Polytype Distribution in Presolar SiC: Microstructural Characterization by Transmission Electron Microscopy.

摘要

Presolar dust grains predate the formation of the solar system, originating from circumstellar outflows and supernova ejecta. The two most abundant forms of presolar grains isolated from primitive meteorites are nm- sized diamond and mm to submm-sized SiC. Both are ubiquitous in primitive chondritic meteorites at 300 - 1800 ppm (diamond) and 1 - 28 ppm (SiC). Silicon carbide is particularly interesting because it is known to form hundreds of different polytype structures and the formation of a particular polytype is sensitive to growth conditions. The first astronomical evidence of SiC in dusty envelopes of carbon stars came from a relatively broad 11.3 mm infrared (IR) feature. Later attempts to identify crystallographic structure of circumstellar SiC from IR spectra of carbon stars have generated controversy over the techniques and interpretation of the data. The outstanding question of polytype has bearing on physical conditions, such as temperatures and pressures. at which SIC condense from circumstellar outflows or supernova ejecta. Therefore, using transmission electron microscopy (TEM), we unambiguously determine the distribution of polytypes in presolar SiC grains, isolated by acid dissolution from the Murchison CM2 carbonaceous meteorite.