| The CM carbonaceous chondrite meteorites formed over 4.5 billion years ago and provide a record of the physical and chemical processes that occurred in the early solar system. The matrix and fine-grained rim (FGR) components, which can constitute more than 50 vol. % of the total material in these meteorites, were profoundly affected by hydration reactions early in their histories. The mechanisms and environment of these reactions are a topic of controversy. Current models include liquid-solid reactions on a parent asteroid and gas-solid reactions in a nebular environment.; Detailed examination of matrix and FGRs is needed to address this controversy. However, their minerals are heterogeneous and intimately mixed at the nanometer scale. Transmission electron microscopy and electron energy-loss spectroscopy were used to obtain information on grain sizes, morphologies, structures, compositions, and oxidation states in the FGRs and matrices of the Murchison, Murray, Mighei, and Cold Bokkeveld CM chondrites. This information was used to determine the minerals characteristic of alteration and to decipher the reactions and conditions responsible for their formation.; This research consists of three investigations focusing on sheet-silicate mineralogy. First, an alteration reaction from cronstedtite to polygonal serpentine and a chrysotile-like phase was inferred in the FGRs of Cold Bokkeveld. The inferred reaction provides a mechanism by which fine-grained material was produced and explains previously reported bulk-compositional trends in the CM chondrites. Second, the ratio of iron (III) to total iron was measured from cronstedtite in the Murchison, Murray, and Cold Bokkeveld CM chondrites, meteorites that respectively experienced mild, moderate, and intense degrees of alteration. The ratios from these meteorites fall within a narrow range, suggesting that they experienced alteration under similar redox conditions. Third, serpentine nanotubes were discovered in the Mighei CM chondrite. Their composition is distinct from previously reported terrestrial and meteoritic materials, and their unique structure potentially served as a container for primordial fluids. Taken as a whole, the data from the three investigations suggest that hydration reactions occurred in asteroidal rather than nebular environments. |
