Abstract: During research on the influence of temperature on cation partitioning in natural Mg-Al-Fe²⁺-Fe³⁺ spinels, some crystals were accidentally oxidized during heat treatment. The oxidation product, studied by means of single-crystal X-ray diffraction, turned out to be a phase retaining the Fd3m parent spinel structure, but with cell edge a and oxygen coordinate u considerably smaller than the parent ones (a ∼ 8.087 as compared with ∼ 8.111 Å; u ∼ 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 valence, the site population was determined and compared with that of untreated and heated samples. It was found that, on oxidation, a charge enrichment in the tetrahedral site T had occurred, this phenomenon following that observed during heating at increasing temperatures also in other spinel series. This continuity was always in the direction of an increase in random charge distribution. Cation vacancies produced during oxidation were restricted to the octahedral site M.

Examination of bulk sections by reflected light microscopy showed a few hematite lamellae as inclusions in the oxidized samples, not detectable by microprobe analysis or single-crystal structural refinement. However, hematite played a marginal part in oxidation. Vacancy-oxygen distances in oxidized spinels were determined from experimental data in the literature.