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.
Common impurities: Mn,Mg,Ca,Li,Cr,Ti,F,K
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 (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).
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