talc n : a fine grained mineral having a soft soapy feel and consisting of hydrated magnesium silicate; used in a variety of products including talcum powder [syn: talcum] v : apply talcum powder to (one's body) [also: talcking, talcked]

English

Etymology

From (talq).

Noun

1. A soft mineral of a soapy feel and a greenish, whitish, or grayish color, usually occurring in foliated masses.

Related terms

• talcum

Talc (derived from the Persian via Arabic talq) is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg3(SiO3)4 or Mg3Si4O10(OH)2. In loose form, it is the widely used substance known as talcum powder. It occurs as foliated to fibrous masses, its monoclinic crystals being so rare as to be almost unknown. It has a perfect basal cleavage, and the folia are non-elastic, although slightly flexible. It is sectile and very soft, with a hardness of 1 (Talc is the softest of the Mohs' scale of mineral hardness, and can be easily scratched by a fingernail). It has a specific gravity of 2.5–2.8, a clear or dusty luster, and is translucent to opaque. Its colour ranges from white to grey or green and it has a distinctly greasy feel. Its streak is white.

Formation

Talc is a metamorphic mineral resulting from the metamorphism of magnesian minerals such as pyroxene, amphibole, olivine and other similar minerals in the presence of carbon dioxide and water. This is known as talc carbonation or steatization and produces a suite of rocks known as talc carbonates.

Talc is primarily formed via hydration and carbonation of serpentine, via the following reaction;

Serpentine + Carbon Dioxide → Talc + Magnesite + Water

2Mg_3Si_2O_5(OH)_4 + 3CO_2 \rarr Mg_3Si_4O_(OH)_2 + 3MgCO_3 + 3H_2O

Talc can also be formed via a reaction between dolomite and silica, which is typical of skarnification of dolomites via silica-flooding in contact metamorphic aureoles;

Dolomite + Silica + Water → Talc + Calcite + Carbon Dioxide

3CaMg(CO_3)_2 + 4SiO_2 + H_2O \rarr Mg_3Si_4O_(OH)_2 + 3CaCO_3 + 3CO_2

Talc can also be formed from magnesian chlorite and quartz in blueschist and eclogite metamorphism via the following metamorphic reaction:

Chlorite + Quartz → Kyanite + Talc + H2O

In this reaction, the ratio of talc and kyanite is dependent on aluminium content with more aluminous rocks favoring production of kyanite. This is typically associated with high-pressure, low-temperature minerals such as phengite, garnet, glaucophane within the lower blueschist facies. Such rocks are typically white, friable, and fibrous, and are known as whiteschist.

Talc is a tri-octahedral layered mineral; its structure is similar to that of pyrophyllite, but with magnesium in the octahedral sites of the composite layers. lung cancer, skin cancer and ovarian cancer. This is a major concern considering talc's widespread commercial and household use. In 1993, a US National Toxicology Program report found that cosmetic grade talc caused tumours in animals, even though it contained no asbestos-like fibres. However, the U.S. Food and Drug Administration (FDA) considers non-asbestiform talc, that is talc which does not contain potentially carcinogenic asbestiform amphibole fibers, to be Generally recognized as safe (GRAS) for use in cosmetics.

See also

• List of minerals
• Serpentinite
• Talc carbonate
• Magnesite
• Blueschist
• Metamorphic rocks
• Asbestos
• French chalk

References

• mineral.galleries.com
• Webmineral.com
• www.mindat.org