Crystal chemistry of some Mg, Cr, V normal spinels from Sludyanka (Lake Baikal, Russia): the influence of V3+ on structural stability

A suite of vanadian magnesiochromites from the Sludyanka metamorphic complex (South Lake Baikal, Russia) were investigated by means of X-ray single-crystal structural refinements and microprobe analyses. Various morphological types of Cr-V-bearing Mg spinels are located in calc-silicate metamorphic rocks, in an assemblage that also contains other Cr-V minerals such as escolaite-karelianite, uvarovite-goldmanite, Cr-V-bearing clinopyroxene, tourmaline, amphibole, mica, etc. Along the suite there is widespread V-Cr substitution (0.14 less than or equal to V3+ less than or equal to 0.95 afu, 1.02 less than or equal to Cr3+ less than or equal to 1.80 afu), and minor, variable Al contents. The Mg content of slightly lesser than 1 afu, is almost constant. Cell parameters and octahedral bond distances increase with V3+. Unexpectedly, the Mg-O tetrahedral bond distance also increases slowly with V3+. This weak dragging effect contributes towards maintaining distortion of the oxygen array with respect to the ideal CCP, thus providing a shielding effect, which reduces V3+-V3+ repulsion. This leads to the energetic stabilization of the structure, in spite of the increase of bond strain with increasing V3+ contents

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