Talc

Article By:

Liou, Juhn G. Geological and Environmental Sciences, Stanford University, Stanford, California.

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.676800

A hydrated magnesium sheet silicate (phyllosilicate) mineral with a composition close to Mg₃Si₄O₁₀(OH)₂. Rather limited substitution of aluminum (Al) or titanium (Ti) for silicon (Si) and of iron (Fe), manganese (Mn), or aluminum for magnesium (Mg) is possible. Steatite is a massive talc aggregate that contains less than 1.5% calcium oxide (CaO), 1.5% iron(II) oxide + iron(III) oxide (FeO + Fe₂O₃) and 4% aluminum oxide (Al₂O₃). Talc commonly is white, but it may appear pale green or grayish, depending on the amount of minor impurities (see illustration). It typically occurs in coarse to fine platy or fibrous aggregates that have a more or less parallel arrangement. Shreds and plates are commonly bent; some tabular crystals exhibit perfect basal cleavage, yielding flexible, slightly elastic lamellae. Talc has a greasy feel and pearly luster and it is one of the hardness standards for rock-forming minerals, with the value 1 on the Mohs scale. Because talc is soft, it can be scratched by fingernails.

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