Abstract

We report evidence for high ferric iron to total iron (Fe3+/∑Fe) ratios in Al-bearing akimotoite coexisting with other high-pressure silicates and Fe-Ni metal from shock melt-veins in the Sixiangkou (L-6) chondrite. The measurements were made using electron energy-loss near-edge structure (ELNES) spectroscopy. The results demonstrate that akimotoite in shock-melt veins of this meteorite has high proportions of Fe3+, with a Fe3+/∑Fe ratio of 0.67(3). In contrast, the coexisting majoritic garnet and ringwoodite, which are the typical Fe-bearing phases in shock veins in this meteorite, are enriched in Fe2+ rather than Fe3+, with Fe3+/∑Fe ratios of 0.10(5) and 0.15(5), respectively. We conclude that the higher affinity of Fe3+ for akimotoite, rather than for the other dense silicate phases, is related strongly to the substitution mechanism of trivalent cations. This mechanism is described as VI(A)Fe3+ + VI(B)Al3+ = VI(A)Mg2+ + VI(B)Si4+ in the ABO3 structural formula of MgSiO3-ilmenite under high pressures and temperatures, and operates even at a low oxygen fugacity where Fe-Ni metal is stable.

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