Oxy-chromium-dravite, NaCr3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Oxy-chromium-dravite, NaCr3(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, calcite, chromphyllite, eskolaite, chromite, uvarovite, chromian phlogopite, and pyroxenes of the diopside-kosmochlor series, Cr-bearing tremolite, Cr-bearing titanite, Cr-bearing rutile, and pyrite. Crystals are emerald green, transparent with a vitreous luster, green streak, and conchoidal fracture. Oxy-chromium-dravite has a VHN hardness of 14540 MPa, a Mohs hardness of approximately 7 1/2, and a calculated density of 3.3 g/cm(3). In plane-polarized light, oxy-chromium-dravite is pleochroic (O = dark green, E = yellow green) and uniaxial negative: omega = 1.765(5), epsilon = 1.715(5). Oxy-chromium-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1121(3), c = 7.3701(1) angstrom, V = 1656.95(5) angstrom(3), Z = 3. The chemical characterization resulted in: SiO2 = 31.73, TiO2 = 0.31, B2O3 = 9.35, Al2O3 = 3.61, Cr2O3 = 36.25, V2O3 = 5.81, MgO = 7.49, Na2O = 2.78, K2O = 0.08, F = 0.78, H2O = 2.16, sum 100.01 wt%. The unit formula is (X)(Na1.00K0.02)(Sigma 1.02)(Y)(Cr1.953+V0.873+Mg0.14Ti0.044+)(Sigma 3.00)(Z)(Cr3.373+Al0.69Mg1.93)(Sigma 6.00)[(T)(Si5.90Al0.10)(Sigma 6.00)O-18](B)(BO3)(3) (V)(OH2.67O0.33)(Sigma 3.00)(W)(O0.54F0.46)(Sigma 1.00). The crystal structure of oxy-chromium-dravite was refined to statistical index R1 for all reflections equal to 1.54% using MoK alpha X-ray intensity data. Oxy-chromium-dravite is related to chromium-dravite, ideally NaMg3Cr6(Si6O18)(BO3)(3)(OH)(3)OH, by the heterovalent substitution Cr3++O2- -> Mg2++OH1-.