Talc

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Talc
Talc.jpg
Crystals of talc
General
Category Silicate mineral
Formula
(repeating unit)
Mg3Si4O10(OH)2
Strunz classification 9.EC.05
Crystal symmetry Either monoclinic 2m or triclinic 1[1]
Unit cell a = 5.291 Å, b = 9.173 Å, c = 5.290 Å; α = 98.68°, β = 119.90°, γ = 90.09°; Z = 2 or
a = 5.287 Å, b = 9.158 Å, c = 18.95 Å, β = 99.3°; Z = 4[1]
Identification
Color Light to dark green, brown, white,grey
Crystal habit Foliated to fibrous masses, rare as platey to pyramidal crystals
Crystal system monoclinic or triclinic[2]
Cleavage Perfect on {001} basal cleavage
Fracture Flat surfaces (not cleavage), fracture in an uneven pattern
Tenacity Sectile
Mohs scale hardness 1 (defining mineral)
Luster Waxlike or pearly
Streak White to pearl black
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
Ultraviolet fluorescence Short UV=orange yellow, long UV=yellow
References [1][3][4]

Talc (derived from Persian: تالکtālk; Arabic: تلكtalk) 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, and in an exceptionally rare crystal form. It has a perfect basal cleavage, and the folia are non-elastic, although slightly flexible. It is the softest known mineral and listed as 1 on the Mohs hardness scale. It can be easily scratched by a fingernail. It is also sectile (can be cut with a knife). 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[edit]

A block of talc

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 via the following reaction;

serpentine + carbon dioxide → talc + magnesite + water
2 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
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.[2]

Occurrence[edit]

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[edit]

Talcum powder
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.

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.[6]

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[edit]

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.[7][8]

The studies reference, by subject: pulmonary issues,[9] lung cancer,[10][11] and ovarian cancer.[12] 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.[10] A 1971 paper found particles of talc embedded in 75% of the ovarian tumors studied.[13]

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.[14]

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.[15]

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%.[16]

See also[edit]

References[edit]

  1. ^ a b c Handbook of Mineralogy
  2. ^ 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.
  3. ^ Talc at Mindat.org
  4. ^ Talc at Webmineral
  5. ^ Luzenac Group report (2006). p.3. (French)
  6. ^ Revealed: The cement that eats carbon dioxide Alok Jha, The Guardian, 31 December 2008
  7. ^ Talcum powder and cancer, Cancerresearch.uk
  8. ^ Talcum Powder and Cancer, American Cancer Society
  9. ^ Hollinger, MA (1990). "Pulmonary toxicity of inhaled and intravenous talc". Toxicology letters 52 (2): 121–7; discussion 117–9. doi:10.1016/0378-4274(90)90145-C. PMID 2198684. 
  10. ^ a b 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. 
  11. ^ NIOSH Worker Notification Program. Health effects of mining and milling talc.. 
  12. ^ Harlow, Cramer, Bell, et al (1992). "Perineal exposure to talc and ovarian cancer risk". Obstetrics and gynecology 80 (1): 19–26. PMID 1603491. 
  13. ^ 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. doi:10.1111/j.1471-0528.1971.tb00267.x. PMID 5558843. 
  14. ^ "NIOSH Pocket Guide to Chemical Hazards". Centers for Disease Control and Prevention. 2011. 
  15. ^ "Is talcum powder asbestos?". The Straight Dope. Feb. 16, 1990. 1
  16. ^ "Code of Federal Regulations". U.S. Food and Drug Administration. 2009. 

External links[edit]