Cation vacancies and the crystal chemistry of breakdown reactions in kimberlitic omphacites
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% vacancy in the M2 site. The breakdown reaction accounting for the apparent alteration is: CaAl/sub 2/Si/sub 4/O/sub 12/..-->..CaAl/sub 2/SiO/sub 6/ + 3SiO/sub 2/. 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.
- Research Organization:
- Los Alamos Scientific Lab., NM
- OSTI ID:
- 6636513
- Journal Information:
- Am. Mineral.; (United States), Vol. 65:11, 12
- Country of Publication:
- United States
- Language:
- English
Similar Records
Argon isotopic studies of minerals in kimberlites, mantle xenoliths and diamonds, from selected southern African localities
Seismic Visibility of Eclogite in the Earth’s Upper Mantle—Implications From High Pressure‐Temperature Single‐Crystal Elastic Properties of Omphacite