Schorl is the commonest member of the tourmaline group.
Formula: NaFe2+3Al6(Si6O18) (BOSub>3)3(OH)3(OH) cyclosilicate (ring silicate)
Specific gravity: 3.18 to 3.22
Streak: Brown Greyish-white to bluish-white.
Colour: Bluish-black to black, sometimes brownish-black, rarely greenish-black.
Plutonic igneous environments
Pegmatites best known
Tourmaline is found in granite and granite pegmatites, in metamorphic rocks and as a primary mineral in hypothermal (high temperature) hydrothermal veins.
In the Erongo region, Namibia, schorl is found in small cavities in granite, associated with quartz, topaz, fluorite, apatite and cassiterite.
Common impurities: Mn,Mg,Ca,Li,Cr,Ti,F,K
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