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We report isotopic and microstructural data on five presolar hibonite grainsidentified in an acid residue of the Krymka LL3.1 ordinary chondrite. Isotopicmeasurements by secondary ion mass spectrometry (SIMS) verified a presolarcircumstellar origin for the grains. Transmission electron microscopy (TEM)examination of the crystal structure and chemistry of the grains was enabled byin situ sectioning and lift-out with a focused-ion-beam scanning-electronmicroscope. Comparisons of isotopic compositions with models indicate that fourof the five grains formed in low-mass stars that evolved through thered-giant/asymptotic-giant branches, whereas one grain formed in the ejecta ofa Type II supernova. Selected-area electron-diffraction patterns show that allgrains are single crystals of hibonite. Some grains contain stacking faults andsmall spreads in orientation that can be attributed to a combination of growthdefects and mechanical processing by grain-grain collisions. The similarstructure of the supernova grain to those from RGB/AGB stars indicates asimilarity in the formation conditions. Radiation damage, if present, occursbelow our detection limit. Of the five grains we studied, only one has the purehibonite composition of CaAl12O19. All others contain minor amounts of Mg, Si,Ti, and Fe. The microstructural data are generally consistent with theoreticalpredictions, which constrain the circumstellar condensation temperature to arange of 1480 K to 1743 K, assuming a corresponding total gas pressure between1 x 10-3 and 1 x 10-6 atm. The TEM data were used to develop a calibration forSIMS determination of Ti contents in oxide grains. Grains with extreme 18Odepletions, indicating deep mixing has occurred in their parent AGB stars, areslightly Ti-enriched compared to grains from stars without deep mixing, mostlikely reflecting differences in grain condensation conditions.

A transmission electron microscopy study of presolar hibonite