TALC, a mineral which in its compact forms is known as steatite, or soapstone. It was probably the p.ayviins XiBos of Theophrastus, described as a stone of silvery lustre, easily cut. The name word comes indirectly from the Arabic talq, and is not connected with Swed. tiilja, to cut. It was confused with mica by the older writers, and even now mica is sometimes known in trade as talc ; while the term was formerly applied to foliated gypsum.
Talc is occasionally found in small hexagonal and rhombic plates with perfect basal cleavage, which are supposed to be monoclinic, and often occurs in foliated masses, sometimes with a curved surface, readily separating into thin, very flexible, non elastic laminae. The plates give a six-rayed percussion-figure. Talc has a hardness of only about 1, and a specific gravity of from 2.6 to 2.8. Its extreme softness and its greasy feel are characteristic. The lustre on the cleavage face is pearly, or sometimes silvery, and one of the old names of the mineral was stella terrae, while German writers sometimes called it Katzen silber. The colour is white, grey, yellow or frequently green.
Talc is a magnesium silicate, It is generally re garded as a hydrous silicate, but the water is expelled only at a very strong heat, and may therefore be regarded as basic. By the action of heat the hardness of the mineral is greatly increased. Pseudomorphs are known after actinolite, pyroxene, etc., and the mineral has probably been generally formed by the alteration of ferro-magnesian silicates. Talc occurs chiefly in crystalline schists, usually associated with chlorite, serpentine and dolomite. Fine examples of apple-green colour are found at Mt. Greiner, in the Zillerthal, Tirol. Talc-schist is a foliated rock composed chiefly of talc, generally associated with quartz and felspar; but all soapy schists are not necessarily talcose.
The steatites of Pliny was a stone resembling fat, but other wise undescribed. Being easily cut, steatite has always been a favourite material with the carver : it was used for Egyptian scarabs and other amulets, which were usually coated with a blue vitreous glaze ; it was employed for Assyrian cylinder-seals and for other ancient signets, and ancient steatite carvings are found among the ruins of Rhodesia. By the Chinese steatite is largely used for ornamental carvings ; but many of their "soap stone" figures are wrought in a compact pyrophyllite (q.v.), which is essentially different from talc. The name agalmatolite is often applied to the material of these figures, and was suggested by M. H. Klaproth from the Greek 6,7aXp.a, an image. Pagodite is an old name for Chinese figure-stone.
Steatite is usually a white, grey, greenish or brown substance, occurring in veins or nodular masses or in lenticular bedded deposits. Pseudomorphs after quartz and dolomite occur near
Wunsiedel in Bavaria. In some cases it is a product of the altera tion of pyroxenic rocks, and the commercial mineral may be very impure. The ease with which steatite may be worked, coupled with its power of resisting heat, has led to its employment for vessels for household use, whence it is called "potstone"—the lapis ollaris of old writers; it is also used, especially in America, for sinks, stoves, firebricks, foot-warmers, tips for gas-burners and electric switchboards, and, when ground, as a filler for paper, for leather-dressing, for covering steam-pipes, as an ingredient in soap, for toilet-powder, for certain paints and as a lubricant. A fine granular variety, known as "French chalk" or "Spanish chalk," is used by tailors for marking cloth ; slate pencils are made of steatite and pyrophyllite; and in Burma steatite pencils are used for writing on black paper. In the oxyhydrogen flame, steatite has been fused and drawn out into threads, like quartz fibres.
Steatite- and talc-schists are widely distributed and have oc casionally been used as building stones. When first raised the stone is soft, but hardens on exposure. Soapstone from Gud brandsdal is used in the cathedral of Trondhjem, Norway. Veins of steatite occur in the serpentine of the Lizard district, Cornwall, and the mineral was used under the name of soap rock in the manufacture of the old Worcester porcelain. In North America its distribution is very extensive.
A fibrous steatite from New York State, used in the manufac ture of paper, is known as agalite. Rensselaerite is a wax-like talcose substance, passing into serpentine, from St. Lawrence county, N.Y., named by E. Emmons in 1837 after S. Van Rens selaer, of Albany, N.Y. Beaconite is an asbestiform talc from Michigan, named by L. W. Hubbard. The term pyrallolite was given by Nils G. Nordenskiold to a mineral from Finland, which appears to be talc pseudomorphous after pyroxene. Talcoid was K. F. Naumann's name for a white lamellar mineral from near Pressnitz in Bohemia. A blue earthy mineral from Silver City, New Mexico, known locally as "native ultramarine," is a mag nesium silicate.
See "Talc and Soapstone" in vol. ii. of Mineral Resources of the U.S. (19°9), and J. H. Pratt, "Economic Papers," No. 3, of Geol. Surv. of N. Carolina (two) ; C. H. Smyth, Jr., "The Fibrous Talc Industry of St. Lawrence Co., N.Y.," in Mineral Industry, vol. ix. (19oo) ; G. P. Merrill, Non-metallic Minerals (1904) ; and R. B. Ladoo, "Talc and Soapstone," U.S. Bur. of Mines, Bull. No. 213 (1923).