talc

Talc
A block of talc
General
Category	Silicate mineral
Chemical formula	Mg3Si4O10(OH)2
Identification
Color	white, grey, green, blue, or silver
Crystal habit	foliated to fibrous masses
Crystal system	Monoclinic or Triclinic[1]
Cleavage	perfect basal cleavage
Fracture	Flat surfaces (not cleavage) fracture in an uneven pattern
Tenacity	Sectile
Mohs scale hardness	1
Luster	waxlike or pearly sometimes smooth
Streak	White
Diaphaneity	Translucent
Specific gravity	2.58 to 2.83
Optical properties	Biaxial (-)
Refractive index	nα = 1.538 - 1.550 nβ = 1.589 - 1.594 nγ = 1.589 - 1.600
Birefringence	δ = 0.051
Pleochroism	Weak in dark varieties
Other characteristics	Fluorescent
References	[2][3][4]

Talc (derived from the Persian talc 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, 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. Talc is not soluble in water, but it is slightly soluble in dilute mineral acids. Its colour ranges from white to grey or green and it has a distinctly greasy feel. Its streak is white.

Soapstone is a metamorphic rock composed predominantly of talc.

Formation

Talc is a metamorphic mineral resulting from the metamorphism of magnesian minerals such as serpentine, pyroxene, amphibole, olivine, 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

Mg₃Si₂O₅ (OH)₄ + 3CO₂ → Mg₃Si₄O₁₀ (OH)₂ + 3 MgCO₃ + 3 H₂O

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

CaMg(CO₃)₂ + 4 SiO₂ + H₂O → Mg₃Si₄O₁₀ (OH)₂ + 3 CaCO₃ + 3 CO₂

Crystal structure of talc

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

chlorite + quartz → kyanite + talc + water

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.^[1]

Occurrence

Talc output in 2005

Talc is a 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 Guiana 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. The French-based Luzenac Group is the world's largest supplier of mined talc.

Uses

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Talcum powder.

A coarse grayish-green high-talc rock is soapstone or steatite and has been used for stoves, sinks, electrical switchboards, etc. It is often used for surfaces of lab counter tops and electrical switchboards because of its resistance to heat, electricity and acids. 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). It is also often used in basketball to keep a player's hands dry. 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 as a glidant. In medicine talc is used as a pleurodesis agent to prevent recurrent pneumothorax. In the European Union the additive number is E553b.

Talc is widely used in the ceramics industry in both bodies and glazes. In low-fire artware bodies it imparts whiteness and increases thermal expansion to resist crazing. In stonewares, small percentages of talc are used to flux the body and therefore improve strength and vitrification. It is a source of MgO flux in high temperature glazes (to control melting temperature). It is also employed as a matting agent in earthenware glazes and can be used to produce magnesia mattes at high temperatures.

ISO standard for quality (ISO 3262)

Patents are pending on the use of magnesium silicate as a cement substitute. Its production requirements are less energy-intensive than ordinary portland cement at around 650C, while it absorbs far more carbon dioxide as it hardens. This results in a negative carbon footprint overall, as the cement removes 0.6 tonnes of CO2 per tonne used. This contrasts with a carbon footprint of 0.4 tonne per tone of conventional cement.^[5]

It is used to as an additive for heroin, to expand volume and weight and thereby increase its street value. With intravenous use, it may lead to talcosis, a granulomatous inflammation in the lungs.

Safety

Several studies have established preliminary links between talc and pulmonary issues,^[6] lung cancer,^[7][8] skin cancer and ovarian cancer.^[9] 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 rats (animal testing) forced to inhale talc for 6 hours a day, five days a week over at least 113 weeks, even though it contained no asbestos-like fibres.^[7] 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.^[10] 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. ^[11]

See also

Wikimedia Commons has media related to: Talc

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

References

External links

• mineral.galleries.com
• CeramicMaterials.Info