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asbestos, common name for any of a variety of silicate minerals within the amphibole and serpentine groups that are fibrous in structure and more or less resistant to acid and fire. Chrysotile asbestos, a form of serpentine that is also known as white asbestos and has curly fibers, is the chief commercial asbestos. Varieties of amphibole asbestos, which has needlelike fibers, are amosite, or brown asbestos, used in insulating materials; crocidolite, or blue asbestos, used for making asbestos-cement products; and tremolite, used in laboratories for filtering chemicals. Asbestos is usually found comprising veins in other rock; in most cases it appears to be the product of metamorphism. The asbestos-producing nations are Russia, Kazakhstan, China, and Brazil. Asbestos is mined both in open quarries and underground.
Since the 1960s, asbestos has been recognized as a potent carcinogen and serious health hazard. Inhalation of airborne asbestos fibers has been established as the cause of asbestosis (thickening and scarring of lung tissue) and as a cause of mesothelioma (a highly lethal tumor of the pleura) as well as of cancers of the lung, intestines, and liver. In 1972, the Occupational Safety and Health Administration began regulating asbestos and strengthening work safety standards. Large class action lawsuits were filed and won against asbestos companies, which had probable prior knowledge of the dangers involved. In 1989, the Environmental Protection Agency imposed a ban on 94% of U.S. asbestos production and imports, to be phased in over a seven year period. Most current asbestos exposure comes from asbestos in older buildings and products such as automobile brakes.
See P. H. Riordon and V. F. Hollister, Geology of Asbestos Deposits (1981); S. S. Chissick and R. Derricott, Asbestos: Properties, Applications and Hazards (1983).
a collective term used to designate a group of fine-fiber minerals of the silicate class that form aggregates made up of extremely fine, flexible fibers. These properties are possessed by minerals of two groups—serpentine and amphibole, known respectively by the names chrysotile asbestos and amphibole asbestos, which differ in atomic structure. Chemically, asbestos minerals are composed of hy-drated silicates of magnesium and iron and partially of calcium and sodium. Most important is chrysotile asbestos, which constitutes 95 percent of the total asbestos in use.
Chrysotile asbestos is a mineral of the serpentine group, Mg6[Si4O10](OH)8; it is a double-layered, sheet-form silicate. One layer consists of silicon oxide tetrahedra, the other of oxide octahedra with magnesium (or sometimes with iron) at the center. In particles it has a greenish-gray color and a silken sheen. Its hardness on the mineralogical scale ranges from 2 to 2.5, and its density is 2,500 kg/m3. The fibers are flexible, possessing a high tensile strength (approximately 3 giganewtons per m2 [300 kilograms-force per mm2]) and a high resistance to fire (the melting point is roughly 1500°C); they are poor conductors of heat and electricity. Fiber length varies from fractions of a millimeter to 50 mm, rarely more, and its thickness is measured in fractions of a micron. Deposits are formed in ultrabasic rocks, rich in magnesium, by the action of hydrothermal solutions connected with younger granites. In the USSR chrysotile asbestos is mined in the Urals (the Bazhenov and Kiembaev deposits), the western part of Kazakhstan (Dzhetygarina), and the Tuva ASSR (Ak-Dovurak). In the northern part of Chita Oblast the Molodezhnyi deposit has been discovered, in which exceptionally long asbestos fibers are encountered. Of great importance are the chrysotile deposits in the serpentine belt of the Eastern and Western Saian mountains, as well as in the Northern Caucasus. Major foreign deposits of chrysotile asbestos have been found in Canada (Quebec Province) and South Africa (Southern Rhodesia and the Republic of South Africa); it is also mined in Czechoslovakia, China, the USA, Italy, France, Finland, Cyprus, Japan, and the Australian Commonwealth. Chrysotile asbestos is suitable for the manufacture of fireproof textile products, heat-insulating products, various fillers for plastics, and asbestos cement.
Amphibole asbestos is represented by several minerals of the amphibole group, such as tremolite, anthophyllite, and crocidolite. Amphibole asbestos is an aggregate of fine, hairlike crystals lying in a parallel or radial formation or without any order at all. The fibers of certain types of amphibole asbestos are brittle. The color and other physical properties depend on its composition. Fiber length ranges up to 5 cm but often is even longer. Deposits of amphibole asbestos are limited to metamorphic rocks; in the USSR they are located in the Urals (Syserf); abroad they are found in Southern Rhodesia and the Republic of South Africa. Amphibole asbestos has a high resistance to acid and seawater and is also a good material for fireproofing and heat-insulating products. It is most widely used in the chemical industry as a filler.
REFERENCESMerenkov, B. Ia. Genezis khrizotil-asbesta. Moscow, 1958.
Mestorozhdeniia khrizotil-asbesta SSSR. Moscow, 1967.
V. P. PETROV