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

多种不同化学计量和晶体结构的铁氧化物在对应于上地幔下部和过渡带的高压条件下稳定存在。这包括Fe4O5和Fe5O6，这两种材料在很大的压力和温度范围内都是稳定的。这些相可能作为地幔组合的附属相出现，并可能参与钻石的形成过程。除了在受冲击的陨石中出现的高压氧化铁多晶外，对来自奥拉帕点的岩石圈下钻石的包裹体中的磁铁矿进行了结构调查，可能具有Fe4O5或Fe5O6的化学计量。Fe5O6是否可以被认为是一个可行的前驱相取决于它容纳少量元素的能力，如在金刚石中的许多包裹体中观察到的Cr。然而，没有关于Fe5O6结构中Cr固溶程度的信息。目前还不清楚Cr的存在是否有助于稳定富镁成分或其他富镁氧化物相，如Mg4Fe2O7，目前只有间接证据证明它们的稳定性。本项目旨在通过实验研究可能与地幔有关的高压fe - mg -氧化物相中Cr的行为。重点将放在含有Cr的o6结构相的稳定性上。该项目有两个主要目标：1)实验确定Fe3Cr2O6和Mg3Cr2O6端元是否稳定，2)确定Cr在（Mg,Fe2+）3Fe3+2O6中的掺入程度。第一部分包括一系列简短的多砧实验，温度为1300°和1500°C，温度为14-20 GPa，使用预合成的铬尖晶石和氧化物作为起始材料。研究Mg3Cr2O6端元的实验也将测试潜在的Mg4Cr2O7相的稳定性，该相的结构目前尚不清楚。实现第二个目标的方法将取决于第一部分的结果，但将涉及使用Fe2+3Fe3+2O6 - Fe2+3Cr2O6二元组合物以及含有Mg的组合物的实验。通过对高压高温下fe5o6型结构中Cr结合的系统了解，我们将能够评估“O6”相是否是岩石圈下钻石中包裹体或潜在前体相的可行候选物。此外，我们将能够提供含cr Fe5O6的晶体化学表征

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