ATOMIC STRUCTURE AND NON-STOICHIOMETRY OF DEFECT-STRUCTURED HIBONITE IN METEORITES

Stoichiometric hibonite (CaAl₁₂O₁₉) consists of spinel blocks alternating with Ca-containing blocks normal to the c axis. There are five Al sites that can be occupied with minor Mg and Ti: three octahedral M1, M4, and M5 sites, a trigonal bipyramidal M2 site, and a tetragonal M3 site. The M1, M3, and M5 sites are arranged in spinel blocks, which are separated by Ca-containing blocks comprising the M2 and M4 sites. However, hibonite in meteorites commonly shows a unique microstructure of numerous defects along the (001) plane, indicative of complex intergrowths of stoichiometric and non-stoichiometric hibonites. Here, we present the results of atomic resolution TEM imaging and X-ray mapping of hibonite in the Allende meteorite to better understand the structure and non-stoichiometry of defect-structured hibonite.

Atomic resolution high angle annular dark-field imaging and X-ray mapping reveal the occurrence of layers having a range of (001) spacings in meteoritic hibonite. Hibonite crystals are dominated by 1.1 nm wide (001) layers that are randomly intergrown with isolated defect layers of 1.6 nm (001) spacing. The common 1.6 nm wide (001) layers are interpreted as wider spinel blocks of non-stoichiometric, Mg-enriched hibonite that contain twice as many M1 and M3 sites and 50% more M5 sites compared to stoichiometric hibonite having 1.1 nm (001) spacing. Mg is concentrated in these defect layers, and occupies the M3 sites and possibly the neighboring M5 sites. In contrast, Ti is concentrated in the M2 sites, but also appears to occupy the M4 sites. In addition to the discovery of an elongated Al-rich spinel inclusion in hibonite, rare 2.0 nm and 2.5 nm wide (001) layers are randomly intergrown within the prominent 1.1 nm wide (001) layers. They lack Ca-containing blocks and are therefore interpreted as distinct lamellae of Al-rich spinel up to 6 unit cells. Collectively, the defect-structured hibonite and Al-rich spinel intergrowths observed in this study imply that both of these phases were metastable, but kinetically stable at high-temperatures.