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Schorl is the commonest member of the tourmaline group.
Formula: NaFe2+3Al6(Si6O18)
(BO3)3(OH)3(OH)
cyclosilicate (ring silicate), borosilicate
Crystal System: Trigonal
Specific gravity: 3.18 to 3.22 measured, 3.244 calculated
Hardness: 7
Streak: Brown Greyish-white to bluish-white.
Colour: Bluish-black to black, sometimes brownish-black, rarely greenish-black.
Common impurities: Mn,Mg,Ca,Li,Cr,Ti,F,K
Environments (tourmaline):
Plutonic igneous environments
Pegmatites best known
Metamorphic environments
Hydrothermal environments
Tourmaline is found in granite and
granite pegmatites, in metamorphic rocks and as a
primary mineral in
hypothermal (high temperature) hydrothermal
veins.
Localities
In the Erongo region, Namibia, schorl is found in small cavities in granite,
associated with quartz, topaz,
fluorite, apatite and
cassiterite
(Journal of the Gemmological Association of Hong Kong 2018 Vol XXXIX p 75). Schorl
pseudomorphs after beryl have been
found here
(KL p227).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, schorl to 45 cm is abundant in the wall zone. In
the interior
zones schorl is partially or completely replaced by muscovite. The
Emmons pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.515).
At the PC Mine, Cataract Mining District, Jefferson county, Montana, USA, schorl occurred as inclusions in
quartz and as fine needles
(R&M 96.6.494).
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