Atomic-scale structure and non-stoichiometry of meteoritic hibonite: A transmissionelectron microscope study

1,2Jangmi Han,3Ichiro Ohnishi,3Akira Yasuhara,2Lindsay P. Keller
American Mineralogist 107 873–884 Link to Article [http://www.minsocam.org/msa/ammin/toc/2022/Abstracts/AM107P0873.pdf]
1Lunar and Planetary Institute, USRA, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A.
2Astromaterials Research and Exploration Science, NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
3JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo, 196-8558, Japan
Copyright: The Mineralogical Society of America

Hibonite (CaAl12O19) is a common refractory mineral in Ca-Al-rich inclusions (CAIs) in primitive
meteorites. 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. HAADF images of hibonite grains reveal lamellar intergrowths of common 1.6 nm spacing,
and less commonly 2.0 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. 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
Meteoritic hibonite shows common Ti and Mg substitution for Al in its structure. Atomic-resolution
EDX maps of hibonite grains in the Allende CAI confirm the preferred site occupancy of Mg on
tetragonal M3 sites in S blocks and of Ti on trigonal bipyramidal M2 and octahedral M4 sites in R
blocks. 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. Electron energy loss spec-
troscopy data from the Ti L2,3 edge show the presence of mixed Ti oxidation states with ~15–20% of Ti
as Ti3+ in hibonite, suggesting a direct substitution of Ti3+ ↔ Al3+ in hibonite. The remaining ~80–85%
of Ti is present as Ti4+ and corresponding EDX analyses are consistent with the well-known coupled
substitution 2Al3+ ↔ Ti4+ + Mg2+ being the major mechanism for Ti and Mg substitution in hibonite.
The formation of planar defects in hibonite occurred during high-temperature nebular condensa-
tion or melting/crystallization processes. The occurrence of non-stoichiometric hibonite in the Allende
CAI deviates from the mineral formation sequence predicted from equilibrium condensation models.
Overall, our atomic resolution TEM observations signify non-equilibrium, kinetic-controlled crystal
growth during the high-temperature formation of refractory solids in the early solar nebula.

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