Monday 17 March 2014 - LPSC 2014

Aki Takigawa
Crystal Structure, Morphology, and Isotopic Compositions of Presolar Al203 Grains in Unequilibrated Ordinary Chondrites

"Corundum, the thermodynamically stable phase of Al2O3, is predicted to be the most abundant refractory dust species condensed in envelopes around oxygen-rich asymptotic giant branch (AGB) stars. Infrared spectroscopic observations have revealed that more than 90% of semi-irregular variables and 20% of Mira variables show single peaks at 13 μm and corundum is the most plausible dust species capable of emitting the 13-μm feature. 

Most presolar Al2O3 grains have been identified from acid-residues of chondrites. The grain morphology and crystal structure of presolar grains may reflect condensation conditions in circumstellar envelopes of AGB stars (e.g., a cooling rate and chemical composition of a surrounding gas) and processing in the interstellar medium (ISM) and protosolar disk (e.g., irradiation with accelerated ions by supernova (SN) shocks, grain-grain collisions, and alteration in meteorite parent bodies). Morphologies and crystal structures of a large number of presolar SiC and graphite grains have been investigated by transmission electron microscopy (TEM). TEM studies of presolar oxide grains (hibonite, spinel, and Al2O3) have been much more limited, however, due to their small sizes and low abundance relative to oxide grains with solar isotopic compositions. There are also few studies on the morphology of presolar Al2O3 grains because isotopic measurements used to identify presolar grains alter the grain surfaces. Presolar amorphous Al2O3, corundum, and crystalline Al2O3 with a hexagonal form have been identified but the relationship between interior crystal structure and morphology of presolar Al2O3 grains has not been studied.

Using scanning electron microscopy (SEM) we obtained detailed secondary electron images, energy dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD) patterns, and cathodoluminescence (CL) spectra of each grain prior to isotopic measurements. Focused ion beam (FIB) lift-out sections were made from the identified presolar grains and the interior structures were observed."

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