Talc

Talc
General
Category	Silicate mineral
Formula (repeating unit)	Mg3Si4O10(OH)2
Strunz classification	9.EC.05
Crystal system	Monoclinic or triclinic[1]
Crystal class	Either prismatic (2m) or pinacoidal (1)[2]
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[2]
Identification
Color	Light to dark green, brown, white, grey
Crystal habit	Foliated to fibrous masses, rare as platey to pyramidal crystals
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 jot 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	[2][3][4]

Talc or talcum is a clay mineral composed of hydrated magnesium silicate with the chemical formula H₂Mg₃(SiO₃)₄ or Mg₃Si₄O₁₀(OH)₂. In loose form, it is (in ratio with or without corn starch) one of the most widely used substances known as baby 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 not elastic, although slightly flexible.

Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 1 as the hardness of talc. As such, talc can easily be scratched by a fingernail. Talc 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 is slightly soluble in dilute mineral acids.^{[citation needed]} 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.

Etymology[edit]

The word "talc" derives from Medieval Latin talcum, which in turn originates from Arabic: طلق‎ ṭalq which in turn was derived from Persian: تالک‎ tālk. In ancient times, the word was used for various related minerals, including talc, mica, and selenite.^[5]

Formation[edit]

A block of talc

Talc is a metamorphic mineral that results from the metamorphism of magnesian minerals such as serpentine, pyroxene, amphibole, and 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 by hydration and carbonation by this reaction:

serpentine2 Mg₃Si₂O₅(OH)₄ + carbon dioxide3CO₂ → talcMg₃Si₄O₁₀(OH)₂ + magnesite3 MgCO₃ + water3 H₂O

Talc can also be formed via a reaction between dolomite and silica, which is typical of skarnification of dolomites by silica-flooding in contact metamorphic aureoles:

dolomite3 CaMg(CO₃)₂ + silica4 SiO₂ + waterH₂O → talcMg₃Si₄O₁₀(OH)₂ + calcite3 CaCO₃ + carbon dioxide3 CO₂

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

chlorite + quartz → kyanite + talc + water

In this reaction, the ratio of talc and kyanite depends 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, and glaucophane within the lower blueschist facies. Such rocks are typically white, friable, and fibrous, and are known as whiteschist.

Talc is a trioctahedral layered mineral; its structure is similar to pyrophyllite, but with magnesium in the octahedral sites of the composite layers.^[1]

Occurrence[edit]

Talc output in 2005

Talc is a common metamorphic mineral in metamorphic belts that 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 stretch 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.

China is the key world talc and steatite producing country with an output of about 2.2M tonnes(2016), which accounts for 30% of total global output. The other major producers are Brazil (12%), India (11%), the U.S. (9%), France (6%), Finland (4%), Italy, Russia, Canada, and Austria (2%, each)^[6].

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.

Conflict mineral[edit]

Extraction in disputed areas of Nangarhar province, Afghanistan, has led the international monitoring group Global Witness to declare talc to be a conflict mineral, as the profits are used to fund the armed confrontation between the Taliban and Islamic State.^[7]

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, including paper making, plastic, paint and coatings, rubber, food, electric cable, pharmaceuticals, cosmetics, and ceramics. A coarse grayish-green high-talc rock is soapstone or steatite, used for stoves, sinks, electrical switchboards, crayons, soap, etc. It is often used for surfaces of laboratory table 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. It is used to coat the insides of inner tubes and rubber gloves during manufacture to keep the surfaces from sticking. Talc, with heavy refinement, has been used in baby powder, an astringent powder used to prevent diaper rash. The American Academy of Pediatrics recommends that parents not use baby powder because it poses a risk of respiratory problems, including breathing trouble and serious lung damage if the baby inhales it. The small size of the particles makes it difficult to keep them out of the air while applying the powder. Zinc oxide-based ointments are a much safer alternative.^[8]

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. Also a limited use of talc as friction-reducing additive in lubricating oils is made.^[9]

Talc is widely used in the ceramics industry in both bodies and glazes. In low-fire art-ware 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 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 positive carbon footprint of 0.4 tonne per tonne of conventional cement.^[10]

Talc is used in the production of the materials that are widely used in the building interiors such as base content paints in wall coatings. Other areas that use talc to a great extent are organic agriculture, food industry, cosmetics, and hygiene products such as baby powder and 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 pulmonary talcosis, a granulomatous inflammation in the lungs.

