A transmission electron microscopy study of presolar spinel

The isotopic and microstructural data indicate that the Al-Mg-rich and Fe-Cr-rich grains experienced different condensation and processing histories. The single-crystal, stoichiometric, nearly pure Mg-Al spinels are generally consistent with equilibrium condensation predictions, which constrain their condensation temperatures between 1161 and 1221 K, assuming total gas pressures of 1 × 10−6 and 1 × 10−3 atm, respectively. Minor stacking disorder is observed in one of the Mg-Al spinels and is probably a result of slight perturbations to crystal growth during condensation in the circumstellar environment or of impact-induced sheer strain as a response to grain-grain collisions, which could have occurred in the circumstellar environment, the interstellar medium, or the solar nebula. The minor Ca in one of the Mg-Al spinels suggests back reaction with the circumstellar gas from which it formed. In comparison, the similarly oriented Fe-Cr-rich grains of the Orgueil assemblage and their homogeneous isotopic compositions are consistent with their condensation as a single circumstellar dust particle. However, the Fe-Cr-rich compositions (nearly chromite) are inconsistent with predictions for equilibrium condensation and suggest a complex cooling history that is not possible to precisely constrain.