Fe4O5 and its solid solutions in several simple systems
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Experiments at high pressures and temperatures reveal the stability of a Fe4O5-type structured phase in several simple chemical systems. On the one hand, the Fe4O5 end-member is stable in the presence of SiO2-rich phases, including stishovite, but contains ≤0.01 Si cations per formula unit. This indicates that Si is essentially excluded from this phase. On the other hand, the Fe4O5 phase can form solid solutions with Mg and Cr and can coexist with silicate phases at the high P–T conditions expected in the transition zone of the mantle (i.e. >~9 GPa). It can coexist with both wadsleyite and Mg-rich ringwoodite and can contain at least 25 mol% Mg2Fe2O5 component. The Fe4O5 phase always contains the least amount of Mg in any given mineral assemblage. Cr-bearing Fe4O5 has been synthesised with up to 46 mol% Fe2Cr2O5 component and can coexist with spinel and/or hematite-eskolatite solid solutions. Substitution of Mg and Cr for Fe2+ and Fe3+, respectively, leads to variations in Fe3+/∑Fe from the ideal value of 0.5 for the Fe4O5 end-member composition, which can influence its redox stability. These cations also have contrasting effects on the unit-cell parameters, which indicate that they substitute into different sites. This initial study suggests that Fe4O5-type structured phases may be stable over a range of P–T–fO2 conditions and bulk compositions, and can be important in understanding the post-spinel phase relations in a number of chemical systems relevant to the Earth’s transition zone. Thus, the presence of even small amounts of Fe3+ could alter the expected phase relations in peridotitic bulk compositions by stabilising this additional phase.
KeywordsFe4O5 Solid solution Transition zone High pressure Fe-oxides Oxidation state
This work was partially supported by the Deutsche Forschungsgemeinschaft grants Wo 652/4-1, Wo 652/9-1 and Fr 1555/4-1. Recent contributions to this study by RJA were supported by ERC starting Grant 307322 to F. Nestola. Discussions with D. Trots are gratefully acknowledged. The comments of three anonymous reviewers helped to improve the presentation of our results.
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