Talc, a mineral composed almost entirely of silica and magnesia, in the proportions of 63.5 silica and 31.7 magnesia, with 4.8 water. Its colours are silvery-white, greenish-white, and green. It has a pearly or semi-metallic lustre, and is unctuous to the touch, in which it differs from mica. It cleaves easily into thin flakes which are transparent and flexible, but not elastic. Its hardness = 1, hence it is readily scratched with the finger-nail. It usually occurs compact and not in good crystalline forms, but belongs either to the rhombic or the monoclinic system. It is also found massive, in beds associated with mica-schists, gneiss, and serpentine. Talc frequently occurs in rocks as an alteration-product, especially of magnesia minerals that contain little or no alumina.
Hence it frequently replaces enstatite, augite, hornblende, &c. It is found in Scotland, Tyrol, the Pyrenees, and various parts of the United States, and is or has been used like Mica (q.v.) for stoves and ovens, for slate-pencils, for surfaces exposed to acids, for stoppers of chemical bottles, &c.—A kind called Indurated Talc, or Talc Slate, has a curved slaty structure, and is not separable into laminae like common talc. It approaches in character to steatite, and is used for similar purposes.