CRYSTAL CHEMISTRY OF THE DRAVITE-CHROMDRAVITE SERIES

Abstract

Five Cr-dravite-chromdravite samples, representative of the high-chromium part of the compositional field observed in tourmalines occurring in rocks surrounding Lake Baikal, were selected and studied by electron microprobe analysis and structural refinement. All examined tourmalines belong to the Alkali group, Oxy-subgroup. Their most striking feature is the exceptionally high Cr content (from 3.2 to 4.41 apfu), which substitutes for Al. The main divalent cation is Mg. Site populations were determined by a minimization procedure which simultaneously takes into account both structural and chemical data. Empirical bond distances for Y Cr 3+ -O (1.978 A) and z Cr 3+ -O (1.970 A) were optimised within the minimization procedure. Results indicate that the crystals are characterized by a quite disordered cation distribution between Y and Z octahedral sites, the various cations showing different degrees of preference. Cr 3+ and Mg populate both sites but show opposite behaviour: Mg has a marked preference for the Z octahedron and Cr 3+ for Y. Al almost exclusively populates Z. Most structural features are related to variations in the Z octahedron, whereas the dimensions of other polyhedra remain almost constant. Z dimensions depend on site populations, and are in particular determined by the z Cr 3+ ⟷ Z Al substitution. In the dravite-chromdravite series, both current and literature data show that the unit cell parameter c is strongly and positively correlated with Z dimensions, while correlation with a is evident only within the Cr-dravite-chromdravite subseries. Y does not actively participate in structural variations and, in particular, no correlation was ever observed between Y dimensions and unit cell parameters. Most structural variations are thus due to Z , while the effects of Y are negligible. This behaviour stands out when examining the a vs. c plot, for the whole series: a strong correlation between the two unit cell parameters was observed in the Cr-dravite-chromdravite subseries, but none within the dravite-Cr-dravite subseries or schorl-dravite series.