Magnesium silicate

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Talc
250px
Crystals of talc
General
CategorySilicate mineral
Chemical formulaMg3Si4O10(OH)2
Identification
ColorLight to dark green, brown, white, grey
Crystal habitFoliated to fibrous masses, rare as platey to pyramidal crystals
Crystal systemmonoclinic or triclinic[1]
CleavagePerfect on {001} basal cleavage
FractureFlat surfaces (not cleavage), fracture in an uneven pattern
TenacitySectile
Mohs Scale hardness1 (defining mineral)
LusterWaxlike or pearly
Refractive indexnα = 1.538 – 1.550
nβ = 1.589 – 1.594
nγ = 1.589 – 1.600
Optical PropertiesBiaxial (-)
Birefringenceδ = 0.051
PleochroismWeak in dark varieties
Ultraviolet fluorescenceShort UV=orange yellow, long UV=yellow
StreakWhite to pearl black
Specific gravity2.58 to 2.83
DiaphaneityTranslucent
References[2][3][4]

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

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

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

chlorite + quartzkyanite + 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

File:2005talc.png
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 which stretches along Pakistan, India, Nepal and Bhutan.

Talc carbonate 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 France-based Luzenac Group is the world's largest supplier of mined talc; its largest talc mine at Trimouns near Luzenac in southern France produces 400,000 tonnes of talc per year, representing 8% of world production.[5]

Uses

File:Talc.GIF
The structure of talc is composed of Si2O5 sheets with magnesium sandwiched between sheets in octahedral sites.

Talc is used in many industries such as paper making, plastic, paint and coatings, rubber, food, electric cable, pharmaceuticals, cosmetics, ceramics, etc. A coarse grayish-green high-talc rock is soapstone or steatite and has been used for stoves, sinks, electrical switchboards, crayons, soap, 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. It is also often used in basketball to keep a player's hands dry. Most tailor's chalk, or French 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 pleural effusion or pneumothorax. In the European Union the additive number is E553b.

Due to its low shear strength, talc is one of the oldest known solid lubricants. There is also a limited use of talc as friction-reducing additive in lubricating oils.[6]

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)

Type Talc content min. wt% Loss on ignition at 1000 °C, wt % Solubility in HCl, max. wt %
A 95 4 – 6.5 5
B 90 4–9 10
C 70 4–18 30
D 50 4–27 30

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 a heating requirement of around 650 °C for talc compared to 1500 °C for limestone to produce Portland cement), while it absorbs far more carbon dioxide as it hardens. This results in a negative carbon footprint overall, as the cement substitute removes 0.6 tonnes of CO2 per tonne used. This contrasts with a carbon footprint of 0.4 tonne per tonne of conventional cement.[7]

Talc is used in manufacturing of wall coatings as a base content Paints, Talc is also using in agriculture for organic farming Agriculture Talc is used in Food. Talc is also used in cosmetics baby powder. Talc is used in detergent powder.

Talc is sometimes used as an adulterant to illegal 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

Talc powder is a household item, sold globally for use in personal hygiene and cosmetics. Some suspicions have been raised about the possibility its use promotes certain types of diseases, mainly cancers of the ovaries and lungs (it is in the same 2B category in the IARC listing as mobile phones and coffee) although this is not widely recognised as an established link.[8][9]

The studies reference, by subject: pulmonary issues,[10] lung cancer,[11][12] and ovarian cancer.[13] One of these, published in 1993, was a US National Toxicology Program report, which found that cosmetic grade talc containing no asbestos-like fibres was correlated with tumour formation in rats (animal testing) forced to inhale talc for 6 hours a day, five days a week over at least 113 weeks.[11] A 1971 paper found particles of talc embedded in 75% of the ovarian tumors studied.[14] Recent research questions if a link does actually exist between the two. [15] [16]

The Occupational Safety and Health Administration and National Institute for Occupational Safety and Health have set occupational exposure limits to respirable talc dusts at 2 mg/m3 over an eight-hour workday.[17]

One particular issue with commercial use of talc is its frequent co-location in underground deposits with asbestos ore, which often leads to contamination of powdered talc products with asbestos fibres. Stringent quality control since 1976 (separating cosmetic and food-grade talc from "industrial" grade talc, which is allowed a certain portion of asbestos contamination) has mostly eliminated this issue, but it remains a continuing hazard requiring mitigation in the mining and processing of talc.[18] A 2010 US FDA survey failed to find asbestos in a variety of talc-containing products.[19]

The US Food and Drug Administration (FDA) considers talc (magnesium silicate) to be generally recognized as safe (GRAS) for use as an anti-caking agent in table salt in concentrations smaller than 2%.[20]

See also

References

  1. 1.0 1.1 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. Handbook of Mineralogy
  3. Talc at Mindat.org
  4. Talc at Webmineral
  5. Luzenac Group report (2006). p.3. Template:Fr
  6. Talc as friction reducing additive to lubricating oil
  7. Revealed: The cement that eats carbon dioxide Alok Jha, The Guardian, 31 December 2008
  8. Talcum powder and cancer, Cancerresearch.uk
  9. Talcum Powder and Cancer, American Cancer Society
  10. Hollinger, MA (1990). "Pulmonary toxicity of inhaled and intravenous talc". Toxicology letters. 52 (2): 121–7; discussion 117–9. PMID 2198684. doi:10.1016/0378-4274(90)90145-C. 
  11. 11.0 11.1 National Toxicology, Program (1993). "NTP Toxicology and Carcinogenesis Studies of Talc (Non-Asbestiform) in Rats and Mice (Inhalation Studies)". National Toxicology Program technical report series. 421: 1–287. PMID 12616290. 
  12. NIOSH Worker Notification Program. "Health effects of mining and milling talc.". 
  13. Harlow, Cramer, Bell; et al. (1992). "Perineal exposure to talc and ovarian cancer risk". Obstetrics and gynecology. 80 (1): 19–26. PMID 1603491. 
  14. Henderson WJ, Joslin CA, Turnbull AC, Griffiths K (1971). "Talc and carcinoma of the ovary and cervix". J Obstet Gynaecol Br Commonw. 78 (3): 266–272. PMID 5558843. doi:10.1111/j.1471-0528.1971.tb00267.x. 
  15. Harlow, BL; Hartge, PA (Apr 1995). "A review of perineal talc exposure and risk of ovarian cancer.". Regulatory toxicology and pharmacology : RTP. 21 (2): 254–60. PMID 7644715. doi:10.1006/rtph.1995.1039. 
  16. Gertig, D. M.; Hunter, D. J.; Cramer, D. W.; Colditz, G. A.; Speizer, F. E.; Willett, W. C.; Hankinson, S. E. (2 February 2000). "Prospective Study of Talc Use and Ovarian Cancer". JNCI Journal of the National Cancer Institute. 92 (3): 249–252. doi:10.1093/jnci/92.3.249. 
  17. "NIOSH Pocket Guide to Chemical Hazards". Centers for Disease Control and Prevention. 2011. 
  18. "Is talcum powder asbestos?". The Straight Dope. Feb. 16, 1990. 1
  19. "Talc Ingredients". U.S. Food and Drug Administration. 2010. 
  20. "Code of Federal Regulations". U.S. Food and Drug Administration. 2009. 

External links

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