Preaccretionary origin of clay minerals in CI chondritic objects? Insights from the Orgueil clay mineralogy and iron oxidation state

Le résumé fourni par la source

Phyllosilicates are the main phases in carbonaceous Ivuna-type (CI) materials and major carriers of water (structural OH) as well as of some organic matter adsorbed on their surfaces. Understanding their formation is therefore crucial for reconstructing the history of water and carbon in the early Solar System. While typically interpreted as products of extensive aqueous alteration under asteroidal conditions, their presence in CI chondrites, whose bulk composition matches that of the solar photosphere, remains a paradox. To investigate their formation processes, we characterized the crystal structure and proportion of phyllosilicates in the Orgueil CI meteorite using X-ray diffraction (XRD) profile modeling on oriented preparations as well as Fe valences by Mössbauer spectroscopy at room and helium temperatures. The XRD modeling confirms the presence of discrete smectite and serpentine, along with two randomly stacked (R0) mixed-layer minerals (MLMs): serpentine-mica (90 %-10 %) and serpentine-smectite-mica (50 %-30 %-20 %), the latter being dominant across all size fractions. Mössbauer spectroscopy reveals that discrete serpentine and the R0 serpentine-mica MLM are Fe²⁺-rich, while the discrete smectite and the R0 serpentine-smectite-mica MLM contain 80 % Fe³⁺. The coexistence of these distinct Fe valences, the unusual serpentine-smectite-mica clay structure, the presence of mica layers within MLMs, and their structural homogeneity across size fractions appear to be incompatible with low-temperature aqueous alteration at equilibrium. Instead, they support a pre-accretionary origin for phyllosilicates, likely involving gas-grain interactions or condensation process.

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