Talc
31422
224308986
2008-07-08T07:39:46Z
Benbest
167797
solubility
{{Infobox mineral
| name = Talc
| category = Minerals
| boxwidth =
| boxbgcolor =
| image = Talc block.jpg
| caption = A block of talc
| formula = [[Magnesium|Mg]]<sub>3</sub>[[Silicon|Si]]<sub>4</sub>[[Oxygen|O]]<sub>10</sub>([[Hydroxide|OH]])<sub>2</sub>
| molweight =
| color = white, gray, green or blue
| habit = foliated to fibrous masses
| system = [[Monoclinic]] or [[Triclinic]]<ref name="DHZ">''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.</ref>
| twinning =
| cleavage = perfect [[basal cleavage]]
| fracture =
| mohs = 1
| luster = waxlike or pearly sometimes smooth
| refractive =
| opticalprop =
| birefringence =
| pleochroism =
| streak = White
| gravity = 2.5-2.8
| melt =
| fusibility =
| diagnostic =
| solubility =
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}}
'''Talc''' (derived from the [[Persian language|Persian]] via [[Arabic language|Arabic]] ''talq'') is a [[mineral]] composed of [[hydrate|hydrated]] [[magnesium]] [[silicate]] with the chemical formula [[Hydrogen|H]]<sub>2</sub>[[Magnesium|Mg]]<sub>3</sub>([[Silicate|SiO]]<sub>3</sub>)<sub>4</sub> or [[Magnesium|Mg]]<sub>3</sub>[[Silicon|Si]]<sub>4</sub>[[Oxygen|O]]<sub>10</sub>([[Hydroxide|OH]])<sub>2</sub>. In loose form, it is the widely used substance known as [[talcum powder]]. It occurs as foliated to fibrous masses, its [[monoclinic]] [[crystal]]s being so rare as to be almost unknown. It has a perfect [[basal cleavage]], and the folia are non-[[elastic (solid mechanics)|elastic]], although slightly flexible. It is [[sectile]] and very soft, with a [[Mohs scale of mineral hardness|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 [[Lustre (mineralogy)|luster]], and is translucent to opaque. Talc is not soluble in water, but it is slightly soluble in dilute [[mineral acid]]s. 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 rock|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 carbonate]]s.
Talc is primarily formed via hydration and carbonation of serpentine, via the following reaction;
''[[Serpentine]] + [[Carbon Dioxide]] → Talc + [[Magnesite]] + [[Water]]''
:<math>2Mg_3Si_2O_5(OH)_4 + 3CO_2 \rarr Mg_3Si_4O_{10}(OH)_2 + 3MgCO_3 + 3H_2O</math>
Talc can also be formed via a reaction between dolomite and silica, which is typical of [[skarn]]ification of dolomites via silica-flooding in contact metamorphic aureoles;
''[[Dolomite]] + [[Silica]] + Water → Talc + [[Calcite]] + Carbon Dioxide''
:<math>3CaMg(CO_3)_2 + 4SiO_2 + H_2O \rarr Mg_3Si_4O_{10}(OH)_2 + 3CaCO_3 + 3CO_2</math>
Talc can also be formed from magnesian chlorite and quartz in [[blueschist]] and [[eclogite]] metamorphism via the following [[metamorphic reaction]]:
''[[Chlorite group|Chlorite]] + [[Quartz]] → [[Kyanite]] + Talc + H<sub>2</sub>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 [[fengite|phengite]], [[garnet]], [[glaucophane]] within the lower [[Blueschist#Blueschist facies|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.<ref name="DHZ" />
== Occurrence ==
[[Image:2005talc.PNG|thumb|left|Talc output in 2005]]
Talc is a very common metamorphic mineral in metamorphic belts which contain [[ultramafic rock]]s, 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 [[Orogeny|orogens]] such as the [[Himalayas]].
[[Talc carbonate]]d ultramafics are typical of many areas of the [[Archaean]] [[craton]]s, notably the [[komatiite]] belts of the [[Yilgarn craton|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.
== Uses ==
{{Unreferencedsection|date=January 2008}}
[[Image:Talcum Powder.JPEG|thumb|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 [[Cosmetics|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 Union]] the additive number is E553b.
== Safety ==
Several studies have established preliminary links between talc and [[pulmonary]] issues,<ref>
{{cite journal
|url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2198684&dopt=Abstract |title=Pulmonary toxicity of inhaled and intravenous talc.
|author=Hollinger |year=1990 }}
</ref> [[lung cancer]],<ref name="NTP">
{{cite journal
|url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12616290&dopt=Abstract
|title=NTP Toxicology and Carcinogenesis Studies of Talc (Non-Asbestiform) in Rats and Mice (Inhalation Studies).
|author=National Toxicology Program|year=1993}}
</ref><ref>
{{cite journal |url=http://www.cdc.gov/niosh/pgms/worknotify/Talc.html
|title=Health effects of mining and milling talc.
|author=NIOSH Worker Notification Program }}(historical)
</ref> [[skin cancer]] and [[ovarian cancer]].<ref>
{{cite journal
|url=http://www.meb.uni-bonn.de/cgi-bin/mycite?ExtRef=MEDL/92293722
|title=Perineal exposure to talc and ovarian cancer risk.
|author=Harlow, Cramer, Bell, et al. |year=1992}}
</ref> 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.<ref name="NTP" /> 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.<ref>
{{cite journal
|url=http://www.ncbi.nlm.nih.gov/pubmed/5558843
|title=Talc and carcinoma of the ovary and cervix.
|author=Henderson WJ, Joslin CA, Turnbull AC, Griffiths K|year=1971
|journal=J Obstet Gynaecol Br Commonw|volume=78|issue=3|pages=266–272}}
</ref>{{Fact|date=June 2007}}
However, the U.S. [[Food and Drug Administration]] (FDA) considers non-asbestiform talc, that is talc which does not contain potentially [[carcinogen]]ic [[asbestos|asbestiform]] [[amphibole]] fibers, to be [[Generally recognized as safe]] (GRAS) for use in cosmetics.
<ref>
{{cite web
| author = CFSAN/Office of Food Additive Safety
| title = Food Additive Status List
| publisher = U.S. Food and Drug Administration
|date=July 2006
| url =http://www.cfsan.fda.gov/~dms/opa-appa.html#ftnT
| accessdate = December 2007 }}
</ref>
== See also ==
{{commonscat|Talc}}
* [[List of minerals]]
* [[Serpentinite]]
* [[Talc carbonate]]
* [[Magnesite]]
* [[Blueschist]]
* [[Metamorphic rocks]]
* [[Asbestos]]
* [[French chalk]]
== References ==
* [http://mineral.galleries.com/minerals/silicate/talc/talc.htm mineral.galleries.com]
* [http://webmineral.com/data/Talc.shtml Webmineral.com]
* [http://www.mindat.org/min-3875.html www.mindat.org]
<references/>
[[Category:Phyllosilicates]]
[[Category:Magnesium minerals]]
[[Category:Symbols of Vermont]]
[[Category:Cosmetic chemicals]]
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