Abstract
A combined polarized optical absorption and 57Fe Mossbauer spectroscopy study of inhomogeneous, Fe and Ti-bearing terrestrial hibonite (Madagascar) has been carried out. Mossbauer data were also obtained on synthetic material prepared under different fo2 inconditions. A strong band at 5400 cm-1 in the near-infrared spectra is attributed to spin-allowed d-d transitions of Fe2+ occupying tetrahedral sites within the spinel blocks of the hibonite crystal structure. There is agreement with the Mossbauer results, showing that ferrous iron orders onto a single, low-coordinated crystallographic site. Ferric iron is distributed over several positions, but shows strongest preference for the large bipyramidal site located outside the spinel blocks. The colour and pleochroism of hibonite in thin section is related to a prominent UV absorption edge, and several broad absorption bands in the visible spectrum ascribed to charge-transfer transitions involving Fe2+, Fe3+ and Ti4+.
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