Crystal chemistry of clinopyroxenes from mantle eclogites: A study of the key role of the M2 site population by means of crystal-structure refinement

Single-crystal X-ray structure refinements of a series of geochemically well-characteized clinopyroxenes from grospydite and eclogite xenoliths from the Roberts Victor and Bobbejaan diamond mines, South Africa, have been carried out. The study was done to clarifu the crystal chemical relations of high-pressure mantle clinopyroxenes and to understand the structural constraints on REE substitution. The jadeite component ranges from 0 to 50 mol%, and cation vacancies are suggested from EMP analyses.

All the omphacitic clinopyroxenes examined belong to the C2/c space group, in agreement with the high temperature of reequilibration and the rapid quenching during kimberlite transport. Relatively high (Fe²⁺ + Mg) contents (up to 0. 19 apfu) are present at the M2 site in many of these specimens, resulting in a volume contraction in the M2 polyhedron. The presence of vacancies doesn't significantly affect the M2 site volume. The overall decrease in the unit-cell volume observed in vacancy-bearing pyroxenes is predominantly caused by the contraction in the Ml polyhedron, resulting from the incorporation of highly charged cations in Ml required to preserve overall charge balance of the structure. Both the chemical substitutions observed at M2 [Ca → (Fe²⁺, Mg) and Na → vacancy] are therefore efrcient ways to allow crystallization of low-volume clinopyroxene. Other crystal chemical features are similar to those reported for metamorphic eclogite clinopyroxenes on the Di-Jd join.

Variations in cation electrostatic potentials, volume, and shape of the M2 polyhedron caused by major elements agree with the observed inverse relationship between Na content and total trivalent REE concentration.