composition and isotopic composition of presolar silicates in MET 00426.
Stroud R. M., Floss C., and Stadermann F. J. (2009)
Lunar & Planet. Sci. 40, Abstract #1063.
Other than nanodiamonds, silicates are the most abundant circumstellar
condensate grains found in meteorites and interplanetary dust
particles. Laboratory analyses of these grains can provide
complementary information to telescope-based IR-spectroscopy studies of
grains in circumstellar disks, as well as inform our understanding of
the circumstellar condensation process. The 10-micron region of the IR
spectra of most circumstellar disks appears devoid of sharp features,
which is consistent with a predominance of amorphous grains. However,
crystalline fractions above 50% have been estimated for some disks.
Previous transmission electron microscopy studies of circumstellar
silicates are limited to ~ 18 grains in total, of which 4 were olivine,
1 a metastable grain with a perovskite-like structure and pyroxene
composition, and the remainder amorphous to finely nanocrystalline. The
laboratory analysis and astronomical observations appear to be in
general agreement over the predominance of amorphous grains. However,
further work is needed to understand whether the laboratory data are
biased by choice of host meteorite, targeting of larger grains, or
other effects, and to look for signatures of post-condensation
processes, such as radiation or grain-grain collisions effects. By
performing coordinated analysis of in situ silicates in primitive
meteorites using secondary ion mass spectrometry for isotopic
identification, Auger electron spectroscopy for high-spatial resolution
surface composition measurements, and transmission electron microscopy
(TEM) for structure and cross-section composition measurements, we aim
to address the range of circumstellar dust primary microstructures and
elemental compositions, and any correlation with the type of progenitor
star. Here we report TEM analysis of two grains from the MET 00426 CR3
chondrite, previously characterized by NanoSIMS and Auger spectroscopy.