title: Polytype distribution in circumstellar silicon carbide.
abstract: The inferred crystallographic class of circumstellar silicon carbide based on astronomical infrared spectra is controversial. We have directly determined the polytype distribution of circumstellar SiC from transmission electron microscopy of presolar silicon carbide from the Murchison carbonaceous meteorite. Only two polytypes (of a possible several hundred) were observed: cubic 3C and hexagonal 2H silicon carbide and their intergrowths. We conclude that this structural simplicity is a direct consequence of the low pressures in circumstellar outflows and the corresponding low silicon carbide condensation temperatures.
authors.affiliation: Materials Science Division, Argonne National Laboratory, Argonne IL, 60439-4838, USA. tdaulton@nrlssc.navy.mil
authors.forename: T L
authors.initials: TL
authors.lastname: Daulton
authors.forename: T J
authors.initials: TJ
authors.lastname: Bernatowicz
authors.forename: R S
authors.initials: RS
authors.lastname: Lewis
authors.forename: S
authors.initials: S
authors.lastname: Messenger
authors.forename: F J
authors.initials: FJ
authors.lastname: Stadermann
authors.forename: S
authors.initials: S
authors.lastname: Amari
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D017554:Carbon Compounds, Inorganic / Q000032:analysis*; D019110:Meteoroids; D008854:Microscopy, Electron; D011312:Pressure; D017655:Silicon Compounds / Q000032:analysis*; D013696:Temperature
chemical_list: D017554:Carbon Compounds, Inorganic; D017655:Silicon Compounds; C022088:silicon carbide