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Formula: Al2SiO4F2
Nesosilicate (insular SiO4 groups)
Crystal System: Orthorhombic
Specific gravity: 3.4 to 3.6 measured
Hardness: 8
Streak: White
Colour: Colourless, white, yellow, blue, green, red, pink, violet, brown
Solubility: Insoluble in hydrochloric and nitric acid; slightly soluble in sulphuric acid
Environments:
Plutonic igneous environments
Pegmatites (typical)
Placer deposits
Metamorphic environments
Basaltic cavities
Hydrothermal environments
Topaz is typically found in acid igneous rocks such as granite,
granite pegmatites
and rhyolite, and their
contact zones (Lauf p191). It is formed by fluorine-bearing vapours given off during the last stages of the
solidification of
siliceous igneous rocks, found in cavities in rhyolite lava,
and pegmatites, where it is
a common and characteristic constituent, associated with beryl.
Topaz is found in some localities as placers,
and also in
contact metamorphic environments, where it is formed by
high-grade metamorphism of
aluminous, quartz-rich and fluorine-bearing sediments, and
associated with tourmaline,
cassiterite, apatite and
fluorite; also beryl,
quartz, mica and
feldspar
(HOM).
Topaz is a common accessory in tin lodes in all parts of the world
(Lauf p191).
Hydrothermal topaz is often associated with intensely altered
granitic rock
in the contact zone between tin-tungsten-molybdenum granites and
rhyolites. In the presence of low pH (acid) fluids,
feldspar and mica alter to form topaz.
Associated minerals are fluorite, muscovite,
quartz,
chlorite and biotite
(AofA).
Topaz can alter to muscovite and
quartz, to
orthoclase or to clay minerals,
and it may replace
feldspar or amphibole
(Lauf p194).
Localities
At the Brumado Mine, Bahia, Brazil, topaz occurs in low-temperature hydrothermal veins (Lauf p194) with
quartz
(Lauf p197).
At the Fazenda Concórdia pegmatite, Mimoso do Sul, Espírito Santo, Brazil, in cavities to 3m across, colourless
to pale blue to grrenish topaz crystals up to 1 kg in weight have been found; some large groups are
overgrown by smoky quartz crystals. Huge
quartz - microcline -
topaz specimens to 350 kg have been recovered. Topaz crystals with overgrowths of
short-prismatic schorl crystals are also known. The Concordia mine
also produced a few extraordinary crystals of golden topaz, including one 9 cm specimen with
a flawless gem-quality crystal of golden topaz attached to a
smoky quartz crystal
(Minrec 54.738-740).
At San Shek Wan, Lantau Island, Islands District, New Territories, Hong Kong, China, it is necessary to visit
the site at
low tide, when the outcrop on the beach is exposed. The outcrop is a sequence of metamorphosed
sandstone, metamorphosed
siltstone and
granite,
and a small skarn body was also identified with abundant
magnetite and quartz veins.
Little red
crystals of garnet were found in the
magnetite, and there were also some yellow columnar crystals of
topaz within the quartz veins
(Geological Society of Hong Kong newsletter 16.1.2, Mineralogy Society of Hong Kong field trip).
At Devil's Peak, Sai Kung District, New Territories, Hong Kong, China, the mineralisation occurred in
quartz veins in the contact zone between a
granite intrusion and acid volcanic rocks. The mine is now closed,
and inaccessible for collecting.
Topaz occurred as crystals up to 2 cm in length in quartz veins with
beryl, fluorite,
muscovite and wolframite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Topaz is found in quartz-mica veins
in the granite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Needle Hill Mine, Needle Hill, Sha Tin District, New Territories, Hong Kong, China, is a tungsten mine, abandoned
in 1967. The principal ore is wolframite, and the principal gangue mineral is
quartz. Molybdenum also occurs. The
mineralisation consists of a series of parallel fissure veins that cut through
granite. Wolframite and
quartz are the main minerals, but galena,
sphalerite, pyrite,
molybdenite and fluorite have also
been found here
(Geological Society of Hong Kong Newsletter 9.3.29-40).
The quartz-wolframite veins are of
high-temperature hydrothermal formation,
and grade into wolframite-bearing
pegmatites.
