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Polytype distribution in presolar SiC:
microstructural characterization by transmission electron
microscopy
Daulton T. L., Bernatowicz T. J., Lewis R. S.,
Messenger S., Stadermann F. J., and Amari S. (2002) Lunar
and Planetary Science XXXIII, Abstract #1127, Lunar and
Planetary Institute, Houston (CD-ROM).
ABSTRACT
Presolar dust grains predate the formation of the solar
system, originating in circumstellar outflows and supernova
ejecta. Their isotopic compositions are characteristic of
the different nucleosynthetic processes that occurred in
their stellar sources at various stages of stellar
evolution. The two most abundant forms of presolar grains,
isolated from primitive meteorites, are nm-sized diamond
and micrometer- to submicrometer-sized SiC. Both are
ubiquitous in primitive chondritic meteorites at 300 - 1800
ppm (diamond) and 1 - 28 ppm (SiC). SiC is particularly
interesting because, in the laboratory, 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 micrometer
infrared (IR) feature attributed to emission by SiC
particles between the transverse and longitudinal optical
phonon frequencies. Later attempts to identify the
crystallographic structure of circumstellar SiC from IR
spectra have generated considerable controversy over the
techniques and interpretation of the dat. The outstanding
question of polytypes in presolar SiC has bearing on the
physical conditions, such as temperatures and pressures, at
which SiC condense from circumstellar outflows and supernova
ejecta.
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