Polyhedral serpentine grains in CM chondrites

We used high-resolution transmission electron microscopy (HRTEM), electron tomograpy, electron energy-loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS) to investigate the structure and composition of polyhedral serpentine grains that occur in the matrices and fine-grained rims of the Murchison, Mighei, and Cold Bokkeveld CM chondrites. The structure of these grains is similar to terrestrial polygonal serpentine, but the data show that some have spherical or subspherical, rather than cylindrical morphologies. We therefore propose that the term polyhedral rather than polygonal be used to describe this material. EDS shows that the polyhedral grains are rich in Mg with up to 8 atom% Fe. EELS indicates that 70% of the Fe occurs as Fe³⁺. Alteration of cronstedtite on the meteorite parent body under relatively oxidizing conditions is one probable pathway by which the polyhedral material formed. The polyhedral grains are the end-member serpentine in a mineralogic alteration sequence for the CM chondrites.