Compressibility of omphacite to 60 kbar; role of vacancies

Compressibilities of two C2/c omphacites from kyanite eclogite xenoliths in South African kimberlites have been measured. Both omphacitesare approximately Jd₅₈ in composition,but one of the crystals contains 6.5% M2-site vacancies,whereas the other containsonly 1.7%. Cell parameters were determined at six pressures between 1 bar and 60 kbar on three single crystals (two omphacites and a nearly pure, natural diopside standard) mounted together in a Merrill-Bassett diamond anvil cell, on a four-circle X-ray diffractometer.Relative compression of the three pyroxenes could be measured precisely because pressure was the same for all crystals. Linear regression analysis of the data yields volumecompressibilities of 0.75,0.70, and 0.65 (all ±0.01) Mbar^-1 for diopside, vacancy-richomphacite, and vacancy-poor omphacite respectively. Both omphacites are significantlyless compressible than diopside, consistent with the stiffer elastic moduli of jadeite. The vacancy-poor omphacite is significantly less compressible than the vacancy-bearing pyroxene of similar jadeite content. Greatest differences in axial compression among the three crystals occur parallel to band may beaccounted for by different distortions of the M2 sites, where the cation vacancies appear to occur.