Abstract

Vanadio-oxy-dravite, NaV3(Al4Mg2)(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 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/cm3. In plane-polarized light, vanadio-oxy-dravite is pleochroic (O = yellow green and E = pale olive green) and uniaxial negative: ω = 1.693(5), ɛ = 1.673(5). Vanadio-oxy-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.0273(3), c = 7.2833(1) Å, V = 1620.24(5) Å3, Z = 3. Crystal-chemical analysis resulted in the empirical structural formula:  
(Na0.70Ca0.230.05K0.02)XΣ1.00(V1.393+Mg1.16Al0.35Fe0.043+Ti0.044+Fe0.022+)YΣ3.00(Al3.74Mg1.28V0.783+Cr0.203+)ZΣ6.00(Si6.00O18)T(BO3)B3(OH)V3[O0.74(OH)0.26]WΣ1.00.

The crystal structure of vanadio-oxy-dravite was refined to an R1 index of 1.70% using 1800 unique reflections collected with MoKα X-radiation. Ideally, vanadio-oxy-dravite is related to oxy-dravite and oxy-vanadium-dravite by the homovalent substitution V3+ ↔ Al3+. 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 V3+ that are determined at Y+Z(V1.5Al5.5), corresponding to NaY(V1.5Al1.5)Z(Al4Mg2)Si6O18(BO3)3(OH)3O, and Y+Z(V5Al2), corresponding to NaY(V3)Z(V2Al2Mg2)Si6O18(BO3)3(OH)3O.

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