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Formula: KFe2+3(Si3Al)O10sF2
Phyllosilicate (sheet silicate), biotite group, F may be replaced by OH,
leading to annite
Crystal System: Monoclinic
Specific gravity: 3.16 to 3.20 measured, 3.23 calculated
Hardness: 3
Streak: Grey
Colour: Iron-black
Luminescence: Not fluorescent
Environments
Localities
At the type locality, the Western suburb, Suzhou City, Suzhou, Jiangsu, China, fluorannite occurs as euhedral to
subhedral sheets and tabular crystals to more than 6 mm across, but predominantly 2 to 4 mm long and 1 to 3 mm wide.
Iron-black colour, submetallic lustre, with a grey streak. The mineral is a rock-forming species in the upper part of
an A-type granite (AM 86.1534-1537). Associated Minerals include quartz,
zircon, fluorite and
magnetite
(Mindat).
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.
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. The fluorannite occurs with
fluorite and zircon in
magnesium-deficient igneous rocks
(MinRec 51.6.809-810).
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