Pyrophyllite

Pyrophyllite is a hydrous aluminum silicate mineral with the approximate formula Al2Si4O10(OH)2. It is a member of the phyllosilicate (sheet silicate) group and is closely related to talc, but with aluminum substituting for magnesium. It forms in low-grade metamorphic rocks and in hydrothermal alteration zones where aluminous rocks interact with silica-rich fluids.

Crystallography and structure: Pyrophyllite occurs in microcrystalline to platy forms with perfect cleavage along the sheets, yielding a pearly to silky luster on cleavage surfaces. It is dioctahedral and structurally similar to talc, but its aluminum-rich octahedral sheets give it different chemical properties.

Physical properties: It has a soft, pearly mineral character with color typically white to gray, pale yellow, or greenish. Mohs hardness is about 1.5–2, and the specific gravity is around 2.9–3.0. It leaves a white streak and shows perfect basal cleavage with a pearly to silky luster.

Occurrence: Pyrophyllite forms in low-grade metamorphic rocks such as slate, phyllite, and schist, as well as in hydrothermal veins that alter aluminous rocks. It is commonly found with quartz, muscovite, chlorite, and talc, and occurs in deposits worldwide.

Uses: As an industrial mineral, pyrophyllite is used as a filler and extender in paints, plastics, and ceramics, and as a refractory material in certain kiln linings. It can also be ground for use in cosmetics and as a decorative rock when suitable.