Phase relations of Mg-Fe oxide spinels and their high-P post-spinel phases have received increasing interest for constraining the P-T conditions of formation of certain inclusions in diamonds or shock events in extraterrestrial samples. However, application to natural samples requires knowledge of how major element substitutions affect phase stabilities. Building from the relations for magnesioferrite (MgFe3+2O4) [1], new multi-anvil experiments were performed to investigate the effect of Al substitution at 10-22 GPa and 1100-1700°C, using Mg(Al0.2Fe3+0.8)2O4 or Mg(Al0.4Fe3+0.6)2O4 bulk compositions.

The presence of Al stabilizes a spinel phase up to ~17 GPa; significantly higher pressure than the ~10 GPa for magnesioferrite [1]. However, the composition varies systematically with P and T; compared to the starting composition it shifts to higher Al contents with increasing P and decreasing T. Initially, the spinel phase co-exists with MgO and (Fe,Al)2O3. At 14 GPa a phase with Mg2(Al,Fe3+)2O5 stoichiometry appears instead of MgO, contrasting with the Fe-Al system [2]. The O5-phase incorporates limited amounts of Al (to ~0.5 cpfu) and persists to 20 GPa at high T. Beginning at 18 GPa, both a high-P-O4-phase and one with Mg3(Al,Fe3+)4O9 stoichiometry often coexist in the sample. The high-P-O4 phase is quenchable with a structure consistent with space group Pnma, like [3] reported for MgFe2O4. Thus, it may have undergone a phase transition during decompression [3]. Both high-P-O4 and the O9-phase seem capable of incorporating Al up to that of the starting composition, in contrast to the Fe-Al system [2]. The occurrence of two high-P phases seems to be related to variable degrees of reduction during the experiment, driving the bulk composition slightly off of the MgFe2O4-MgAl2O4 join.

1Uenver-Thiele et al. (2017) Am Min, 102, 632–642

2Uenver-Thiele et al. (2024) Am Min, 109, 1062-1073

3Ishii et al. (2020) Geophys Res Lett, 47, e2020GL087490