Nine omphacitic clinopyroxenes with a jadeitic content between 25% and 57% from the Udachnaya and Zagadochnaya diamond-bearing kimberlites were investigated using single-crystal X-ray diffraction, electron microprobe analysis, and Mössbauer and infrared spectroscopy. Crystal-structure refinements, incorporating constraints from chemical analyses and Mössbauer data, show a significant concentration of vacancies at the M2 site in three of the samples. The cell volumes of the non-stoichiometric samples plot below the linear trend defined by stoichiometric diopside-jadeite compositions, where the deviation from the linear trend is positively correlated with the concentration of vacancies. Charge-balance appears to be achieved primarily through substitution of Al3+ for Mg2+ on the M1 site, which causes a contraction of the M1 site volume compared to stoichiometric clinopyroxenes with the same composition. Vacancies are not associated with OH incorporation, as the sample with the highest concentration of vacancies contains only 89 ppm (wt) H2O. Significant hydrogen loss through iron oxidation during ascent is ruled out based on the low Fe3+ concentrations. H content in omphacite samples from kimberlites could provide a sensitive tool for determining fH2O during diamond formation.

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