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TOPAZ

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Originally appearing in Volume V27, Page 48 of the 1911 Encyclopedia Britannica.
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TOPAZ , a See also:

mineral usually found in connexion with granitic rocks and used, when See also:fine, as a See also:gem-See also:stone. It is believed that the topaz of See also: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 See also:island in the Red See also:Sea known as roirb. .os vi)o-os, was the mineral which is now termed See also:chrysolite or See also:peridot (q.v.). The See also:Hebrew pitdah, translated " topaz " in the Old Testament, may also have been the chrysolite. Topaz crystallizes in the orthorhombic See also:system, usually with a prismatic See also:habit (See also:figs. 1 and 2). Many of the crystals, like those from See also:Saxony and See also:Siberia, are See also:rich in faces, and See also:present with the prisms a complicated See also:combination of pyramids and domes. The faces of the See also:prism-See also:zone are usually striated vertically. Doubly-terminated crystals are rare, and sometimes apparently See also:hemimorphic. The mineral presents a perfect cleavage transverse to the See also:long See also:axis of the prism, and the cleavage-See also:plane often has a pearly lustre. The chemical See also:composition of the topaz has given rise to much discussion, but it is now generally regarded as an See also:aluminium fluo-silicate having the See also:formula Al2F2SiO4. It was shown by See also:Professor S.

L. Penfield and Mr J. C. See also:

Minor that the See also:fluorine may be partially replaced by hydroxyl. When strongly heated topaz suffers considerable loss of See also:weight. See also:Sir D. See also: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 See also:hydrocarbons. The topaz, when pure, may be colourless, and if cut as a brilliant has been mistaken for See also: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 See also:lower refractivity and dispersive See also:powers: moreover, being an orthorhombic mineral, it possesses See also:double See also:refraction. From See also:phenacite and from See also: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 See also:

heating, by See also:friction or by pressure. Colourless limpid topazes are known in See also:Brazil as pingos d'agoa, or " drops of See also:water," whilst in See also:England they pass in See also:trade as " See also:minas novas," from a locality in the See also:state of Minas Geraes in Brazil. Coloured topazes usually present various shades of yellow, See also:blue or See also:brown. The pleochroism is fairly marked, the See also:colour of the See also:sherry-yellow crystals from Brazil being generally resolved by thedichroscope into a brownish-yellow and a See also:rose-See also:pink. The colour in many cases is unstable, and the brown topazes of Siberia are specially liable to suffer See also:bleaching by exposure to sunlight. In 1750 a Parisian jeweller named Dumelle discovered that the yellow Brazilian topaz becomes pink on exposure to a moderate See also:heat, and this treatment has since been extensively applied, so that nearly all the pink topaz occurring in See also:jewelry has been artificially heated. Such " burnt topaz " is often known as " Brazilian See also:ruby," a name applied also to the natural red topaz, which, however, is excessively rare. " Brazilian See also:sapphire " is the See also:term sometimes given to blue topaz, but the colour is usually See also:pale. The delicate See also:green topaz has been incorrectly called See also:aquamarine, which is a name applicable only to the sea-green See also:beryl (q.v.). According to A. K. Coomfiraswamy, yellow sapphire is often sold as topaz in See also:Ceylon, where yellow topaz is unknown, whilst pink See also:corundum is frequently called there " See also:king topaz." The topaz is cut on a leaden See also:wheel, and polished with See also:tripoli.

It is generally step-cut, or table-cut, but its beauty is best See also:

developed when in the See also:form of a brilliant. Cut topazes of large See also:size are known, and it is said that the See also:great " See also:Braganza diamond " of See also:Portugal is probably a topaz. Topaz usually occurs in granitic and gneissose rocks, often in See also:greisen, and is commonly associated with cassiterite, See also:tourmaline and beryl. It seems to have been formed, in many cases, by pneumatolytic See also:action. In the See also:west of England it is found in See also:Cornwall, notably at St See also:Michael's See also:Mount and at Cligga See also:Head near St See also:Agnes. It occurs also in See also:Lundy Island. The finest See also:British topaz is found in the See also:Cairngorm See also:group of mountains in the central See also:Highlands, especially at See also:Ben a Buird. Rolled pebbles occur in the See also:bed of the See also:Avon in See also:Banffshire. Beautiful, though small, crystals occur in the drusy cavities of the See also:granite of the Mourne Mountains in See also:Ireland. The famous topaz-rock of the Schneckenstein, near See also:Auerbach, in Saxony, yields pale yellow crystals, formerly cut for jewelry, and it is said that these do not become pink on heating. Fine topazes occur in See also:Russia, at several localities in the Urals and in the Adun-chalon Mountains, near See also:Nerchinsk, in Siberia. A very fine See also:series from the See also:Koksharov collection is in the British Museum.

Beautiful crystals of topaz are found in See also:

Japan, especially at Takayama in the See also:province of Mino, and at Tanokamiyama in Omi province. Ceylon and See also:Burma occasionally yield topazes. Brazil is a famous locality, the well-known sherry-yellow crystals coming from Ouro Preto, formerly called See also:Villa Rica, the See also:capital of Minas Geraes, where they occur in a kaolinitic See also:matrix, resulting from the alteration of a See also:mica-schist, which is regarded by Professor O. A. See also:Derby as a metamorphosed igneous rock. Topaz occurs in the See also:tin-drifts of New See also:South See also:Wales, especially in the New England See also:district; it has been discovered in the See also:Coolgardie See also:goldfield, West See also:Australia; and it is found also in the tinfields of See also:Tasmania and on See also:Flinders Island in See also:Bass's Strait. Fine topaz has been worked near See also:Pike's See also:Peak in See also:Colorado, and in See also:San Diego See also:county, See also:California. The mineral occurs in See also:rhyolite at Nathrop in See also:Chaffee county and See also:Chalk See also:Mountain in See also:Summit county, Colorado, and in See also:trachyte near See also:Sevier See also:Lake, See also:Utah. The occurrence of topaz in these volcanic rocks is very notable, and contrasts with its See also:common occurrence in granites. It is found in like manner in rhyolite at San Luis See also:Potosi in See also:Mexico; and beautiful little limpid crystals accompany stream-tin at See also:Durango. Common topaz occurs in coarse crystals at many localities. A columnar variety from the tin-districts of Saxony and Bohemia, and from Mt Bischoff in Tasmania, is known as pycnite (amcvbs, dense) ; whilst a coarse opaque topaz from granite near See also:Falun, in See also:Sweden, has been termed pyrophysalite (aup, See also:fire; ¢uvaw, to See also:blow), in allusion to its behaviour when heated.

" See also:

Oriental topaz " is the name sometimes given to yellow corundum, a mineral readily distinguished from true topaz by See also:superior hardness and See also:density. Yellow and See also:smoke-tinted See also:quartz, or cairngorm, is often known as " Scotch topaz " or " See also:Spanish topaz," according to its locality; but these, on the contrary, are inferior in hardness and density. The See also:chief See also:differences between the three minerals may be seen in the following table, in which they are arranged in See also:order of hardness, density and refractivity: 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 See also:Crystallization Hexagonal Orthorhombic Hexagonal Chemical composition SiO2 Al2F2SiO4 Al2Os (F. W.

End of Article: TOPAZ

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