Pyrophyllite is a phyllosilicate mineral species belonging to the clay family and composed of aluminium silicate hydroxide: AlSi2O5OH. It occurs in two more or less distinct varieties, namely, as crystalline folia and as compact masses; distinct crystals are not known.
The folia have a pronounced pearly lustre, 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 before the blowpipe, exfoliates and swells up to many times its original volume, hence the name pyrophyllite, from the Greek irip (fire) and ??? (a leaf), given by R. Hermann in 1829. The color of both varieties is white, pale green, greyish or yellowish; they are very soft (hardness of 1 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. The compact variety of pyrophyllite is used for slate pencils and tailors chalk (French chalk), and is carved by the Chinese into small images and ornaments of various kinds. Other soft compact minerals (steatite and pinite) used for these Chinese carvings are included with pyrophyllite under the terms agalmatolite and pagodite.
Pyrophyllite occurs in phyllite and schistose rocks, often associated with kyanite, of which it is an alteration product. Pale green foliated masses, very like talc in appearance, are found at Beresovsk near Ekaterinburg in the Urals, and at Zermatt in Switzerland. The most extensive deposits are in the Deep River region of North Carolina, where the compact variety is mined, and in South Carolina and Georgia. Major deposits of pyrophyllite occur within region of Ottosdal, South Africa, where it is mined for the production of a variety of manufactured goods and blocks are quarried and marketed as "Wonderstone" for the carving of sculptures. In Australia, pyrophyllite has been mined at three sites near Pambula on the Sapphire Coast of NSW.
It is added to clay to reduce thermal expansion when firing but it has many other industry uses when combined with other compounds, such as in insecticide and for making bricks.