Structure, Elemental Composition and Isotopic Composition of Presolar Silicates in Met

Introduction: Other than nanodiamonds, silicates are the most abundant circumstellar condensate grains found in meteorites and interplanetary dust particles [1]. 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 circumstel-lar 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 [2]. However, crystalline fractions above 50% have been estimated for some disks [3]. Previous transmission electron microscopy studies of circumstellar silicates are limited to ~ 18 grains in total, of which 4 were olivine [4-7], 1 a me-tastable grain with a perovskite-like structure and py-roxene composition [8], and the remainder amorphous to finely nanocrystalline [6, 9-12]. 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 mi-crostructures 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 [13]. Experimental: We targeted three grains identified as isotopically anomalous for focused ion beam lift-out and subsequent TEM study. The isotopic compositions of the grains were measured using the Cameca NanoSIMS at Washington University. Elemental spectra of the surface of these grains were obtained with the Washington University Auger Nanoprobe, and quantified based on sensitivity factors for olivine and pyroxene standards.