Pyrophyllite

Pyrophyllite is a phyllosilicate composed of aluminium hydroxide : Al2Si4O10(OH)2. It occurs in two forms (habits ): crystalline folia and compact masses; distinct crystals are not known.

The folia have a pronounced pearly luster , owing to the presence of a perfect cleavage parallel to their surfaces: they are flexible but not elastic, and are usually arranged radially in fan-like or spherical groups. This variety, when heated , exfoliates and swells up to many times its original volume, hence the name pyrophyllite, from the Greek pyros (fire)[clarify in 1829. The color of both varieties is white, pale green, greyish or yellowish; they are very soft (hardness of 1.0 to 1.5) and are greasy to the touch. The specific gravity is 2.65 - 2.85. The two varieties are thus very similar to talc.