talc

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talc A member of the 2:1 phyllosilicates (sheet silicates) with composition Mg6[Si4O10]2(OH)4; sp. gr. 2.58–2.83; hardness 1 (it has the lowest hardness on Mohs's scale of hardness); monoclinic; rare crystals are tabular, often massive; white to green; cleavage perfect hardness; massive talc (soapstone or steatite) can be formed during the low-grade metamorphism of siliceous dolomites; and as a secondary mineral during hydrothermal alteration of ultrabasic igneous rocks along shear planes. It is associated with serpentization with serpentine changing to talc and magnetite by addition of CO2. It is used extensively as a mineral filler.

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