Vanadio-oxy-dravite, NaV3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Vanadio-oxy-dravite, NaV 3 (Al 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 mica, 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 green, transparent with a vitreous luster, pale green streak, and conchoidal fracture. Vanadio-oxy-dravite has a Mohs hardness of approximately 7½, and a calculated density of 3.14 g/cm 3 . In plane-polarized light, vanadio-oxy-dravite is pleochroic (O = yellow green and E = pale olive green) and uniaxial negative: ω = 1.693(5), ε = 1.673(5). Vanadio-oxy-dravite is rhombohedral, space group R 3 m , with the unit-cell parameters a = 16.0273(3), c = 7.2833(1) Å, V = 1620.24(5) Å 3 , Z = 3. Crystal-chemical analysis resulted in the empirical structural formula: X(Na 0.70 Ca 0.23 □ 0.05 K 0.02 ) Σ1.00 Y (V 3+ 1.39 Mg 1.16 Al 0.35 Fe 3+ 0.04 Ti 4+ 0.04 Fe 2+ 0.02 ) Σ3.00 Z(Al 3.74 Mg 1.28 V 3+ 0.78 Cr 3+ 0.20 ) Σ6.00 T (Si 6.00 O 18 ) B (BO 3 ) 3 V (OH) 3 W [O 0.74 (OH) 0.26 ] Σ1.00 . The crystal structure of vanadio-oxy-dravite was refined to an R 1 index of 1.70% using 1800 unique reflections collected with Mo K α X-radiation. Ideally, vanadio-oxy-dravite is related to oxy-dravite and oxy-vanadium-dravite by the homovalent substiution V 3+ ↔ Al 3+ . Tourmaline with chemical compositions classified as vanadio-oxy-dravite can be either Al dominant or V dominant as a result of the compositional boundaries along the solid solution between Al and V 3+ that are determined at Y+Z (V 1.5 Al 5.5 ), corresponding to Na Y (V 1.5 Al 1.5 ) Z (Al 4 Mg 2 )Si 6 O 18 (BO 3 ) 3 (OH) 3 O, and Y+Z (V 5 Al 2 ), corresponding to Na Y (V 3 ) Z (V 2 Al 2 Mg 2 )Si 6 O 18 (BO 3 ) 3 (OH) 3 O.