Interstellar grains in meteorites: I. Isolation of SiC, graphite and diamond; size distributions of SiC and graphite

Abstract

A procedure has been developed for isolating three types of interstellar grains from primitive meteorites, in >90% purity and yields of generally ≥70%, and is here applied to the Murchison C2M meteorite. Silicates are dissolved in HF-HCl, kerogen (macromolecular organic matter) is destroyed by Cr 2O 7 =, KOH, and H 2O 2, and microdiamonds (~400 ppm) are recovered as a colloid. Graphite (<1 ppm) is isolated by density and size separations. Spinel in the residue is dissolved in H 2SO 4, leaving SiC (~6 ppm), hibonite, and corundum. The size distribution of SiC has been measured in the range 0.2 to 6 μm. Over part of this range, it can be fit either to a power-law or a log-normal distribution, but the deficiency of small grains strongly favors the latter. Statistics are more limited for graphite spherules, but they, too, follow a log-normal distribution, and so does interstellar diamond ( Lewis et al., 1989). Apparently the primary condensation process in stellar atmospheres consistently yields a log-normal distribution. The power-law distribution commonly inferred for interstellar grains may have been produced by secondary processes such as fragmentation.