OXIDATION MECHANISMS IN AL-MG-FE SPINELS - A 2ND-STAGE - ALPHA-FE2O3 EXSOLUTION

A two-stage model of oxidation was devised to explain the observed var iations in crystallographic parameters in two artificially oxidized na tural spinels. In the first stage, oxygen is added to the crystal boun dary as cations are preserved, with Fe rising in total valence and vac ant sites being formed. In the second stage, oxygen is preserved and a lpha-Fe2O3 intergrowth occurs, at the expense of the oxygen of the par ent spinel structure. On the basis of this model, crystallochemical fo rmulae were calculated and cations partitioned in the various conditio ns. It was found that, both before and after oxidation, the spinel sit e population varies continuously in the direction of an increase in ra ndom charge distribution, depending on the increase of heat to the cry stals. This trend was found to be reversible. Cation vacancies produce d during oxidation are distributed between tetrahedral site T and octa hedral site M.