Diaphite structures of Campo del Cielo cliftonites indicate a terrestrial collision‐induced moderate shock process

Abstract

Cliftonites, polycrystalline aggregates of graphite with unusual cuboid morphology, are important carbon components of certain iron meteorites. Although they consist predominantly of sp²-bonded carbon, recent studies suggest that those from the Canyon Diablo (IAB) meteorite also include composite sp²- and sp³-bonded structures, named diaphites. Here, we investigate the nanostructure of cliftonites in a Campo del Cielo specimen and demonstrate that these cliftonites also contain a nanocomposite mixture of well-ordered 3R graphite regions interfingered with type 1 diaphite structure, consisting of <01–10> projected graphite and <011> projected diamond domains. This finding suggests that certain pieces of the Campo del Cielo meteorite experienced moderate shock pressures (> ~10 GPa), which exceed the 4–10 GPa pressure range previously reported for the main meteorite. We propose that a portion of Campo del Cielo cliftonites provides evidence for the shock-induced diamondization of graphite and the “projectile decapitation” process during terrestrial impact. The complexity of the initial carbonaceous material, combined with the wide range of pressures encountered during terrestrial impact events, may explain the diversity of nanostructures in the Campo del Cielo and Canyon Diablo cliftonites. Our findings could assist in the development of a pressure/shock classification system for characterizing impact events in graphite-bearing meteorites.