talc

talc

a white, grey, brown, or pale green mineral, found in metamorphic rocks. It is used in the manufacture of talcum powder and electrical insulators. Composition: hydrated magnesium silicate. Formula: Mg₃Si₄O₁₀(OH)₂. Crystal structure: monoclinic

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

talc

[talk]

(mineralogy)

Mg₃Si₄O₁₀(OH)₂ A whitish, greenish, or grayish hydrated magnesium silicate mineral crystallizing in the monoclinic system; it is extremely soft (hardness is 1 on Mohs scale) and has a characteristic soapy or greasy feel.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

talc

A soft mineral composed of hydrous magnesium silicate; a major ingredient of soapstone; used on roll roofing to prevent sticking in the roll.

McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Talc

 

a widespread mineral of the class of phyllosilicates, with the formula Mg₃ [Si₄O₁₀](OH)₂. The magnesium may be replaced by Fe, Ni, Al, or Cr. Varieties of talc include minnesotaite, which contains Fe (50–80 atomic percent), willemseite, which is essentially nickeliferous talc, and steatite, or soapstone, which is compact massive talc.

The crystal structure of talc is a superposition of layers about 9 angstroms thick, consisting of two silicon oxide layers with Mg and OH ions between them. The charges of the atoms in the layers are balanced, and the layers are held together only by weak van der Waals’ bonds. The symmetry of the unit cell is triclinic, but any relative displacement of the layers causes a transition to a less ordered, monoclinic structure. Talc forms foliated or compact fine-grained masses and, less frequently, irregular lamellar crystals, up to 10 cm in size, which readily break into flexible transparent lamellae along cleavage planes.

Talc may be white or various shades of green. It is soft, with a hardness of 1 on Mohs’ scale, and has a greasy feel. Its density is 2,776–2,824 kg/m³; minnesotaite has a density of 3,010 kg/m³. Although talc is formed mainly at great depths under high temperatures and pressures, crystallization is possible at the earth’s surface in a strongly alkaline medium. Commercial deposits of talc are genetically associated with ultrabasic rocks or with magnesium-carbonate rocks, such as dolomite and magnesite. Talc occurs in talcose rocks, with admixtures of carbonates or chlorite, which are formed during normal metamorphism of magnesium silicate rocks, and as the more valuable pure talcites, formed by hydrometamorphic reactions involving contact between magnesium and siliceous rocks, such as gneisses, slates, or quartzites.

In the USSR, talc is obtained from the Shabrovskii talc-magnesite and the Miass talcite deposits in the Central Urals and from the Onot steatite talc deposit in the Vostochnyi Saian. A western Baikal talc-bearing region has been discovered. Abroad, major deposits of talc exist in Canada (Madoc), the USA (Gouverneur), and France (Luzenac).

Talc is used as an extender in the rubber, paper, and paint and varnish industries. It is also used in medicine and in the perfume and cosmetics industries; in technology it is used as a solid lubricant. An important application of talc is in ceramics, especially ceramic products used in radio insulation. Nonferrous steatite talc is an ornamental stone.

REFERENCE

Mestorozhdeniia tal’ka SSSR. Moscow, 1973.

P. P. SMOLIN

Inhalation of talc dust may lead to talcosis, an illness of the pneumonoconiosis group. Talcosis is observed in workers in the talc extraction and processing industries, as well as in the perfume and rubber industries. As a rule, it is a benign, slowly progressing illness, which has little effect on general health and work capacity. The most important means for its prevention are mechanization and automation of labor, dust suppression, dust collection, and the use of respirators.

REFERENCE

Borshchevskii, Iu. M., V. G. Artamonova, and V. M. Retnev. Silikatozy i gigiena iruda. Leningrad, 1974.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.