Abstract

Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with quartz, Cr-V-bearing tremolite and muscovite-celadonite-chromphyllite-roscoelite, diopside-kosmochlor-natalyite, Cr-bearing goldmanite, escolaite-karelianite, dravite-oxy-vanadium-dravite, V-bearing titanite and rutile, ilmenite, oxyvanite-berdesinskiite, shreyerite, plagioclase, scapolite, zircon, pyrite, and an unnamed oxide of V, Cr, Ti, U, and Nb. Crystals are emerald green, transparent with a vitreous luster, pale green streak, and conchoidal fracture. Vanadio-oxy-chromium-dravite has a Mohs hardness of approximately 7½, and a calculated density of 3.3 g/cm3. In plane-polarized light, vanadio-oxy-chromium-dravite is pleochroic (O = dark green, E = pale green) and uniaxial negative: w = 1.767(5), e = 1.710(5). Vanadio-oxy-chromium-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1260(2), c = 7.3759(1) Å, V = 1661.11(4) Å3, Z = 3. Crystal chemistry analysis resulted in the empirical structural formula:  
(Na0.89K0.060.05)X(V2.773+Mg0.17Fe0.063+)Y(Cr1.853+Al1.59V0.783+Mg1.78)Z[(Si5.95Al0.05)O18]T(BO3)B3(OH2.91O0.09)V(O0.86F0.14)W.

The crystal structure of vanadio-oxy-chromium-dravite was refined to a statistical index R1 of 1.16% using 2543 unique reflections collected with MoKa X-radiation. Ideally, vanadio-oxy-chromium-dravite is related to oxy-chromium-dravite and oxy-vanadium-dravite by the homovalent substitution V3+ ↔ Cr3+. Tourmaline with chemical compositions classified as vanadio-oxy-chromium-dravite can be either Cr3+-dominant or V3+-dominant as a result of the compositional boundaries along the solid solution between Cr3+ and V3+ that are determined at Y+Z(V5Cr2), corresponding to NaY(V3)Z(V2Cr2Mg2)Si6O18(BO3)3(OH)3O, and Y+Z(V1.5Cr5.5), corresponding to NaY(V1.5Cr1.5)Z(Cr4Mg2)Si6O18(BO3)3(OH)3O.

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