Work

Chrome incorporation in high-pressure Fe–Mg oxides

Year: 2024

Type: article

Abstract: The occurrence of Cr-bearing oxide phases as inclusions in diamonds and in extraterrestrial materials has the potential to serve as an indicator of formation conditions. However, such an application requires detailed knowledge of phase stabilities and the influence that Cr may have on its stability. To this end, the incorporation of Cr in high-pressure post-spinel Fe–Mg oxide phases was experimentally investigated at pressures of 14–22 GPa and temperatures between 1100 and 1600 °C using a multi-anvil press. We find that neither the Fe 3 Cr 2 O 6 nor the Mg 3 Cr 2 O 6 endmember composition is stable over the expected range of pressure and temperature where Fe 5 O 6 itself is known to be stable. Further experiments along the Fe32+Fe23+O 6 –Fe32+Cr 2 O 6 binary indicate only small amounts of Cr substitution are possible: ∼ 0.12 cations Cr per formula unit or ∼ 6 mol % Fe32+Cr 2 O 6 component. In contrast, complete solid solution is apparent across both the Fe22+(Cr,Fe 3+ ) 2 O 5 and the Mg 2 (Cr,Fe 3+ ) 2 O 5 binaries, and there are indications of complete solution in the entire (Mg,Fe 2+ ) 2 (Cr,Fe 3+ ) 2 O 5 quaternary system. The O 5 -structured phase usually coexists with (Fe,Mg)O. At 16–20 GPa, a post-spinel phase with O 4 stoichiometry was occasionally encountered, having either a modified Ca-ferrite- (mCF-FeCr 2 O 4 ) or a Ca-titanate-type (CT-MgCr 2 O 4 ) structure. In one high-temperature experiment at 1600 °C, an unquenchable Mg-rich phase with a reconstructed Mg 4 Fe23+O 7 stoichiometry occurred together with Mg 2 (Cr,Fe 3+ ) 2 O 5 . In one experiment at 1100 °C and 16 GPa with a bulk composition of Mg 2 (Cr 0.6 ,Fe0.43+) 2 O 5 , an assemblage of O 5 phase + eskolaite–hematite solid solution + periclase was obtained together with minor amounts of the CT-type phase and a β -(Cr,Fe)OOH phase. The occurrence of these two minor phases in this low-temperature experiment is an indicator of variable reaction kinetics amongst the starting materials, which caused chemical heterogeneities to develop at the onset of the experiment. The structural systematics of Fe22+(Cr,Fe 3+ ) 2 O 5 and (Fe 2+ ,Mg) 2 (Cr,Fe 3+ ) 2 O 5 solid solutions were investigated. It is notable that the Fe 3+ and Cr endmembers have somewhat different crystal structures, belonging to space groups Cmcm (no. 63) and Pbam (no. 55), respectively. The phase transition occurs around the midpoint of the Fe 3+ –Cr joins. In spite of complexities in the behavior of the unit-cell parameters, the variation in molar volume with composition deviates only slightly from linearity. Wüstite and periclase coexisting in our experiments reveal the incorporation of up to 9 wt % and 25 wt % Cr 2 O 3 , respectively. This is consistent with the minor Cr contents reported for some ferropericlase inclusions in natural diamond. The limited solubility of other cations in Fe 5 O 6 limits the likelihood of it being an accessory phase in the Earth's deep upper mantle and transition zone, except in Fe-rich environments. In contrast, the O 5 phase appears to be more flexible in accommodating a range of divalent and trivalent cations, suggesting that this phase is more likely to be stabilized, potentially where redox reactions related to diamond formation occur.

Source: European Journal of Mineralogy

DOI: 10.5194/ejm-36-845-2024