A 29Si MAS NMR spectroscopic study of three samples of MgSiO3 perovskite with 95% isotopically enriched 29Si prepared at 24 GPa and 1800 °C shows the following. (1) There is only one 29Si peak, with a chemical shift of −191.7 ppm. (2) This peak is extremely narrow, with a Lorentzian shape and a full-width at half-height of 19–27 Hz (0.19–0.27 ppm). (3) The static peak shows no singularities attributable to chemical shift anisotropy (CSA). The peak breadth is probably substantially influenced by 29Si dipole-dipole interaction. These results demonstrate that MgSiO3 with the perovskite structure is extremely well ordered and that its Si coordination polyhedron is a very symmetrical octahedron. There is no evidence for Mg-Si disorder, for Si on the central site, or for more than one next-nearest-neighbor Si environment. Thus, configurational entropy does not play a role in stabilizing this phase. The presence of small amounts of an amorphous phase in all our samples suggests that the octahedral Fe detected by Jackson et al. (1987) in an Mg0.88Fe0.12SiO3 perovskite sample may be located in a similar amorphous phase, eliminating the need for charge balancing by eightfold-coordinated Si in perovskite, which we do not detect.

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