Polytype Distribution in Circumstellar Silicon Carbide
T. L. Daulton,12*
T. J. Bernatowicz,3
R. S. Lewis,4
S. Messenger,3
F. J. Stadermann,3
S. Amari3
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.
1 Materials Science Division, Argonne National
Laboratory, Argonne IL, 60439-4838, USA.
2 Marine
Geosciences Division, Naval Research Laboratory, Stennis Space Center,
MS 39529-5004, USA.
3 Laboratory for Space
Sciences, Washington University, St. Louis, MO 63130-4899, USA.
4 Enrico Fermi Institute, University of Chicago,
Chicago, IL 60637-1433, USA.
*
Present address: Naval Research Laboratory.
To whom correspondence should be addressed. E-mail:
tdaulton{at}nrlssc.navy.mil
