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phyllosilicate (sheet silicate, layered silicate) A large group of silicate minerals which are characterized by possessing layers of [SiO4]4− tetrahedra linked together to form a flat sheet with the composition [Si4O10]n. This group includes micas, chlorite, clays, talc, and serpentine, which are soft minerals and of variable but generally low density. They form at low temperatures and some, particularly the clays, replace primary minerals as a result of hydrothermal alteration or weathering. They are an essential constituent of argillaceous sedimentary rocks and some lowgrade metamorphic rocks.