Matrix mineralogy of the Orgueil CI carbonaceous chondrite

A petrographic and transmission electron microscopic study of the Orgueil CI carbonaceous chondrite shows that the matrix consists mainly of Fe-bearing, Mg-rich serpentine and smectite (saponite) as well as a poorly crystallized Fe-rich material that contains minor, variable S and Ni; the Fe-rich material is probably ferrihydrite (a ferric hydroxide). The ferrihydrite occurs in small particles (<80 Å in diameter), and the S and Ni are probably adsorbed on its surfaces. The serpentine and saponite occur in poorly crystallized, fine crystallites ("fine phyllosilicates") that are intimately associated with the ferrihydrite. Coarsely crystallized phyllosilicates ("coarse phyllosilicates") occur in clusters relatively free of ferrihydrite. The ferrihydrite is a major matrix constitutent and probably accounts for most of the Fe³⁺ as well as the superparamagnetic fraction in the Orgueil matrix. The textures suggest that the fine phyllosilicates resulted from alteration of the coarse phyllosilicates. The alteration probably involved a substantial amount of water and coincided with the period of sulfate vein formation. During the aqueous alteration magnetite grains, disaggregated from "framboids", and Fe-(Ni) sulfides were replaced by ferrihydrite. Simultaneously, aqueous solutions reacted with the coarse phyllosilicates and precipitated ferrihydrite, producing intimate, pervasive mixtures with the fine phyllosilicates. The matrix mineralogy of the Orgueil chondrite suggests that Orgueil experienced considerably différent alteration conditions from the CM chondrites. The mineralogical and petrological differences between CI and CM chondrites can not simply be explained as the result of different degrees of aqueous alteration.