A hydrated aluminium silicate with composition Al₂Si₄O₁₀(OH)₂. The mineral is commonly white, grayish, greenish, or brownish, with a pearly to waxy appearance and greasy feel. It occurs as compact masses, as radiating aggregates (see illus.), and as foliated masses. Pyrophyllite belongs to the layer silicate (phyllosilicate) group of minerals. The atomic (crystal) structure is similar to that of talc, Mg₃Si₄O₁₀(OH)₂, with two of the Mg positions occupied by Al and the third position vacant. The layers are electrically neutral and are held together by weak secondary bonds and long-range ionic (electrostatic) forces. The mineral is soft (hardness 1–1½ on the Mohs scale) and has easy cleavage parallel to the structural layers. The mineral is highly stable to acids. Thermal decomposition begins around 750–800°C (1400–1500°F), and recrystallization to mullite and cristobalite occurs at 1200°C (2200°F) and higher temperatures.