite) is the lithified version of that sand. Both are examples of siliceous abrasives of moderate hardness. Silica sand is used for sandblasting and for glass grinding. Historically, sandstone has been shaped into grindstones, whetstones, and millstones. Because high-quality sandstones were deposited in shallow seas during virtually all the geological periods, the reserves of silica sand and sandstone of commercial quality in the United States are enormous. Nevertheless, siliceous material for polishing and pulverizing has been replaced to a large extent by steel balls. The market share of silica sand as a sandblasting medium has declined because of health concerns related to the breathing of silica dust, which can lead to a condition called silicosis.
Other siliceous abrasives include diatomite, pumice, tripoli, flint, and chert. Diatomite, or diatomaceous earth, is an accumulation of the siliceous remains of shell-secreting freshwater and marine algae (diatoms). Because it is lightweight and porous, diatomite finds its most important uses as a filtering medium in water purification and waste treatment plants and as a filler (extender) in paint and paper. As an abrasive it is used in scouring soaps and powders, toothpaste, and metal-polishing pastes. The United States possesses the world’s most important reserves of...
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Soft Abrasives (Encyclopedia of Global Resources)
The soft abrasives include feldspar, clay, dolomite, chalk, and talc. They are primarily used for the polishing and buffing of metals. Feldspar, mined from granite pegmatites, is also crushed and used in soaps and scouring powders.
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Synthetic Abrasives (Encyclopedia of Global Resources)
Beginning in about 1900, a variety of manufactured abrasives were developed that have gradually replaced natural abrasives in the marketplace. In addition to lower cost, manufactured abrasives have the advantages of being tailored to meet specific industrial needs and of being produced in uniform quality. Among the important manufactured abrasives are synthetic diamond, cubic boron nitride, fused aluminum oxide, silicon carbide, alumina-zirconia oxide, and steel shot and grit. Synthetic diamonds were first produced in 1955, the result of a process that fuses graphite and metallic catalysts at extremely high temperature and pressure. Cubic boron nitride, first synthesized in 1957, is the next hardest substance after diamond and has challenged synthetic diamond as an abrasive in many industrial applications. Fused aluminum oxide is formed at high temperatures in an electric furnace by the fusing of either bauxite or corundum. Uses include tumbling, polishing, and blasting. It is also used in coated abrasives. Silicon carbide is fused from a mixture of quartz sand and coke; it finds its primary uses as a coated abrasive, in polishing and buffing media, and in wire saws for the cutting of stone. One of the primary uses of steel shot and grit is as a blasting medium. The automobile industry is the largest consumer of artificial abrasives, and the economic fortunes of the two industries are closely tied together.
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Further Reading (Encyclopedia of Global Resources)
Giese, Edward, and Thomas Abraham. New Abrasives and Abrasives Products, Technologies, Markets. Norwalk, Conn.: Business Communications, 1997.
Hayes, Teresa L., Debra A. Celinski, and Rebecca Friedman. Abrasives Products and Markets. Cleveland, Ohio: Freedonia Group, 2000.
Jensen, Mead Leroy, and Alan M. Bateman. Economic Mineral Deposits. 3d ed. New York: Wiley, 1979.
Kogel, Jessica Elzea, et al., eds. “Abrasives.” In Industrial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006.
U.S. Geological Survey. Manufactured Abrasives: Statistics and Information. http://minerals.usgs.gov/minerals/pubs/commodity/abrasives
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Abrasives (Encyclopedia of Science)
Abrasives are materials used to wear down, smooth, clean, shape, or polish a surface. The perfectly smooth surfaces needed in telescope mirrors, for example, are produced by grinding a piece of glass or metal with abrasives. One of the most familiar abrasives is sandpaper, heavy paper coated with a thin layer of aluminum oxide or silicon carbide. Sandpaper can be made in many grades, from coarse to very fine. The grade of sandpaper depends on the size of abrasive particles it contains.
Abrasives can be either natural or synthetic materials. Until the twentieth century, humans used naturally occurring materials, such as sandstone, quartz, emery, corundum, diamonds, and garnet, as abrasives. Then, in 1891, American inventor Edward G. Acheson (1856931) produced silicon carbide by heating a mixture of clay and coke. For 50 years, silicon carbide (also known as Carborundum was the second-hardest substance known, after diamond. This property made it popular as an abrasive. Other synthetic compounds, such as aluminum oxide, boron carbide, and boron nitride are also used now as abrasives.
A material can serve as an abrasive as long as it is harder than the material on which it is to be used. Diamond is the best abrasive of all because it is harder than all other substances. A measure of a material's hardness is the Mohs scale. This 0-to-10...
(The entire section is 472 words.)