Talc, a silicious mineral occurring in two forms, crystalline and massive. The massive variety was formerly called steatite (soapstone), and was regarded as a distinct species; but it has the same chemical composition as crystalline talc, viz., 4MgO,5Si024/3 H20, or silica 62.14, magnesia 32.92, and water 4.94 per cent., being a hydrous silicate of magnesia. Talc is commonly in the form of lamellar crystals, which cleave perfectly in one direction, but are usually too incompletely developed to allow of an exact determination of their crystalline system; but hexagonal prisms and plates are occasionally found. They are optically biaxial. The lamellae are very flexible but not elastic, and are unctuous to the touch. Talc is very soft, being only 1 on the scale of 10; sp. gr. from 2.6 to 2.8; lustre pearly; color apple-green to white, also greenish gray to dark green, subtranslucent. Talc, either in a foliated or lamellar form, or massive as steatite, is common, forming extensive beds in some localities. It is often associated with serpentine, talcose, or chloritic schist.

Steatite or soapstone is known in different localities as potstone (Topfstein), lapis ollaris, rensselaerite, and French chalk (craie de Briangon). There are extensive quarries at Grafton, Athens, Westfield, and Marlborough, Vt.; at Francestown, Pelham, and Keene, N. H.; at Middlefield, Chester, and other places in Massachusetts; in Maryland near Baltimore, in Virginia near Washington and in Loudon co., and in Guilford co., N. C, and numerous other places, in metamorphic rocks. It is used for making stoves, ovens, and hearths; for sizing-rollers in cotton factories, on account of its not being affected by acids; and sometimes as a lubricant for journals. It is also used for slate pencils and crayons, and for the stoppers of chemical vessels. The American aborigines use it for culinary articles, and the Chinese for the carving of idols.