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

• Published in: The American mineralogist Vol. 92; no. 10; pp. 1545 - 1549
• Main Authors: Miyajima, Nobuyoshi, El Goresy, Ahmed, Dupas-Bruzek, Catherine, Seifert, Friedrich, Rubie, David C, Chen Ming, Chen Ming, Xie Xiande, Xie Xiande
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.10.2007; De Gruyter; Walter de Gruyter GmbH
• Subjects: akimotoite; Cations; chondrites; coexisting minerals; electron microscopy; electron microscopy data; Fe-bearing silicates; ferric iron; Ferrites; garnet group; high pressure; ilmenite; Iron; L chondrites; L-6 chondrite; majorite; melting; metals; metamorphism; Meteorite; meteorites; meteorites and tektites; mineral assemblages; Minerals; nesosilicates; Nickel; olivine group; ordinary chondrites; orthosilicates; oxides; Petrology; pressure; ringwoodite; SEM data; shock metamorphism; shock-melt vein; Silica; Silicates; Sixiangkou Meteorite; Spectrum analysis; stony meteorites; valency; Veins (geology)
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am.2007.2546

We report evidence for high ferric iron to total iron (Fe3+/ΣFe) 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+/ΣFe 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+/ΣFe 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.