Abstract
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 ≤ V3+ ≤ 0.95 afu, 1.02 ≤ Cr3+ ≤ 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.
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Acknowledgments.
A preliminary draft of this manuscript was kindly reviewed by A. Della Giusta and S. Lucchesi, which the authors greatly appreciated comments by reviewers significantly improved the manuscript. The authors also wish to express their gratitude to Mr Carampin, who accomplished microprobe analyses. Ms G. Walton revised the English text. This research was supported by MURST and the Russian Foundation for Basic Research (RFBR), projects 02-05-65329, 01-05-97234.
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Lavina, B., Reznitskii, L. & Bosi, F. Crystal chemistry of some Mg, Cr, V normal spinels from Sludyanka (Lake Baikal, Russia): the influence of V3+ on structural stability. Phys Chem Minerals 30, 599–605 (2003). https://doi.org/10.1007/s00269-003-0354-y
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DOI: https://doi.org/10.1007/s00269-003-0354-y