Ferric iron in Al-bearing akimotoite coexisting with iron-nickel metal in a shock-melt vein in an L-6 chondrite

Abstract

We report evidence for high ferric iron to total iron (Fe 3+ /∑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 Fe 3+ , with a Fe 3+ /∑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 Fe 2+ rather than Fe 3+ , with Fe 3+ /∑Fe ratios of 0.10(5) and 0.15(5), respectively. We conclude that the higher affinity of Fe 3+ 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) Fe 3+ + VI(B) Al 3+ = VI(A) Mg 2+ + VI(B) Si 4+ in the ABO 3 structural formula of MgSiO 3 -ilmenite under high pressures and temperatures, and operates even at a low oxygen fugacity where Fe-Ni metal is stable.