Sterile talc powder[edit]

Sterile talc powder (NDC 63256-200-05) is a sclerosing agent used in the procedure of pleurodesis. This can be helpful as a cancer treatment to prevent pleural effusions (an abnormal collection of fluid in the space between the lungs and the thoracic wall). It is inserted into the space via a chest tube, causing it to close up, so fluid cannot collect there. The finished product has been sterilized by gamma irradiation.

Safety[edit]

Talc powder is a household item, sold globally for use in personal hygiene and cosmetics. Suspicions have been raised that its use contributes to certain types of disease, mainly cancers of the ovaries and lungs. It is classified as a group 3 agent in the IARC listing.^[11] Reviews by Cancer Research UK and the American Cancer Society conclude that some studies have found a link, but other studies have not.^[12][13]

The studies discuss pulmonary issues,^[14] lung cancer,^[15][16] and ovarian cancer.^[17] 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 tumor formation in rats forced to inhale talc for 6 hours a day, five days a week over at least 113 weeks.^[15] A 1971 paper found particles of talc embedded in 75% of the ovarian tumors studied.^[18] Research published in 1995 and 2000 concluded that it was plausible that talc could cause ovarian cancer, but no conclusive evidence was shown.^[19][20]

Industrial grade[edit]

In the United States, 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. At levels of 1000 mg/m³, inhalation of talc is considered immediately dangerous to life and health.^[21]

Food grade[edit]

The United States Food and Drug Administration considers talc (magnesium silicate) to be generally recognized as safe (GRAS) for use as an anticaking agent in table salt in concentrations smaller than 2%.^[22]

Asbestos link[edit]

One particular issue with commercial use of talc is its frequent co-location in underground deposits with asbestos ore. Stringent quality control since 1976, including separating cosmetic- and food-grade talc from "industrial" grade talc, has eliminated this issue, but it remains a potential hazard requiring mitigation in the mining and processing of talc.^[23] A 2010 US FDA survey failed to find asbestos in a variety of talc-containing products.^[24]

Litigation[edit]

In 2006 the International Agency for Research on Cancer classified talcum powder as a possible human carcinogen if used in the female genital area. Yet no federal agency in the US acted to remove talcum powder from the market or add warnings.^[25]

In February 2016, as the result of a lawsuit against Johnson & Johnson (J&J), a St. Louis jury awarded $72 million to the family of an Alabama woman who died from ovarian cancer. The family claimed that the use of talcum powder was responsible for her cancer.

In May 2016, a South Dakota woman was awarded $55 million as the result of another lawsuit against J&J.^[26] The woman had used Johnson & Johnson's Baby Powder for more than 35 years before being diagnosed with ovarian cancer in 2011.

In October 2016, a St. Louis jury awarded $70.1 million to a Californian woman with ovarian cancer who had used Johnson's Baby Powder for 45 years.^[27]

In August 2017, a Los Angeles jury awarded $417 million to a Californian woman, Eva Echeverria, who developed ovarian cancer as a "proximate result of the unreasonably dangerous and defective nature of talcum powder", her lawsuit against Johnson & Johnson stated.^[28]

At least 1,200 to 2,000 other talcum powder-related lawsuits are pending.^[27][29]

On October 20, 2017, Los Angeles Superior Court judge Maren Nelson, dismissed the verdict of Eva Echeverria against Johnson & Johnson and its talc-containing consumer products for causing cancer. The judge stated that Eva Echeverria proved there is "an ongoing debate in the scientific and medical community about whether talc more probably than not causes ovarian cancer and thus (gives) rise to a duty to warn", but not enough to sustain the jury’s imposition of liability against Johnson & Johnson stated, and concluded that Echeverria did not adequately establish that talc causes ovarian cancer. ^{[30] [31]}

In July 2018, a court in St. Louis awarded a $4.7bn claim ($4.14bn in punitive damages and $550m in compensatory damages) against J&J, concluding that the company had suppressed evidence of asbestos in its products for more than four decades.^[32]

See also[edit]

• Sillimanite
• Serpentinite

References[edit]

Wikimedia Commons has media related to Talc.

Look up talc in Wiktionary, the free dictionary.