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Pyrope, Mg3Al2Si3O12, being an important phase in the interior of the Earth, has been investigated many times. However, there is long-standing disagreement on the behavior of the Mg cation located in the triangular dodecahedron of the garnet crystal structure. Two interpretations have been advanced: (1) that the Mg cation has a large amplitude of thermal vibration (i.e., shows dynamic disorder or a “rattling” motion) and (2) that the Mg cation is statically disordered positionally around the special 24 c Wyckoff position. Nakatsuka et al. (2011) presented X-ray single-crystal diffraction results on pyrope and argued for interpretation (2). However, their proposal is contradicted by a number of published studies, some of which are not cited in their report. The physical behavior of Mg in pyrope can be analyzed using various approaches (details are found in the studies cited below), that is: (1) through X-ray and neutron diffraction, (2) by various spectroscopic measurements, (3) by computational modeling, and (4) using thermodynamic considerations.In terms of diffraction (point 1), a main contention centers on analyzing the behavior (i.e., the mean square displacements) of the Mg cation as described by its large atomic displacement parameters (adp’s) as obtained in structure refinements. It is clear from all the conflicting interpretations in the literature that this is not a simple matter. The key, here, is in making measurements as a function of temperature and evaluating the adp’s in a physically interpretable manner, as attempted by Nakatsuka et al. (2011) and other workers (Armbruster et al. 1992; Armbruster and Geiger 1993; Pavese et al. 1995; Artioli et al. 1997). In short (see Nakatsuka et al. for details), if the adp’s for Mg decrease in magnitude with decreasing temperature and extrapolate close …

american mineralogist

Static disorders of atoms and experimental determination of Debye temperature in pyrope: Low- and high-temperature single-crystal X-ray diffraction study—Discussion