A monoclinic mineral, (Mg,Fe,Al)₆(Si,Al)₈O₂₀(OH)₄.8H₂O ; mica group; basal cleavage; soft; pearly; a hydrothermal or weathering alteration of biotite; expands 6 to 20 times by thermal exfoliation; occurs in clay sizes in soils and as crystals and megacrysts in ultramafic rocks; in Montana, North Carolina, South Carolina, Wyoming, Virginia, Colorado, and South Africa.