A white, fine-grained potassium mica occurring in small scales as an alteration product of various aluminosilicate minerals, having a silky luster, and found in various metamorphic rocks (esp. in schists and phyllites) or in the wall rocks, fault gouge, and vein fillings of many ore deposits. It is commonly muscovite or very close to muscovite in composition, but may also include paragonite and illite.