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Talc

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

                       Talc
                  A block of talc
                      General
   Category            Mineral
   Chemical formula    Mg[3]Si[4]O[10]( OH)[2]
                  Identification
   Colour              white to gray or green
   Crystal habit       foliated to fibrous masses
   Crystal system      Monoclinic
   Cleavage            perfect basal cleavage
   Mohs Scale hardness 1
   Luster              waxlike or pearly
   Streak              White
   Specific gravity    2.5-2.8

   Talc (derived from the Persian via Arabic talq) is a mineral composed
   of hydrated magnesium silicate with the chemical formula H[2]Mg[3](
   SiO[3])[4] or Mg[3]Si[4]O[10]( OH)[2]. 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). 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
   gray 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_{10}(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_{10}(OH)_2 +
          3CaCO_3 + 3CO_2

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

   Chlorite + Quartz → Kyanite + Talc + H[2]O

   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.

Occurrence

   Talc is a very common metamorphic mineral in metamorphic belts which
   contain ultramafic rocks, such as soapstone (a high-talc rock), and
   within whiteschist and 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.

   Talc carbonated ultramafics are typical of many areas of the Archaean
   cratons, notably the komatiite belts of the Yilgarn Craton in Western
   Australia. Talc-carbonate ultramafics are also known from the Lachlan
   Fold Belt, eastern Australia, from Brazil, the Guyana Shield, and from
   the 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

   Talcum Powder
   Talcum Powder

   A coarse grayish-green high-talc rock is soapstone or steatite and has
   been used for stoves, sinks, electrical switchboards, etc. Talc finds
   use as a cosmetic (talcum powder), as a lubricant, and as a filler in
   paper manufacture. Talc is used in baby powder, an astringent powder
   used for preventing rashes on the area covered by a diaper (see diaper
   rash). Most tailor's 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.

Safety

   Several studies have established preliminary links between talc and
   pulmonary issues, lung cancer, skin cancer and ovarian cancer. This is
   a major concern considering talc's widespread commercial and household
   use. However, no conclusive study has yet been made to determine either
   the toxicity and/or carcinogenic nature of talc and the long history of
   safe use suggests that these concerns are unfounded. 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.
   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. 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.

   Retrieved from " http://en.wikipedia.org/wiki/Talc"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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