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Related to E553b: French chalk, talcs
talc,mineral ranging in color from white through various shades of gray and green to the red and brown of impure specimens, translucent to opaque, and having a greasy, soapy feel. It is a hydrous silicate of magnesium, Mg3Si4O10(OH)2, and usually contains small quantities of nickel, iron, and aluminum as impurities. It occurs commonly in folia (thin layers), but is also found in coarsely granular, finely granular, or cryptocrystalline masses. Soapstonesoapstone
, metamorphic rock of which the characteristic and usually chief mineral is talc, but which also contains varying parts of chlorite, mica, tremolite, quartz, magnetite, and iron compounds.
..... Click the link for more information. , or steatite, is a massive, granular form of talc. French chalk is a fine-grained variety. Talc is usually associated with chlorite schists, serpentine, dolomite, and other metamorphic rocks; it is apparently a secondary mineral formed by the alteration of other magnesium silicates. There are important deposits of talc in Austria, Italy, France, and Canada and in the United States in California, North Carolina, Texas, Georgia, and Montana. Talc is used in making paper (as a filler), paints, face and talcum powder, soap, fireproof roofing, foundry facings, lubricants, linoleum and oilcloth, electrical insulation, and pottery.
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.