¹Maxime Pineau,²Maximilien Mathian,^1,3Fabien Baron,¹Benjamin Rondeau,¹Laetitia Le Deit,²Thierry Allard,¹Nicolas Mangold
American Mineralogist 107, 1453-1469 Open Access Link to Article [http://www.minsocam.org/msa/ammin/toc/2022/open_access/AM107P1453.pdf]
¹Laboratoire de Planétologie et Géodynamique, UMR CNRS 6112, Université de Nantes, Université d’Angers, Nantes, France
²Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR CNRS 7590, Sorbonne Université, Paris, France
³Institut de Chimie des Milieux et Matériaux de Poitiers, UMR CNRS 7285, Université de Poitiers, Poitiers, France
Copyright: The Mineralogical Society of America

Kaolinite is an Al-rich phyllosilicate commonly observed on Earth as a product of the chemical
weathering of aluminosilicates. It has also been detected on the martian surface by orbital remote
sensing observations. While the determination of the geological processes of formation of terrestrial
kaolinite (i.e., hydrothermal activity, continental surface weathering, diagenesis) involves the coupling of field observation and multiple laboratory measurements, only geomorphology and associated
minerals are generally available to determine their geological origin on Mars. Kaolinite crystallinity
depends on many physicochemical parameters reflecting its conditions of crystallization. To determine if the near-infrared (NIR) spectral signature of kaolinite enables estimation of its crystallinity
and furthermore if this method can be used to identify the geological processes involved in kaolinite
formation, we carried out an in-depth analysis of NIR spectra of reference terrestrial kaolinites that
formed in various geological contexts. We calculated second and third derivatives for each spectrum
to highlight subtle variations in the spectral properties of kaolinite. This allowed the identification of
27 spectral contributions for the 4500 and 7000 cm−1 Al-OH-related regions of absorption bands. The
position shifts and shape variations of these spectral contributions were intimately linked to variations
of crystallinity, which was qualitatively estimated using Hinckley and Liétard XRD (dis)order indices.
The results obtained show that the NIR signature of kaolinite is influenced by the stacking disorder of
layers that has some influence on the vibrations of the interfoliar and inner Al-OH groups. Our study
also confirms that: (1) well-ordered kaolinites are not restricted to hydrothermal deposits; (2) kaolinites
from a similar sedimentary or pedogenetic context often display contrasting degrees of crystalline
order; and (3) poorly ordered kaolinites are more likely to have a sedimentary or pedogenetic origin.
Finally, this work highlights that obtaining spectra with sufficient spectral resolution could help to
estimate the crystallinity of kaolinite and, in the best cases, its geological origin, both on Earth and
Mars, especially with in situ NIR measurements.

^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.