Microstructural evidence for a disequilibrium condensation origin for hibonite‐spinel inclusions in the ALHA77307 CO3.0 chondrite

Abstract

Two hibonite-spinel inclusions (CAIs 03 and 08) in the ALHA77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique combined with transmission electron microscopy. These hibonite-spinel inclusions are irregularly shaped and porous objects and consist of randomly oriented hibonite laths enclosed by aggregates of spinel with fine-grained perovskite inclusions finally surrounded by a partial rim of diopside. Melilite is an extremely rare phase in this type of CAI and occurs only in one inclusion (CAI 03) as interstitial grains between hibonite laths and on the exterior of the inclusion. The overall petrologic and mineralogical observations suggest that the hibonite-spinel inclusions represent high-temperature condensates from a cooling nebular gas. The textural relationships indicate that hibonite is the first phase to condense, followed by perovskite, spinel, and diopside. Texturally, melilite condensation appears to have occurred after spinel, suggesting that the condensation conditions were far from equilibrium. The crystallographic orientation relationships between hibonite and spinel provide evidence of epitaxial nucleation and growth of spinel on hibonite surfaces, which may have lowered the activation energy for spinel nucleation compared with that of melilite and consequently inhibited melilite condensation. Hibonite contains abundant stacking defects along the (001) plane consisting of different ratios of the spinel and Ca-containing blocks within the ideal hexagonal hibonite structure. This modification of the stacking sequence is likely the result of accommodation of excess Al in the gas into hibonite due to incomplete condensation of corundum from a cooling gas under disequilibrium conditions. We therefore conclude that these two hibonite-spinel inclusions in ALHA77307 formed by high-temperature condensation under disequilibrium conditions.

Understanding the Formation of Defect-Structured Hibonite in Chondritic Meteorites: A FIB/TEM Study

“…These features are comparable to those observed in hibonite from different carbonaceous chondrite groups, and can be interpreted as complex intergrowths of stoichiometric and non-stoichiometric (Mg-enriched and Ti-depleted) hibonites [2][3][4][5][6]. Such defect-structured hibonite appears more stable than the thermodynamically predicted equilibrium assemblages of stoichiometric hibonite with corundum or spinel in high-temperature environment [3].Hibonite consists of spinel blocks alternating with Ca-containing blocks normal to the c axis. There are five Al sites that can be substituted with various amounts of Mg and Ti: three octahedral sites (M1, M4, and M5), a trigonal bipyramidal site (M2), and a tetragonal site (M3).…”

“…Our previous transmission electron microscope (TEM) studies of hibonite in Ca-Al-rich inclusions (CAIs) and their rims revealed the presence of numerous stacking defects along the (001) plane and correlated non-stoichiometry, which are interpreted as complex intergrowths of stoichiometric and non-stoichiometric, Mg-rich hibonites [2,3]. In this study, we present the TEM analyses of hibonite found from carbonaceous chondrites and synthesized from a set of annealing experiments.…”

“…However, the role of Ti-Al substitution in the formation of wider Ca-containing blocks is unclear. The Ti-Al substitution may be not important in the formation of stacking defects in hibonite, but only required for the charge coupled substitution with Mg 2+ (Ti 4+ + Mg 2+ = 2Al 3+ ) [2][3][4][5][6].…”

“…In this study, we present the TEM analyses of hibonite found from carbonaceous chondrites and synthesized from a set of annealing experiments. Our goals were to better understand the effect of Mg and Ti on the formation of stacking defects in hibonite and the site occupancy and substitution mechanism of Mg and Ti in defectstructured hibonite.We studied hibonite from CM, CO, CV carbonaceous chondrites that contains a range of MgO and TiO2 contents up to ~4 wt% and ~8 wt%, respectively, and often shows fine scale zoning [2][3][4]. In addition, our experimental hibonite samples were prepared by reacting 2CaO-Al2O3 eutectic melt ± 5 wt% MgO ± CaTiO3 with a pure alumina crucible at 1530°C for 4 hours or 5 days, followed by air quenching [5,6].…”

Corundum–hibonite inclusions and the environments of high temperature processing in the early Solar System

“…Additionally, the observed planar defects in hibonite are an indicator of the Ca deficiency in hibonite. Stoichiometric, defect-free hibonite consists of a sequence of one spinel block alternating with one Ca-containing block, while defect-rich regions in hibonite contain extra spinel blocks (Schmid and De Jonghe, 1983;Han et al, 2015). Under Ca-depleted conditions, Al in the gas phase may have been incorporated into the hibonite as extra spinel blocks, resulting in the planar defects in ideally structured hibonite.…”