On the PT\(fO_{2}\) stability of Fe4O5, Fe5O6 and Fe4O5-rich solid solutions

  • Robert Myhill
  • Dickson O. Ojwang
  • Luca Ziberna
  • Daniel J. Frost
  • Tiziana Boffa Ballaran
  • Nobuyoshi Miyajima
Original Paper


The high-pressure phases Fe4O5 and Fe5O6 have recently been added to the list of known iron oxides. As mixed-valence phases, it has been suggested that they could form in the Earth’s mantle once the dominant minerals become saturated in ferric iron. The possibility that Fe4O5 could exist in the mantle is also supported by the fact that it forms extensive solid solutions with both Mg2+ and Cr3+. In this study, we present the results of high-pressure and high-temperature multi-anvil experiments performed between 5 and 24 GPa at 1000–1400 °C aimed at constraining the stability field of the Fe4O5 phase. We combine these results with published phase equilibria, equation of state and Fe–Mg partitioning data to estimate the thermodynamic properties of Fe4O5, Fe5O6 and the (Mg,Fe)2Fe2O5 solid solution. Using our thermodynamic model, the oxygen fugacity at which the high-pressure iron oxides become stable is calculated and the redox stability of (Mg,Fe)2Fe2O5 in an assemblage of olivine and pyroxene is calculated as a function of the bulk Fe/(Fe + Mg) ratio. Fe4O5 and (Mg,Fe)2Fe2O5 are stable at oxygen fugacities higher than the diamond stability field and are, therefore, unlikely to be found as inclusions in diamonds. The stability field of Fe5O6, on the other hand, extends to oxygen fugacities compatible with diamond formation. Using the Mg–Fe solid solution model, we show that Fe4O5-structured phases would be restricted to aluminium-poor environments in the mantle such as dunites or silica–iron oxide-rich sediments transported into the mantle via subduction.


Fe4O5 Fe5O6 Thermodynamics Oxygen fugacity Mantle Deep Earth 



This study was supported by a Humboldt Fellowship awarded to RM and the ERC Advanced Grant ACCRETE project (Contract Number 290568). The authors would like to thank Stefan Ubelhack and Heinz Fischer for assembly and capsule cutting and Hubert Schulze and Raphael Njul for the preparation of run products. They also thank Chris Ballhaus and two anonymous reviewers for their insightful comments which helped to significantly improve the manuscript.

Supplementary material

410_2016_1258_MOESM1_ESM.docx (607 kb)
Supplementary material 1 (DOCX 606 kb)


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Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  1. 1.Bayerisches GeoinstitutUniversität BayreuthBayreuthGermany
  2. 2.Inorganic and Structural Chemistry, Department of Materials and Environmental Chemistry, Arrhenius LaboratoryStockholm UniversityStockholmSweden
  3. 3.School of Earth SciencesUniversity of BristolBristolUK

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