The core structure of presolar graphite onions

Of the "presolar particles" extracted from carbonaceous meteorite dissolution residues, i.e., of those particles that show isotopic evidence of formation in the neighborhood of other stars prior to the origin of our solar system, a subset of the onion-like graphite spheres has an interesting concentric graphene core-graphite rim structure. We show here that single graphene sheet defects in the onion cores (e. g., cyclopentane loops) may be observable edge-on by high-resolution electron microscopy. This could allow a better evaluation of models for their formation and in particular could strengthen the possibility that growth of these assemblages proceeds atom by atom with the aid of such in-plane defects, under conditions of growth (e. g., radiation fluxes or grain temperature) that discourage the graphite layering that dominates subsequent formation of the graphite rims.