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sericite A white variety of muscovite or paragonite and member of the phyllosilicates (sheet silicates) with the formula K2Al4[Si3AlO10]2(OH)2 formed from the alteration of feldspar by either hydrothermal alteration or later-stage weathering. It appears as small scales or flakes or compact aggregates which give the feldspar a ‘cloudy’ appearance in thin section.