Physics
Scientific paper
Jun 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975gecoa..39..857b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 39, Issue 6-7, pp.857-864
Physics
5
Scientific paper
Cr 3+ ions have a strong preference for octahedral sites in mineral structures and discriminate against tetrahedral sites, whereas Cr 2+ ions favor distorted environments, according to crystal field theory. All structure-types proposed for the transition zone and lower mantle contain six-coordinated sites, so that Cr 3+ ions may be distributed over several phases in the Earth's interior. In the upper mantle, however, stabilization energies derived from crystal field spectra suggest that the enrichment of Cr 3+ follows the sequence: spinel > garnet > pyroxene or olivine. High-spin Cr 2+ ions, which may occur at very high pressures and low oxygen fugacities, are predicted to be stabilized in deformed sites of the olivine and Sr 2 PbO 4 structure-types. Low-spin Cr 2+ ions may also occur in the CaFe 2 O 4 structure-type in the lower mantle. It is proposed that some chromite inclusions in forsterites from the upper mantle are formed by pressure-released oxidation of Cr 2+ ions originally present in olivine or spinel modifications of Mg 2 SiO 4 .
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