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

目前已知，各种化学计量比和晶体结构不同的铁氧化物在高压条件下是稳定的，对应于上地幔下部和过渡带。这包括Fe4O5和Fe5O6，众所周知，这两种物质在很大的压力和温度范围内都是稳定的。这些相可能作为辅助相出现在地幔组合中，并可能参与金刚石的形成过程。除了冲击陨石中出现的高压氧化铁外，还对从ORapa到前驱物相的亚石圈钻石包裹体中的磁铁矿进行了结构研究，可能具有Fe4O5或Fe5O6化学计量比。Fe5O6是否可以被认为是一个可行的前驱物相取决于它容纳微量元素的能力，如在钻石中的许多夹杂物中观察到的铬。然而，目前还没有关于Fe_5O_6结构中铬固溶体的程度的信息。目前还不清楚铬的存在是否有助于稳定富镁成分，甚至是其他富镁氧化物相，如镁铁氧体，目前只有间接证据证明它们的稳定性。这项拟议的项目旨在通过实验研究铬在高压铁镁氧化物相中的行为，这可能与地幔有关。重点将放在含铬的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(Mg，Fe2+)3Fe3+2O6中铬的掺入程度。第一部分以预合成的铬尖晶石和氧化物为原料，在14-20 Gpa、1300°C和1500°C的温度下进行了一系列短时间的多顶锤实验。研究Mg3Cr2O6端元的实验也将使潜在的Mg4Cr2O7相的稳定性测试成为可能，其结构目前尚不清楚。实现第二个目标的方法将取决于第一部分的结果，但将涉及使用沿Fe2+3Fe3+2O6-Fe2+3Cr2O6二元化合物以及含镁化合物的实验。通过了解高压和高温下在Fe5O6型结构中掺入铬的系统学知识，我们将能够评估“O6”相是作为包裹体或作为潜在的前驱体相在亚球层金刚石中被发现的一个可行的候选。此外，我们将能够提供含铬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