Dravite

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Formula: NaMg₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH)
Cyclosilicate (ring silicate), borosilicate, tourmaline group, forms a series with elbaite, schorl, uvite and fluor-buergerite.
Crystal System: Trigonal
Specific gravity: 3.03 to 3.18 measured, 3.038 calculated
Hardness: 7
Streak: Light brown, rarely white
Colour: Pale brown to dark-brown to brownish-black, also dark-yellow, blue
Luminescence: May fluoresce weakly under short wave UV
Electrical: Pyroelectric and piezoelectric
Common impurities: Fe,Mn,Ti,Ca,Cr,V,K,F
Environments:

Pegmatites rarely
Sedimentary environments
Metamorphic environments commonly
Hydrothermal environments

Dravite occurs commonly in metamorphosed limestone or mafic igneous rocks with metasomatically introduced boron; it occurs rarely in pegmatites, and as authigenic overgrowths in sedimentary rocks. Associated minerals include quartz, calcite, dolomite, epidote, microcline, albite, muscovite, fluorite and titanite (HOM). Green-to-red colour change in chromium-bearing dravite is caused by the Usambara effect (Mindat).

Localities

In the Moldanubian Zone pegmatites, Vlastějovice, Kutná Hora District, Central Bohemian Region, Czech Republic, pegmatites with pockets lined with crystals of smoky quartz, feldspars, muscovite, and black tourmaline commonly occur in migmatised biotite–sillimanite gneisses and felsic granulites. Thin muscovite-rich veins, 1–10 cm thick, with the assemblage muscovite ≥ albite, quartz and tourmaline are closely associated spatially. Variable prismatic to lens-shaped tourmaline crystals and their aggregates, up to 30 cm across, from pockets and muscovite-rich veins exhibit striking zoning:
core (foitite – schorl – magnesio-foitite),
intermediate zone (schorl – dravite)
narrow outer rim (schorl – dravite).
tourmaline from muscovite veins is relatively homogeneous (dravite – magnesio-foitite) (CM 50.895-912).

At Syros Island, Syros, South Aegean, Greece, metamorphic iron-bearing dravite occurs in glaucophane schist, associated with omphacite (EJM 16.817-823).

At the Forshammar Pegmatite Quarries, Ramsberg, Lindesberg, Örebro County, Sweden, green to greyish green dravite crystals up to 10 cm across, with distinct optical zoning, occur with quartz, blocky albite and muscovite. The dravite contains inclusions of zircon and xenotime-(Y), and it is cut by veinlets of muscovite and hydroxylbastnäsite-(Ce). The pegmatite core is likely a product of an early magmatic process about 1.8 Ga. The intermediate zone, rim and tourmaline veins originated during the late magmatic to hydrothermal stage. hydroxylbastnäsite-(Ce) and muscovite are apparently the final products of the hydrothermal process (CM 50.825-841).

At Silver Knob, Fish Camp, Mariposa county, California, USA, vanadium-rich dravite occurs with up to 4.9% by weight of V₂O₃ (Dana).

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