Is 600° C enough to produce air oxidation in Cr-spinels?

Eleven natural crystals of Cr-bearing spinels containing Cr 0.1-1.5 and total iron 0.2 – 1.0 atoms per formula unit, have been studied by X-ray single crystal diffraction at ambient conditions and after heating at 600°C for 7 and 28 days in order to verify if they suffered oxidation. SEM-BSD analyses revealed the absence of zoning as a possible indication of oxidation. Structural analyses show that no vacancies are present after heating. In crystals with low Fetot and Cr# the main effects of heating are related to Mg and Al exchange among the two sites; in spinels with 0.75<Cr<1.0, Fetot content comprised between 0.25 and 0.45 a.p.f.u., and Al higher than 0.95 a.p.f.u. both Mg - Al and Fe3+-Mg exchange occurs; crystals with Fetot > 0.45 a.p.f.u. and Cr > 1.2 a.p.f.u. show very limited inversion degree due to the high Cr content. In this case, movement of ferric iron among the sites seems to be dependent upon its initial content in the site. Air oxidation seems to be absent in the studied natural spinels annealed at 600°C.