Behaviour of Fe4O5–Mg2Fe2O5 solid solutions and their relation to coexisting Mg–Fe silicates and oxide phases
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Experiments at high pressures and temperatures were carried out (1) to investigate the crystal-chemical behaviour of Fe4O5–Mg2Fe2O5 solid solutions and (2) to explore the phase relations involving (Mg,Fe)2Fe2O5 (denoted as O5-phase) and Mg–Fe silicates. Multi-anvil experiments were performed at 11–20 GPa and 1100–1600 °C using different starting compositions including two that were Si-bearing. In Si-free experiments the O5-phase coexists with Fe2O3, hp-(Mg,Fe)Fe2O4, (Mg,Fe)3Fe4O9 or an unquenchable phase of different stoichiometry. Si-bearing experiments yielded phase assemblages consisting of the O5-phase together with olivine, wadsleyite or ringwoodite, majoritic garnet or Fe3+-bearing phase B. However, (Mg,Fe)2Fe2O5 does not incorporate Si. Electron microprobe analyses revealed that phase B incorporates significant amounts of Fe2+ and Fe3+ (at least ~ 1.0 cations Fe per formula unit). Fe-L2,3-edge energy-loss near-edge structure spectra confirm the presence of ferric iron [Fe3+/Fetot = ~ 0.41(4)] and indicate substitution according to the following charge-balanced exchange: Si4+ + Mg2+ = 2Fe3+. The ability to accommodate Fe2+ and Fe3+ makes this potential “water-storing” mineral interesting since such substitutions should enlarge its stability field. The thermodynamic properties of Mg2Fe2O5 have been refined, yielding H°1bar,298 = − 1981.5 kJ mol− 1. Solid solution is complete across the Fe4O5–Mg2Fe2O5 binary. Molar volume decreases essentially linearly with increasing Mg content, consistent with ideal mixing behaviour. The partitioning of Mg and Fe2+ with silicates indicates that (Mg,Fe)2Fe2O5 has a strong preference for Fe2+. Modelling of partitioning with olivine is consistent with the O5-phase exhibiting ideal mixing behaviour. Mg–Fe2+ partitioning between (Mg,Fe)2Fe2O5 and ringwoodite or wadsleyite is influenced by the presence of Fe3+ and OH incorporation in the silicate phases.
KeywordsFe4O5 Mg2Fe2O5 Phase B Phase relations Deep mantle High pressure High temperature
This work was supported by the Deutsche Forschungsgemeinschaft through grants WO 652/20-1 and BO 2550/7-1 to ABW and TBB, respectively. E. Alig is thanked for helping with obtaining the X-ray powder diffraction patterns. We are grateful to Thomas Kautz, Nicki Siersch and Svyatoslav Shcheka for their help with the multi-anvil experiments at the University of Frankfurt and the Bayerisches Geoinstitut. Heidi Höfer is thanked for her help with the microprobe analysis. We also acknowledge the comments by Bob Myhill and two anonymous reviewers which helped to improve the manuscript.
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