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Encyclopedia Britannica - Main :: TOO-TUM |
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TOPAZ , a mineral usually found in connexion with granitic rocks and used, when fine, as a gem-stone. It is believed that the topaz of modern mineralogists was unknown to the ancients, and that the stone described under the name of ro1r6. ios, in allusion to its occurrence on an island in the Red Sea known as roirb. .os vi)o-os, was the mineral which is now termed chrysolite or peridot (q.v.). The Hebrew pitdah, translated " topaz " in the Old Testament, may also have been the chrysolite. Topaz crystallizes in the orthorhombic system, usually with a prismatic habit (figs. 1 and 2). Many of the crystals, like those from Saxony and Siberia, are rich in faces, and present with the prisms a complicated combination of pyramids and domes. The faces of the prism-zone are usually striated vertically. Doubly-terminated crystals are rare, and sometimes apparently hemimorphic. The mineral presents a perfect cleavage transverse to the long axis of the prism, and the cleavage-plane often has a pearly lustre. The chemical composition of the topaz has given rise to much discussion, but it is now generally regarded as an aluminium fluo-silicate having the formula Al2F2SiO4. It was shown by Professor S. L. Penfield and Mr J. C. Minor that the fluorine may be partially replaced by hydroxyl. When strongly heated topaz suffers considerable loss of weight. Sir D. Brewster found in topaz numerous microscopic cavities containing fluids, some of which have received the names of brewsterlinite and cryptolinite. Possibly some of the liquid inclusions may be hydrocarbons.The topaz, when pure, may be colourless, and if cut as a brilliant has been mistaken for diamond. It has, too, the same specific gravity, about 3.5. It is, however, greatly inferior in hardness, the hardness of topaz being only 8; and it has lower refractivity and dispersive powers: moreover, being an orthorhombic mineral, it possesses double refraction. From phenacite and from rock-crystal, for which it may be mistaken, it is distinguished by being biaxial and by having a much higher specific gravity. The topaz becomes electric by heating, by friction or by pressure. Colourless limpid topazes are known in Brazil as pingos d'agoa, or " drops of water," whilst in England they pass in trade as " minas novas," from a locality in the state of Minas Geraes in Brazil. Coloured topazes usually present various shades of yellow, blue or brown. The pleochroism is fairly marked, the colour of the sherry-yellow crystals from Brazil being generally resolved by thedichroscope into a brownish-yellow and a rose- pink
pink
The topaz is cut on a leaden wheel, and polished with tripoli
Topaz usually occurs in granitic and gneissose rocks, often in greisen, and is commonly associated with cassiterite, tourmaline and beryl. It seems to have been formed, in many cases, by pneumatolytic action. In the west of England it is found in Cornwall
Mount
capital of Minas Geraes, where they occur in a kaolinitic matrix, resulting from the alteration of a mica-schist, which is regarded by Professor O. A. Derby as a metamorphosed igneous rock. Topaz occurs in the tin-drifts of New South Wales, especially in the New England district; it has been discovered in the Coolgardie goldfield, West Australia; and it is found also in the tinfields of Tasmania and on Flinders Island in Bass's Strait. Fine topaz has been worked near Pike's Peak in Colorado, and in San Diego county, California. The mineral occurs in rhyolite at Nathrop in Chaffee county and Chalk Mountain in Summit county, Colorado, and in trachyte near Sevier Lake, Utah. The occurrence of topaz in these volcanic rocks is very notable, and contrasts with its common occurrence in granites. It is found in like manner in rhyolite at San Luis Potosi
" Oriental topaz " is the name sometimes given to yellow corundum, a mineral readily distinguished from true topaz by superior hardness and density. Yellow and smoke-tinted quartz, or cairngorm, is often known as " Scotch topaz " or " Spanish topaz," according to its locality; but these, on the contrary, are inferior in hardness and density. The chief
Scotch True Oriental Topaz. Topaz. Topaz. Hardness . . 7 8 9 Specific gravity . . 2.6 3'5 4 Refractive indices 1.54, I.55 I.61, I.62 I.76, I.77 Crystallization Hexagonal Orthorhombic Hexagonal Chemical composition SiO2 Al2F2SiO4 Al2Os (F. W. R.'') End of Article: TOPAZ If you wish, you can link directly to this article.
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