Abstract
The degree of alteration in clinopyroxenes from eclogitic inclusions in South African kimberlites is directly related to the deviation from stoichiometry of remnant unaltered areas in individual grains. Deviations from stoichiometry are reconciled by allowing up to 9 percent vacancy in the М2 site. The breakdown reaction accounting for the apparent alteration is:
The products of this reaction are observed in X-ray precession photographs as a second Ca-Tschermak's pyroxene intergrown with the host omphacite plus quartz powder rings. I infer that a vacancy-containing pyroxene is stabilized by pressure but is highly unstable at lower pressures. A general FORTRAN program which breaks pyroxene compositions into end-members has been written to include a vacancy-containing end-member.
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