Common clay mineral and important member of the 2:1 group of phyllosilicates (sheet silicates) with the formula K1–1.5 Al4[Si7–6.5 Al1–1.5 O20](OH)4, which possesses an overall negative charge due to incomplete charge balance; sp. gr. 2.6–2.9; hardness 1–2; monoclinic; crystals form tiny flakes; formed by the weathering decomposition or hydrothermal alteration of muscovite or feldspar.