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Encyclopedia Britannica - Main :: SUS-TAV |
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TALC , a mineral which in its compact forms is known as steatite, or soapstone. It was probably the ayvitres A Wos of Theophrastus, described as a stone of silvery lustre, easily cut. The word talc, sometimes written talk, is said to come from the Arabic talc, and not to be connected, as has been fancifully suggested, with the Swedish talja, " to cut." Talc and mica were confused by the older writers, and even at the present day mica is sometimes known in trade as talc; whilst the term was formerly applied also to foliated gypsum. Talc is found occasionally in small hexagonal and rhombic plates, with perfect basal cleavage, and they are supposed to be monoclinic. Talc 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 r, 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 Katzensilber. The colour is white, grey, yellow or frequently green. The mineral has strong birefringence and a small optic axial angle. Talc is a magnesium silicate H2Mg3Si4O12. It is generally regarded 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, &c., 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 Mount
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; and it was employed for Assyrian cylinder-seals and for other ancient signets. 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 ayaXpa, " an image." Pagodite is an old name for Chinese figure-stone. Ancient steatite carvings are found among the ruins of Rhodesia. 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 alteration 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. Among the uses of steatite may be mentioned its employment, especially in America, for sinks, stoves, firebricks, foot-warmers, tips for gas-burners and electric switchboards: when ground it is used 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 steatite is used by tailors for marking cloth under the name of " French chalk " or " Spanish chalk." 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
Steatite and talc-schists are widely distributed, and have occasion-ally been used as building stones. When first raised the stone is soft, but hardens on exposure. Soapstone from Gudbrandsdal is used in the cathedral of Trondhjem in Norway. Veins of steatite occur in the serpentine of the Lizard district in Cornwall
in the British Isles mention may be made of Crohy Head and Gartan near Letterkenny in co. Donegal, Ireland; the Shetland isles, the Hebrides (Harris) and Shiness in Sutherland
York
Hampshire ; Stockbridge, Windsor co., Vermont; Lynnfield, Massachusetts; near Lafayette, Pennsylvania
A fibrous steatite from New York
See " '1ilr1c and Soapstone " in vol. ii. of Mineral Resources of the U.S. (Washington, 1909), and J. H. Pratt
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