Oxy-vanadium-dravite, NaV3(V4Mg2)(Si6O18)(BO3)3(OH)3O: Crystal structure and redefinition of the "vanadium-dravite" tourmaline

"Vanadium-dravite" NaMg3V6(Si 6O18)(BO3)3(OH)3OH (IMA number 1999-050) has been redefined as oxy-vanadium-dravite with end-member formula NaV3(V4Mg2)Si6O18 (BO3)3(OH)3O. The new name and the new formula have been approved by the CNMNC (IMA proposal 11-E). Oxy-vanadium-dravite occurs in the metamorphic rocks of the Sludyanka complex (southern Baikal region, Russia). The crystal structure of oxy-vanadium-dravite has been refined for the first time using single-crystal X-ray data, with a statistical index R1 for all reflections converging to 1.44%. The structure is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1908(4), c = 7.4143(2) Å, V = 1683.21(7) Å3, Z = 3. The chemical characterization resulted in the empirical structural formula:(Equavation presented) Ideally, the oxy-vanadium-dravite is related to oxy-dravite and oxy-chromium-dravite by the homovalent substitution V3+ → Al and V3+ → Cr3+ (respectively) at the Y and Z sites. The occurrence of solid-solutions among V3+, Cr3+, and Al have been observed in tourmalines from metamorphic rocks of the Sludyanka complex. Significant chemical variations in V3+, Cr3+, and Al were also observed within zoned crystals from Sludyanka, not belonging to the holotype specimen.