Chromium orthopyroxene

In the case of Cu2+ and Cr2+, compounds of which are susceptible to Jahn-Teller distortions ( 6.3 table 6.1), these cations are predicted to show strong preferences for the most distorted orthopyroxene M2 and amphibole M4 sites. A similar explanation accounts for the observed enrichments of Mn3+ ions in the distorted andalusite Ml, alkali amphibole M2, epidote M3 and, perhaps, epidote Ml sites (table 6.1). The presence of significant amounts of chromium in olivines from the Moon and as inclusions in diamond may be due to the presence of Cr2+ ions, and not Cr3+, in the distorted Ml and M2 sites of the olivine structure (Bums, 1975b), in which Jahn-Teller stability may be attained. A similar factor accounts for the stability and site occupancy of the Cr2+ ion in the orthopyroxene M2 site (table 6.1). 

Barnes, S. J. (1986) The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure. Geochim. Cosmochim. Acta, 50,1889-909. 

In detail, however, the picture is not so simple. All mantle peridotites (whether massive peridotites or xenoliths) are metamorphic rocks that have had a complex subsolidus history after melt extraction ceased. As well as subsolidus recrystallization, peridotites have undergone enormous amounts of strain during their emplacement in the lithosphere. Massive peridotites show modal heterogeneity on the scale of centimeters to meters, caused by segregation of the chromium-diopside suite of dikes, which are then folded back into the peridotite as deformation continues. The net result is more or less diffuse layers or bands in the peridotite, which may be either enriched or depleted in the material of the chromium-diopside suite, i.e., in climopyroxene and orthopyroxene in various proportions, minor spinel, and sulfide. This process should cause approximately linear correlations of elements versus MgO, broadly similar to, but not identical with, those caused by melt extraction. Indeed, there is... 

---