Topaz

cassiterite

Formula: Al2SiO4F2 nesosilicate (insular SiO4 groups)
Specific gravity: 3.5 to 3.6
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 (characteristic)
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).

At the Brumado Mine, Brazil, topaz occurs in low-temperature hydrothermal veins (Lauf p194) with quartz (Lauf p197).

At Yaogangxian, Hunan, China, topaz is associated with fluorite and quartz.

At the Krusne hory Mountains, Bohemia, Czech Republic, topaz occurs in granite (Lauf p194).

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 aquamarine (variety of beryl), or with biotite, quartz, fluorite, tourmaline and rutile.

In the Joss plateau, Nigeria, topaz occurs in tin-bearing granite (Lauf p194).

At Mardan, northwest Pakistan, topaz occurs with quartz in calcite veins in calcareous rocks (Lauf p192).

At Adun Chilon, Russia, topaz crystals as big as 4kg have been found.

At Volodarsk-Volynski, 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. Sericite (variety of muscovite) 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).

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