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

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.