Polytype Distribution in Circumstellar Silicon Carbide

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Science  07 Jun 2002:
Vol. 296, Issue 5574, pp. 1852-1855
DOI: 10.1126/science.1071136

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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.

  • * Present address: Naval Research Laboratory.

  • To whom correspondence should be addressed. E-mail: tdaulton{at}

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