Topaz is reported to occur in greisenised
granite bordering the
wolframite-bearing quartz veins and
pegmatites
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Lin Fa Shan deposit, Tsuen Wan District, New Territories, Hong Kong, China, is located in a remote area of the Tai Mo Shan
Country Park, on a steep west facing slope of Lin Fa Shan, just above the abandoned village of Sheung Tong. The
surrounding hillsides are covered with shallow excavations, representing past searches for
wolframite, the natural ore of
tungsten. The abandoned workings are extremely dangerous with unsupported tunnels, open shafts and no maintenance since
their closures in 1957; the workings should not be entered
(http://industrialhistoryhk.org/lin-shan).
Topaz is sometimes found associated with
muscovite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
At the Yaogangxian Mine, Yaogangxian W-Sn ore field, Yizhang county, Chenzhou, Hunan, China, topaz is associated
with fluorite and quartz in a
greisen-skarn deposit
(R&M 80.1.56).
At the Ore Mountains (Erzgebirge; Krušné hory), Bohemia, Czech Republic, topaz occurs in
granite
(Lauf p194).
At Schneckenstein, Tannenbergsthal, Muldenhammer, Vogtlandkreis, Saxony, Germany, a topaz
pseudomorph
after orthoclase with Carlsbad twinning has been found
(KL p221).
At Erongo, Namibia, topaz is associated with quartz,
orthoclase, fluorite and
schorl, and sometimes inclusions of schorl are found in the topaz.
At Klein Spitzkoppe, Namibia, topaz is associated
with microcline and beryl variety
aquamarine, or with
biotite,
quartz, fluorite,
tourmaline and rutile.
The fine colourless crystals of topaz, often over 10 cm in length, are found in
pegmatites in alkali
granite. Yellowish and, rarely, pale blue crystals are also found
(Minrec 36.4.328-333).
In the Joss plateau, Nigeria, topaz occurs in tin-bearing granite
(Lauf p194).
At the Mardan District, Khyber Pakhtunkhwa Province, Pakistan, topaz occurs with
quartz in
calcite veins in calcareous rocks
(Lauf p192).
At the Adun-Cholon Range, Nerchinsky District, Zabaykalsky Krai, Russia, topaz crystals as big as 4kg have
been found.
At the Verkhne-Talskoye occurrence, Krasnoyarsk Krai, Russia, topaz has been found in
gold-bearing river placers
(GAHK 2019 p15).
At Khoroshiv Raion (Volodarsk-Volynskii), Zhytomyr Oblast, Ukraine, an initial crystallisation occurred at
360oC to 540oC, followed
by a secondary crystallisation at 180oC to 350oC. Very few pockets here contain both
beryl and topaz. Beryl crystallises first, at
a higher temperature, and as
the temperature falls the solution becomes more acid. By the time the topaz starts to form the acidity is high enough
to start to dissolve the beryl.
Inclusions of fluorite in the topaz are quite common, also inclusions of
quartz, feldspar,
albite, columbite and elpasolite
K2NaAlF6 have
been reported. Muscovite variety sericite
pseudomorphs after topaz have
been found here.
The largest topaz crystal from this locality found so far was a massive 117kg
(MinRec 40.6.498-502).
In Cornwall, England, UK, topaz is a common associate of cassiterite and
wolframite both in granite and in
sedimentary rocks
(Lauf p191).
At Devils Head, Pikes Peak, Colorado, USA, topaz occurs in pegmatite cavities associated with
quartz, microcline,
albite, fluorite and
cassiterite
(Lauf p193).
At the SK Star #2 Mining Claim, The Cove, Topaz Mountain, Thomas Range, Juab County, Utah, USA, topaz occurs
in rhyolite. The topaz is typically black in colour because of
numerous dark inclusions, but sherry-coloured, grey and colourless topaz crystals are also occasionally found.
No other occurrences of black topaz are known in the Thomas Range. Analysis of thin sections of inclusions in
the topaz indicates the presence of tungsten-rich
ixiolite, magnetite,
fluorite and fluorannite.
Calcium-fluorine-rich fluid inclusions were also observed.
Topaz has been found as radiating groups of sherry-coloured crystals a few hundred feet up the hillside from the black
topaz occurrence. These topaz clusters contain an unusually high amount of
titanium
(MinRec 51.6.809-810).
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