Talc
Talc, or talcum, is a clay mineral composed of hydrated magnesium silicate, with the chemical formula. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent and lubricant. It is an ingredient in ceramics, paints, and roofing material. It is a main ingredient in many cosmetics. It occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. It has a perfect basal cleavage and an uneven flat fracture, and it is foliated with a two-dimensional platy form.
The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 1 as the hardness of talc, the softest mineral. When scraped on a streak plate, talc produces a white streak, though this indicator is of little importance, because most silicate minerals produce a white streak. Talc is translucent to opaque, with colors ranging from whitish grey to green with a vitreous and pearly luster. Talc is not soluble in water, and is slightly soluble in dilute mineral acids.
Soapstone is a metamorphic rock composed predominantly of talc.
Etymology
The word talc derives from tālk. In ancient times, the word was used for various related minerals, including talc, mica, and selenite.Formation
Talc dominantly forms 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:
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:
Talc can also be formed from magnesium chlorite and quartz in blueschist and eclogite metamorphism by the following metamorphic reaction:
Talc is also found as a diagenetic mineral in sedimentary rocks where it can form from the transformation of metastable hydrated magnesium-clay precursors such as kerolite, sepiolite, or stevensite that can precipitate from marine and lake water in certain conditions.
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. The crystal structure of talc is described as TOT, meaning that it is composed of parallel TOT layers weakly bonded to each other by weak van der Waals forces. The TOT layers in turn consist of two tetrahedral sheets strongly bonded to the two faces of a single trioctahedral sheet. It is the weak bonding between TOT layers that gives talc its perfect basal cleavage and softness.
The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion is available to bond with the trioctahedral sheet.
The trioctahedral sheet has the structure of a sheet of the mineral brucite. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite sheet, bonding the tetrahedral sheets tightly to the trioctahedral sheet.
Tetrahedral sheets have a negative charge, since their bulk composition is. The trioctahedral sheet has an equal positive charge, since its bulk composition is The combined TOT layer thus is electrically neutral.
Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic or triclinic symmetry. However, the original hexahedral symmetry is discernible in the pseudotrigonal character of talc crystals.
Occurrence
Talc is a common metamorphic mineral in metamorphic belts that contain ultramafic rocks, such as soapstone, 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 2.2 million tonnes, which accounts for 30% of total global output. The other major producers are Brazil, India, the U.S., France, Finland, Italy, Russia, Canada, and Austria.
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
Extraction in disputed areas of Nangarhar province, Afghanistan, has led the international monitoring group Global Witness to declare talc a conflict resource, as the profits are used to fund armed confrontation between the Taliban and Islamic State.Uses
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, etc. It is often used for surfaces of laboratory table tops and electrical switchboards because of its resistance to heat, electricity, and acids.In finely ground form, talc finds use as a cosmetic, 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. Talcum powder, 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 avoid using baby powder because it poses a risk of respiratory problems, including breathing trouble and serious lung damage if inhaled. The small size of the particles makes it difficult to keep them out of the air while applying the powder. Zinc oxide-based ointments may offer safer and more alternative for prevention of diaper rash.
Soapstone is often used as a marker 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 may be used in the processing of white rice as a buffing agent in the polishing stage.
Because of its low shear strength, talc is one of the oldest known solid lubricants. It also has some limited use as a friction-reducing additive in lubricating oils.
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. 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
| 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, 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 tonnes per tonne of conventional cement.
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
Sterile talc powder is a sclerosing agent used in the procedure of pleurodesis. This can be helpful as a cancer treatment to prevent pleural effusions. It is inserted into the space via a chest tube, causing it to close up, so fluid cannot collect there. The product can be sterilized by dry heat, ethylene oxide, or gamma irradiation.Safety
Suspicions have been raised that talc use contributes to certain types of disease, mainly cancers of the ovaries and lungs. According to the IARC, talc containing asbestos is classified as a group 1 agent, talc use in the perineum is classified as group 2B, and talc not containing asbestos is classified as group 2A. Reviews by Cancer Research UK and the American Cancer Society conclude that some studies have found a link, but other studies have not.The studies discuss pulmonary issues, lung cancer, and ovarian cancer. 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. A 1971 paper found particles of talc embedded in 75% of the ovarian tumors studied. In 2018, Health Canada issued a warning against inhaling talcum powder or women's using it perineally.
In contrast, however, research published in 1995 and 2000 concluded that, although it was plausible that talc could cause ovarian cancer, no conclusive evidence had been shown. Further, a 2008 European Journal of Cancer Prevention review of ovarian cancer and talc use studies pointed out that, although many of them examined the duration, frequency, and accumulation of hygienic talc use, few found a positive association among these factors and some found a negative one: "It may be argued that the overall null findings associated with talc-dusted diaphragms and condom use is more convincing evidence for a lack of a carcinogenic effect, especially given the lack of an established correlation between perineal dusting frequency and ovarian tissue talc concentrations and the lack of a consistent dose-response relationship with ovarian cancer risk." Instead, the authors credited powdered talc with "a high degree of safety."
Similarly, in a 2014 article published in a leading cancer journal, the Journal of the National Cancer Institute, researchers reported the results of a survey of 61,576 postmenopausal women, more than half of whom had used talc powder perineally. The researchers compared the subjects' reports of their own talc use with their reports of having had ovarian cancer diagnosed by their doctors, and found, regardless of subjects' age and tubal ligation status, "Ever use of perineal powder... was not associated with risk of ovarian cancer compared with never use," nor was any greater individual cancer risk associated with longer use of talc powder. On this basis, the article concluded, "perineal powder use does not appear to influence ovarian cancer risk." The Cosmetic Ingredient Review Expert Panel concluded in 2015 that talc, in the concentrations currently used in cosmetics, is safe.
In July 2024, the International Agency for Research on Cancer listed talc as "probably" carcinogenic for humans. The study is based on limited evidence it could cause ovarian cancer in humans.