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a variety of white mica, usually muscovite, KAl2[AlSi3O10](OH)2, less commonly, paragonite, NaAl2[AlSi3O10](OH)2. Sericite often contains less potassium (sodium) and more water, SiO2, and MgO and approaches the composition of hydromicas, such as illite and phengite. It is usually encountered as small flakes and scales with a silky luster. As a secondary mineral, sericite is very common in hydrothermally altered igneous and metamorphic rocks, in sericite schists, and in the selvages of ore bodies, where it occurs with quartz, carbonates, chlorite, sulfides, barite, talc, and, less commonly, with fluorite and tourmaline.