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pyrophyllite Uncommon silicate mineral Al2[Si4O10](OH)2 belonging to the phyllosilicates (sheet silicates) and with properties similar to muscovite; sp. gr. 2.65–2.90; hardness 1–2; occurs as a secondary product from the hydrothermal alteration of feldspar, and as foliated masses (see FOLIATION) in metamorphic schists. It has been mined as a substitute for talc.