Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)(3)(OH)(3)O, 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 TA, and a calculated density of 3.3 g/cm(3). In plane-polarized light, vanadio-oxy-chromium-dravite is pleochroic (O = dark green, E = pale green) and uniaxial negative: omega = 1.767(5), epsilon = 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) angstrom, V=1661.11(4) angstrom(3), Z = 3. Crystal chemistry analysis resulted in the empirical structural formula: (X)(Na0.89K0 06 square(0.05)) (Y)(V2.773+Mg0.17Fe0.063+) (Z)(Cr1.853+Al1.59V0.783+Mg1.78) (T)[(Si5.95Al0.05)O-18] B(BO3)(3) (V)(OH2.91O0.09) (W)(O0.86F0.14). The crystal structure of vanadio-oxy-chromium-dravite was refined to a statistical index R1 of 1.16% using 2543 unique reflections collected with MoK alpha 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 Na-Y(V-3)(Z)(V2Cr2Mg2)Si6O18(BO3)(3)(OH)(3)O, and (Y+Z)(V1.5Cr5.5), corresponding to Na-Y(V1.5Cr1.5)(Z)(Cr4Mg2)Si6O18(BO3)(3)(OH)(3)O.