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

Vanadio-oxy-chromium-dravite, NaV 3 (Cr 4 Mg 2 )(Si 6 O 18 )(BO 3 ) 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-celadonitechromphyllite- 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/cm 3 . In plane-polarized light, vanadio-oxy-chromium-dravite is pleochroic (O = dark green, E = pale green) and uniaxial negative: ω = 1.767(5), ε = 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) Å 3 , Z = 3. Crystal chemistry analysis resulted in the empirical structural formula: X (Na 0.89 K 0.06□0.05 ) Y (V 3+ 2.77 Mg 0.17 Fe 3+ 0.06 ) Z (Cr 3+ 1.85 Al 1.59 V 3+ 0.78 Mg 1.78 ) T [(Si 5.95 Al 0.05 )O 18 ] B (BO 3 ) 3 V (OH 2.91 O 0.09 ) W (O 0.86 F 0.14 ). The crystal structure of vanadio-oxy-chromium-dravite was refined to a statistical index R1 of 1.16% using 2543 unique reflections collected with MoKa X-radiation. Ideally, vanadio-oxy-chromiumdravite is related to oxy-chromium-dravite and oxy-vanadium-dravite by the homovalent substitution V 3+ ↔ Cr 3+ . Tourmaline with chemical compositions classified as vanadio-oxy-chromium-dravite can be either Cr 3+ -dominant or V 3+ -dominant as a result of the compositional boundaries along the solid solution between Cr 3+ and V 3+ that are determined at Y+Z (V 5 Cr 2 ), corresponding to Na Y (V 3 ) Z (V 2 Cr 2 Mg 2 ) Si 6 O 18 (BO 3 ) 3 (OH) 3 O, and Y+Z (V 1.5 Cr 5.5 ), corresponding to Na Y (V 1.5 Cr 1.5 ) Z (Cr 4 Mg 2 )Si 6 O 18 (BO 3 ) 3 (OH) 3 O.