Talc (derived from the Persian via Arabic talq) 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, its monoclinic crystals being so rare as to be almost unknown. It has a perfect basal cleavage, and the folia are non-elastic, although slightly flexible. It is sectile and very soft, with a 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 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.
Talc is a 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 carbonates.
Talc is primarily formed via hydration and carbonation of serpentine, via the following reaction;
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;
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 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.
Talc carbonated 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.
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 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.
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.