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Member of the 2:1 phyllosilicates (sheet silicates) with composition (Mg,Ca)(Mg,Fe2+)5(Al,Fe3+) [(Si,Al)8O20](OH)4. 8H2O and closely related to the smectite clay minerals; sp. gr. about 2.3; hardness about 1.5; monoclinic; yellow or brown; pearly lustre; crystals flat and platy; cleavage perfect basal {001}; occurs as an alteration product of biotite, from the hydrothermal alteration of biotite and phlogopite, and from the alteration of ultrabasic rocks; expands greatly on heating. It is used extensively for insulation and as a lubricant. See also clay minerals.

Subjects: Earth Sciences and Geography.

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