| Abstrakt: |
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)2O3forms and remains stable up to 16–20 GPa and 1200–1550 °C. Fe2(Al,Fe)2O5adopts the CaFe3O5-type structure with the Cmcmspace group. At 18–22 GPa and T> 1300 °C the assemblage Fe3(Fe,Al)4O9+ (Al,Fe)2O3becomes stable. Fe3(Fe,Al)4O9is isostructural with Fe7O9, having the monoclinic structure of the C2/mspace group. At T<1300 °C, Fe3(Fe,Al)4O9+ (Al,Fe)2O3gives way to the assemblage of a hp-Fe(Fe,Al)2O4+ (Al,Fe)2O3. This hp-Fe(Fe,Al)2O4phase 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)2O4can also be applied to bulk compositions richer in Al with the only difference being that larger amounts of an (Al,Fe)2O3phase are present. |
