Stabilitäten und Eigenschaften von Cr-haltigen Hochdruck-Oxidphasen mit Fokus auf (Mg,Fe2+)3Fe2O6-Mischkristallen – Hinweise auf mögliche Vorläuferphasen in Einschlüssen in Diamant
Zusammenfassung der Projektergebnisse
In this project, we set out to experimentally determine if the high-pressure post-spinel endmember compositions Fe2+3Cr2O6 and Mg3Cr2O6 are stable, as well as to determine the extent of Cr incorporation in (Mg,Fe2+)3Fe3+2O6 solid solutions. To our surprise, neither Fe2+3Cr2O6 nor Mg3Cr2O6 were found to be stable over a wide range in pressure and temperature where they would be expected based on the known stability of Fe5O6 and related oxide phases. Instead, the assemblages Fe2+2Cr2O5 + wüstite and Mg2Cr2O5 + MgO were stable. In Mg-bearing compositions, traces of MgCr2O4 with a Ca-titanate type (CT) structure appeared at 20 GPa. In terms of the phase stability of Cr-Fe3+ solid solutions, we found that Cr is for all practical purposes excluded from the Fe5O6 structure, with only ~ 6 mol % Fe2+3Cr2O6 capable of being incorporated. In contrast, there is complete solid solution across the Fe2+2Fe3+2O5 – Fe2Cr2O5 binary. This is in spite of the fact that the two endmembers have somewhat different crystal structures (Cmcm, no. 63 for Fe2+2Fe3+2O5 and Pbam, no. 55 for Fe2Cr2O5). Our experiments constrain the Cmcm to Pbam phase transition in Fe2(Fe,Cr)2O5 solid solutions to lie between 47 and 64 mol% Fe2Cr2O5 component. For Mg-bearing compositions, since the Mg3Fe3+2O6 endmember is also not stable (Woodland et al. 2023), we conclude that Mg3(Cr,Fe3+)2O6 solid solutions are also unstable. This was confirmed by several experiments where the assemblage Mg2(Cr,Fe3+)2O5 + MgO appeared instead. Like the Fe2+-bearing system, solid solution is complete across the Mg2(Cr,Fe3+)2O5 join. In one experiment at high temperature (1600 °C) an unquenchable phase appeared along with Mg2(Cr,Fe3+)2O5. Microprobe analysis with a defocussed beam yielded a composition consistent with (Mg3.93,Fe2+0.07)(Cr1.11,Fe3+0.89)O7 stoichiometry. An unquenchable phase with Mg4Fe3+2O7 stoichiometry has been previously proposed in several experimental studies. Our work here indicates significant amounts of Cr can also enter this phase. An experiment at 1100 °C produced the O5 phase together with an eskolaite-hematite solid solution and periclase. In addition to these three phases traces of the CT polymorph of Mg(Cr,Fe)2O4 and a β-(Cr,Fe)OOH phase with space group Pmn21 were also detected by powder XRD and microprobe and then subsequently confirmed using synchrotron radiation and applying the multigrain single-crystal XRD technique. Microprobe analyses from this sample reveal that Cr partitions preferentially into the O5 phase compared to the eskolaite-hematite solid solution. However, the occurrence of the CT-phase and β-(Cr,Fe)OOH indicates that the starting materials reacted at different rates at the start of the experiment, forming local chemical heterogeneities that resulted in different local equilibria being established within the sample. The molar volume of Fe2+2Fe3+2O5 – Fe2+2Cr2O5 solid solutions is quite well behaved across the binary, even though a phase transition occurs in the middle of the join. A weighted fit with a symmetric solution model yields a small interaction parameter of Wv = 0.32(10) cm3 mol-1, along with endmember molar volumes of 53.73(1) cm3 mol-1 for Fe4O5 and 52.79(1) cm3 mol-1 for Fe2Cr2O5. The variation in the unit-cell parameters with composition indicates complex intersite interactions that may include electron hopping between Fe3+ and Fe2+. In Mgbearing compositions, the unit-cell parameters and molar volumes appear to behave close to linearly across the join. Our results are indicative of complete solution in the entire (Mg,Fe2+)2(Cr,Fe3+)2O5 quaternary system. In coexisting wüstite, up to 9 wt% Cr2O3 was found to be incorporated, which translates to 0.11 cations Cr an one oxygen basis. In the Mg-bearing experiments, periclase was generally present as a coexisting phase and contained up to 25 wt. % Cr2O3 (i.e. ~0.16 cations Cr on an one-oxygen basis). Through this study and previous work of our group, we have found a surprising difference in crystal chemical behavior of Fe5O6 and Fe4O5, even though these endmember compositions appear to have similarly large P-T stability fields. The O6 structure has a very restricted ability to accept cations other than Fe2+ and Fe3+. This was documented previously for Mg2+ and now here for Cr3+ and means that the O5 phase has more flexibility in terms of being able to vary its Fe3+/∑Fe and therefore to remain stable over a range of oxygen fugacity. This would imply that the O5 phase is generally of much more relevance to the mantle assemblage than the O6 phase. An exception would be in those local environments that are anomalously Fe-rich, where Fe5O6 itself could be stable.
Projektbezogene Publikationen (Auswahl)
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Cr incorporation in post-spinel Fe2+-Fe3+ oxides. 18th International Symposium on Experimental Mineralogy, Petrology and Geochemistry (EMPG-XVIII), 12-15 June 2023, Milano, Italy
Schumann K., Uenver-Thiele L., Woodland A. B., Melai C. & Boffa Ballaran T.
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Cr incorporation in post-spinel Mg-Fe3+ oxides. 18th International Symposium on Experimental Mineralogy, Petrology and Geochemistry (EMPG-XVIII), 12-15 June 2023, Milano, Italy
Rosbach K., Woodland A. B., Uenver-Thiele L., Melai C. & Boffa Ballaran T.
