A monoclinic mineral, KAl₂(Si₃Al)O₁₀(OH,F)₂; mica group; pseudohexagonal; perfect basal cleavage; forms large, transparent, strong, electrically and thermally insulating, stable sheets; a common rock-forming mineral in silicic plutonic rocks, mica schists, gneisses, and commercially in pegmatites; also a hydrothermal and weathering product of feldspar and in detrital sediments. Also spelled: moscovite. Sericite is fine-grained muscovite, commonly in connection with hydrothermal alteration, but sericite also includes paragonite and illite. Illite is a common syn. for fine-grained muscovite in clay mineralogy