^1,2Venkateswara Rao Manga,^1,2Krishna Muralidharan,^1,2Thomas J. Zega
American Mineralogist 107, 1470 – 1476 Link to Article [http://www.minsocam.org/msa/ammin/toc/2022/Abstracts/AM107P1470.pdf]
¹Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Boulevard, Tucson, Arizona 85721, U.S.A.
²Department of Materials Science and Engineering, University of Arizona, 1235 E. James E. Rogers Way, Tucson, Arizona 85721, U.S.A.
Copyright: The Mineralogical Society of America

We report a first-principles-based thermodynamic investigation of the interplay between cation
inversion and twinning in MgAl2O4 spinel (MAS). We examine the atomic-scale structure of (111)
twins and characterize the local octahedral and tetrahedral distortions. We observe that the asymmetric
nature of polyhedral distortions about the (111) twin plane causes anisotropy in cation inversion energies
near the planar fault. The predicted enthalpies and entropies of inversion reveal that in comparison to
the Kagome layer, the anti-site occupancies of Al and Mg, i.e., cation inversion, on the mixed-cationlayer near the twin boundary are more favorable and stable in the entire range of temperature of twin
stability. Structurally, such a stable inversion is necessitated by the minimization in the polyhedral
distortions, especially by the octahedral distortion, which exhibits a reduction of four orders of magnitude relative to the polyhedra with no inversion. The fundamental understanding obtained on the
thermodynamics of the twin-cation inversion interplay in conjunction with the kinetics of inversion
was used as a basis for developing a thermochronometer for deducing the temperature of twinning
in MAS. This work serves as an important steppingstone for experimental characterization of MAS
structures within a host of Earth and planetary materials. In the case of the latter, our results enable
the use of planar faults, such as twins, as important markers for deducing the physical and chemical
landscape that MAS experienced in its evolution and transport within the solar protoplanetary disk.