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Jangmi Han,Lindsay P. Keller,Ichiro Ohnishi,Akira Yasuhara 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10
Refractory Ca-Al-rich inclusions (CAIs) in primitive meteorites consist of complex assemblages of high-temperature minerals [1], which are predicted to form by gas-solid condensation reactions in the early solar nebula [2]. Their mineralogy and petrography thus provide a window into events that occurred at the birth of our Solar System ~4.567 billion years ago [3]. Hibonite (CaAl12O19) is a common refractory mineral in CAIs [1], which is predicted as the second phase, after corundum, to condense from a cooling gas of solar composition [2]. Transmission electron microscope (TEM) studies have identified enigmatic planar defects in different occurrences of hibonite in the Allende meteorite that give rise to strong streaking along c* in electron diffraction patterns. Atomic resolution high-angle annular dark-field (HAADF) imaging and energy dispersive X-ray (EDX) analyses were used to determine the nature and origin of these planar features [4]. HAADF images of hibonite grains reveal lamellar intergrowths of common 1.6 nm spacing, and less commonly 2.0 nm and 2.5 nm spacings, interspersed in stoichiometric hibonite showing 1.1 nm (002) spacing. Stoichiometric hibonite consists of alternating Ca-containing (“R”) and spinel-structured (“S”) blocks stacked in a sequence RS [5]. In contrast, the 1.6 nm layers result from a doubled S block such that the stacking sequence is RSS, while in the widest defect observed, the stacking sequence is RSSSS. These intergrowths are epitaxial and have coherent, low-strain boundaries with the host hibonite. In addition, atomic resolution EDX maps of hibonite grains in the Allende CAI show that Mg is highly concentrated, but Ti is absent, in the planar defects where wider S blocks show Al-rich compositions compared to stoichiometric MgAl₂O₄ spinel. Therefore, Mg likely played the major role in the formation and metastability of planar defects in hibonite. The formation of planar defects in hibonite occurred during high-temperature condensation or melting/crystallization processes in the early solar nebula. The occurrence of non-stoichiometric hibonite intergrown with Al-rich spinel in the Allende CAI deviates from the mineral formation sequence predicted from equilibrium condensation models, suggesting that these phases formed metastably in place of thermodynamic equilibrium Ca-Al-oxide assemblages. Therefore, our atomic resolution TEM observations signify non-equilibrium, kinetic-controlled crystal growth during high-temperature formation of refractory solids in the early solar nebula [4,6].
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