  We report isotopic and microstructural data on five presolar hibonite grains
identified in an acid residue of the Krymka LL3.1 ordinary chondrite. Isotopic
measurements by secondary ion mass spectrometry (SIMS) verified a presolar
circumstellar origin for the grains. Transmission electron microscopy (TEM)
examination of the crystal structure and chemistry of the grains was enabled by
in situ sectioning and lift-out with a focused-ion-beam scanning-electron
microscope. Comparisons of isotopic compositions with models indicate that four
of the five grains formed in low-mass stars that evolved through the
red-giant/asymptotic-giant branches, whereas one grain formed in the ejecta of
a Type II supernova. Selected-area electron-diffraction patterns show that all
grains are single crystals of hibonite. Some grains contain stacking faults and
small spreads in orientation that can be attributed to a combination of growth
defects and mechanical processing by grain-grain collisions. The similar
structure of the supernova grain to those from RGB/AGB stars indicates a
similarity in the formation conditions. Radiation damage, if present, occurs
below our detection limit. Of the five grains we studied, only one has the pure
hibonite composition of CaAl12O19. All others contain minor amounts of Mg, Si,
Ti, and Fe. The microstructural data are generally consistent with theoretical
predictions, which constrain the circumstellar condensation temperature to a
range of 1480 K to 1743 K, assuming a corresponding total gas pressure between
1 x 10-3 and 1 x 10-6 atm. The TEM data were used to develop a calibration for
SIMS determination of Ti contents in oxide grains. Grains with extreme 18O
depletions, indicating deep mixing has occurred in their parent AGB stars, are
slightly Ti-enriched compared to grains from stars without deep mixing, most
likely reflecting differences in grain condensation conditions.
