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Polytype distribution of circumstellar silicon
carbide: Microstructural characterization by transmission
electron microscopy
Daulton T. L., Bernatowicz T. J., Lewis R. S., Messenger
S., Stadermann F. J., and Amari S. (2003) Geochimica et
Cosmochimica Acta, 67, No. 24, pp. 4743-4767.
doi:10.1016/S0016-7037(03)00272-2
ABSTRACT
Silicon carbide (SiC) is a particularly interesting
species of presolar grain because it is known to form on the
order of a hundred different polytypes in the laboratory,
and the formation of a particular polytype is sensitive to
growth conditions. Astronomical evidence for the formation
of SiC in expanding circumstellar atmospheres of asymptotic
giant branch (AGB) carbon stars is provided by infrared (IR)
studies. However, identification of the crystallographic
structure of SiC from IR spectra is controversial. Since
>95% of the presolar SiC isolated from meteorites formed
around carbon stars, a determination of the structure of
presolar SiC is, to first order, a direct determination of
the structure of circumstellar SiC. We therefore determined
the polytype distribution of presolar SiC from the Murchison
CM2 carbonaceous meteorite using analytical and
high-resolution transmission electron microscopy (TEM).
High-resolution lattice images and electron diffraction of
508 individual SiC grains demonstrate that only two
polytypes are present, the cubic 3C (beta-SiC) polytype
(79.4% of population by number) and the hexagonal 2H
(alpha-SiC) polytype (2.7%). Intergrowths of these two
polytypes are relatively abundant (17.1%). No other
polytypes were found. A small population of
one-dimensionally disordered SiC grains (0.9%), whose high
density of stacking faults precluded classification as any
polytype, was also observed. The presolar origin of 2H
alpha-SiC is unambiguously established by
tens-of-nanometers-resolution secondary ion mass
spectroscopy (NanoSIMS). Isotopic maps of a
TEM-characterized 2H alpha-SiC grain exhibit non-solar
isotopic compositions of 12C/13C = 64
+ 4 and 14N/15N = 575 +
24. These measurements are consistent with mainstream
presolar SiC thought to originate in the expanding
atmospheres of AGB carbon stars. Equilibrium condensation
calculations together with inferred mineral condensation
sequences predict relatively low SiC condensation
temperatures in carbon stars. The laboratory observed
condensation temperatures of 2H and 3C SiC are generally the
lowest of all SiC polytypes and fall within the predictions
of the equilibrium calculations. These points account for
the occurrence of only 2H and 3C polytypes of SiC in
circumstellar outflows. The 2H and 3C SiC polytypes
presumably condense at different radii (i.e.,
temperatures) in the expanding stellar atmospheres of AGB
carbon stars.
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