Crystallogenesis of Mixed-Valence Fe-Serpentines: Implications for Their Formation during the Aqueous Alteration of Carbonaceous Chondrites' Parent Body

Abstract:

(Fe³⁺,Fe²⁺)-bearing serpentines close to the ideal endmember cronstedtite (Fe²⁺₂,Fe³⁺) (Si, Fe³⁺) O5 (OH)₄, are major components of CM2-type carbonaceous chondrites. Along with other hydrated minerals, they mostly formed during the first million years of the Solar System by aqueous alteration on the meteorite parent body. These secondary minerals could provide constraints to the processes of alteration. Here we developed a two-step protocol (room temperature gel precipitation / hydrothermal growth, in anoxic conditions) for the synthesis of Fe-serpentines with a controlled Fe²⁺/Fe³⁺ ratio in order to improve our understanding of Fe-serpentines formation. XRD analyses of the gels and electron microscopy (SEM, TEM) suggest the formation of Fe-serpentine seeds at room temperature. These germs have integrated significant amounts (up to 24%) of tetrahedral Fe³⁺, as indicated by Mössbauer spectroscopy. Hydrothermal growth at 60°C yields a clear improvement of crystallinity, further suggesting that Fe-serpentines form at low temperatures, lower than Mg-serpentines. We report that among the samples, whose composition covers the solid solution between greenalite (Fe²⁺-serpentine) and cronstedtite, crystallinity improves with the Fe/Si and the Fe³⁺ content, which respective roles remain to be evaluated. In chondrites’ parent body, Fe is mostly released by the aqueous alteration of metallic alloys and Fe²⁺-bearing anhydrous silicates (mostly olivine and pyroxene) and sulfides. We suggest that the formation of cronstedtite, which is associated with the early stages of parent body alteration, might have been kinetically favored by the oxidation of Fe. This raises the question of the processes involved, in the anoxic chondritic environment.