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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 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 combination of pyramids and domes
.
The faces of the prism-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 long See also: axis of the prism, and the cleavage-See also: 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 See also: 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 ofSee also: weight
.
See also: 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 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 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, blue or See also: brown
.
The pleochroism is fairly marked, the 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 heat, and this treatment has since been extensively applied, so that nearly all the pink topaz occurring in 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 pale
.
The delicate See also: green topaz has been incorrectly called 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 west of England it is found in See also: Cornwall, notably at St Michael's See also: Mount and at Cligga See also: Head near St See also: Agnes
.
It occurs also in 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 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 Nerchinsk, in Siberia
.
A very fine series from the Koksharov collection is in the British Museum
.
Beautiful crystals of topaz are found in See also: Japan, especially at Takayama in the 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 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 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 Pike's See also: Peak in See also: Colorado, and in See also: San Diego county, 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 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 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 Sweden, has been termed pyrophysalite (aup, 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 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 chief 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
.
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