CATION AND VACANCY DISTRIBUTION IN AN ARTIFICIALLY OXIDIZED NATURAL SPINEL

During research on the influence of temperature on cation partitioning in natural Mg-Al-Fe2+-Fe3+ spinels, some crystals were accidentally o xidized during heat treatment. The oxidation product, studied by means of single-crystal X-ray diffraction, turned out to be a phase retaini ng the Fd3m parent spinel structure, but with cell edge a and oxygen c oordinate u considerably smaller than the parent ones (a similar to 8. 087 as compared with similar to 8.111 Angstrom; u similar to 0.2609 vs . 0.2617-0.2636) and with vacant sites due to oxidation.Assuming that the oxidation process must occur due to the addition of oxygen to the crystal boundary as cations are being preserved and rising in total va lence, the site population was determined and compared with that of un treated and heated samples. It was found that, on oxidation, a charge enrichment in the tetrahedral site T had occurred, this phenomenon fol lowing that observed during heating at increasing temperatures also in other spinel series. This continuity was always in the direction of a n increase in random charge distribution. Cation vacancies produced du ring oxidation were restricted to the octahedral site M. Examination o f bulk sections by reflected light microscopy showed a few hematite la mellae as inclusions in the oxidized samples, not detectable by microp robe analysis or single-crystal structural refinement. However, hemati te played a marginal part in oxidation. Vacancy-oxygen distances in ox idized spinels were determined from experimental data in the literatur e.