Stabilities and properties of high-pressure oxide phases containing Cr with focus on solid solutions of (Mg,Fe2+)3Fe2O6 – testing for possible precursor phases for inclusions in diamond

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A variety of Fe-oxides with different stoichiometries and crystal structures are now known to be stable at high-pressure conditions corresponding to the lower part of the upper mantle and transition zone. This includes Fe4O5 and Fe5O6, both of which are known to be stable over a wide range in pressure and temperature. Such phases are potential candidates to occur as accessory phases in the mantle assemblage and can be involved in the process of diamond formation. In addition to high-pressure Fe-oxide polymorphs occurring in shocked meteorites, structural investigations of magnetite in an inclusion from a sublithospheric diamond from Orapa point to a precursor phase having been present, possibly with a Fe4O5 or Fe5O6 stoichiometry. Whether Fe5O6 can be considered a viable precursor phase depends its ability to accommodate minor elements like Cr that have been observed in many inclusions in diamonds. However, no information is available on the extent of Cr solid solution in the Fe5O6 structure. It is also unknown if the presence of Cr might help stabilise Mg-rich compositions or even other Mg-rich oxide phases like Mg4Fe2O7, for which only indirect evidence for their stability currently exists.This proposed project aims to experimentally investigating the behavior of Cr in high-pressure Fe-Mg-oxide phases that may be relevant for the Earth’s mantle. The focus will be on the stability of O6-structured phases containing Cr. The project has two major goals: 1) to experimentally determine if the Fe3Cr2O6 and Mg3Cr2O6 end members are stable, and 2) to determine the extent of Cr incorporation in (Mg,Fe2+)3Fe3+2O6. The first part involves a short series of multi-anvil experiments at 14-20 GPa and temperatures of 1300° and 1500°C using pre-synthesised Cr-spinels and oxides as starting materials. Experiments investigating the Mg3Cr2O6 end member will also enable a test of the stability of a potential Mg4Cr2O7 phase, the structure of which is currently unknown. The approach to the second goal will depend on the results from the first part, but will involve experiments using compositions along the Fe2+3Fe3+2O6 – Fe2+3Cr2O6 binary as well as compositions containing Mg. Through knowledge of the systematics of Cr incorporation in the Fe5O6-type structure at high pressures and temperatures we will be able to assess whether the "O6"-phase is a viable candidate to be found as an inclusion or as a potential precursor phase in sublithospheric diamonds. In addition we will be able to provide a crystal chemical characterization of Cr-bearing Fe5O6