Talc

Talc (derived from the Persian via Arabic talq) is a green and it has a distinctly greasy feel. Its streak is white.

Formation

Talc is a talc carbonates.

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

Magnesite + Water

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;

Calcite + Carbon Dioxide

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

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 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 magnesium in the octahedral sites of the composite layers.^[1]

Occurrence

  Talc is a very common metamorphic mineral in metamorphic belts which contain blueschist metamorphic terranes. Prime examples of whiteschists include the Franciscan Metamorphic Belt of the western United States, the western European Alps especially in Italy, certain areas of the Musgrave Block, and some collisional orogens such as the Himalayas.

ophiolite belts of Turkey, Oman and the Middle East.

Notable economic talc occurrences include the Mount Seabrook talc mine, Western Australia, formed upon a polydeformed, layered ultramafic intrusion.

Uses

  A coarse grayish-green high-talc rock is chalk is talc, as is the chalk often used for welding or metalworking.

Talc is also used as food additive or in pharmaceutical products. In medicine talc is used as a pleurodesis agent to prevent recurrent pneumothorax. In the European Community the additive number is E553b.

Recently it has come into popularity as a base for cocaine for several reasons. The first, and most common though, is that its chemical properties interfere with the sense of smell in most dogs, causing the drug to be next to undetectable by drug dogs.

Safety

Several studies have established preliminary links between talc and pulmonary issues,^[2] lung cancer,^[3][4] skin cancer and ovarian cancer.^[5] 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.^[3] Scientists have been aware of the toxicity of talc since the late 1960s, and in 1971 researchers found particles of talc embedded in 75 percent of the ovarian tumors studied.^{[citation needed]} However, the U.S. amphibole fibers, to be Generally recognized as safe (GRAS) for use in cosmetics. ^[6]

See also

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

References

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

1. ^ ^{a b} An Introduction to the Rock-Forming Minerals, second edition, by W.A. Deer, R.A. Howie, and J. Zussman, 1992, Prentice Hall, ISBN 0-582-30094-0.
2. ^ Hollinger (1990). "Pulmonary toxicity of inhaled and intravenous talc.".
3. ^ ^{a b} National Toxicology Program (1993). "NTP Toxicology and Carcinogenesis Studies of Talc (Non-Asbestiform) in Rats and Mice (Inhalation Studies).".
4. ^ NIOSH Worker Notification Program. "Health effects of mining and milling talc.".(historical)
5. ^ Harlow, Cramer, Bell, et al. (1992). "Perineal exposure to talc and ovarian cancer risk.".
6. ^ CFSAN/Office of Food Additive Safety (July 2006). Food Additive Status List. U.S. Food and Drug Administration. Retrieved on December 2007.