The phase relations of Al-bearing magnetite were investigated between 6–22 GPa and 1000–1550 °C using a multi-anvil apparatus. This study demonstrates that the spinel-structured phase persists up to ~9–10 GPa at 1100–1400 °C irrespective of the amount of hercynite (FeAl2O4) component present (20, 40, or 60 mol%). At ~10 GPa, the assemblage Fe2(Al,Fe)2O5 + (Al,Fe)2O3 forms and remains stable up to 16–20 GPa and 1200–1550 °C. Fe2(Al,Fe)2O5 adopts the CaFe3O5-type structure with the Cmcm space group. At 18–22 GPa and T >1300 °C the assemblage Fe3(Fe,Al)4O9 + (Al,Fe)2O3 becomes stable. Fe3(Fe,Al)4O9 is isostructural with Fe7O9, having the monoclinic structure of the C2/m space group. At T <1300 °C, Fe3(Fe,Al)4O9 + (Al,Fe)2O3 gives way to the assemblage of a hp-Fe(Fe,Al)2O4 + (Al,Fe)2O3. This hp-Fe(Fe,Al)2O4 phase is unquenchable; a defect-bearing spinel-structured phase was recovered instead, and it contained numerous lamellae parallel to {100} or {113} planes and notably less Al than the initial starting composition. While low-pressure spinel can have a complete solid solution between Fe3+-Al, the post-spinel phases have only very limited Al solubility, with a maximum of ~0.1 cpfu Al in hp-Fe(Fe,Al)2O4, ~0.3 cpfu in Fe2(Fe,Al)2O5, and ~0.4 cpfu in Fe3(Fe,Al)4O9, respectively. As a result, the phase relations of Fe(Fe0.8Al0.2)2O4 can also be applied to bulk compositions richer in Al with the only difference being that larger amounts of an (Al,Fe)2O3 phase are present.

Coexisting rhombohedral-structured phases demonstrate that the binary miscibility gap established at low pressure between hematite and corundum is still valid up to 20 GPa. Since iron oxides (e.g., magnetite) with variable Al contents are found in extraterrestrial rocks or as inclusions in diamond, constraints on their high-P-T-fO2 stability might help unravel their formation conditions.

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