Distributions of trivalent transition metal ions in mineral assemblages

7 Distributions of trivalent transition metal ions in mineral assemblages 

Trends in CFSE data derived from the absorption spectral measurements described in chapter 5 may be used to predict or interpret element partitioning involving AT3 cations between coexisting aluminosilicate minerals. In many cases, however, the CFSE data represent average values for cations in several Al3+ sites in individual crystal structures. Nevertheless, the stabilization energies are applicable to interphase partitioning trends based on bulk chemical data for coexisting minerals. 

The CFSE data for Ti3+-bearing minerals summarized in table 5.9 decrease in the order 

Ti3+ ions attain highest CFSE in minerals hosted by A1 relative to Mg phases. 

On Earth, the Ti(m) oxidation state is unstable. However, Ti3+-bearing minerals are well-characterized in some meteorites and Moon rocks, generally coexisting with Ti4 in such phases as calcic pyroxene, ulvospinel, hibonite and ilmenite. In hibonite, CaAlI20I9, a refractory phase in carbonaceous chondrites, EPR and optical spectral data indicate that Ti3+ ions are present (Hunger and Stolper, 1986 Live et al., 1986). The trivalent Ti ions may be stabilized in the five-coordinated trigonal bipyramidal M5 site of the hibonite structure 

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