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.