Mineralogy and texture of Fe-Ni sulfides in CI1 chondrites: Clues to the extent of aqueous alteration on the CI1 parent body

Astrophysique du solideTo better understand the role of aqueous alteration on the CI1 parent body, we have analyzed the texture, composition and mineral associations of iron nickel sulfides in four of the five known CI1 chondrites. The most commonly-occurring sulfide present in the CI1 chondrites is the iron-deficient Fe,Ni sulfide pyrrhotite ([Fe,Ni]$_{1−x}$S), that has a composition close to that of stoichiometric troilite (FeS). Three of the CI1s (Alais, Ivuna and Tonk) also contain pentlandite ([Fe,Ni]$_9S_8$), although pentlandite is a rare phase in Ivuna. Cubanite (CuFe$_2S_3$) was found in both Alais and Ivuna in this study, although it has also been reported in Orgueil (MacDougall and Kerridge, 1977). The pyrrhotite grains in all four chondrites form hexagonal, rectangular or irregular shapes, and show no evidence of Ni or Co zoning. The pyrrhotite grains in Orgueil and Ivuna are, in general, smaller, and show more “corrosions,” or “embayments,” than those in Alais or Tonk. We suggest that the precursor sulfide present in the CI1 chondrites was troilite which, during brecciation and oxidation on the parent body at a temperature of 100°C or less, converted the troilite to magnetite and pyrrhotite with pentlandite inclusions. Subsequently, continued alteration on the parent body removed pentlandite—partially from Alais, Tonk and Ivuna, completely from Orgueil—leaving behind pyrrhotite with spaces (“corrosions”) where the pentlandite had been. Ni derived from the pentlandite was incorporated into ferrihydrite, onto the surface of which the Ni,Na sulfate Ni-bloedite formed. Based on the size and abundant “corrosions” within pyrrhotite grains, combined with observations from other authors, we conclude that Orgueil and Ivuna have undergone a greater degree of alteration than Alais and Tonk. Further work is needed to assess the conditions under which pentlandite would be dissolved preferentially to pyrrhotite, as the study of terrestrial literature indicates that the latter mineral is preferentially removed