1911 Encyclopædia Britannica/Pyrophyllite

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PYROPHYLLITE, a mineral species belonging to the clay family, and composed of hydrous aluminium silicate HAl (SiO3)2. 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 πῦρ (fire) and φύλλον (a leaf), given by R. Hermann in 1829. The colour of both varieties is white, pale green, greyish or yellowish; they are very soft (H. = 1–2) and are greasy to the touch. The specific gravity is 2.8–2.9. The two varieties are thus very similar respectively to talc (q.v.) and its compact variety steatite, which is, however, a hydrous magnesium silicate. 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 schistose rocks, often associated with cyanite, 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.