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In this study, the MgAlO_2.5 components within aluminous bridgmanite samples was characterized using ²⁷Al nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra exhibited six-, five -, and four -fold coordinated Al species in addition to near eight-fold coordinated Al substituting for Mg (Fig. 1).  Single-oxygen vacancy sites form Five-fold coordinated Al, but four-fold coordination must result from the short-range ordering of oxygen vacancies, producing oxygen-vacancy clusters  (Fig. 2). Characterizing the occurrence of such oxygen-vacancy structures is an essential prerequisite for understanding how transport properties change with depth and composition in the lower mantle.

Fig. 1. 2D ²⁷Al satellite-transition MAS spectrum of aluminous bridgmanite, which reveals four distinct Al environments typical for ^IVAl, ^VAl, and ^VIAl in addition to Al on the A site of bridgmanite (^AAl)

Fig. 2. (a-f) Schematic representations of Al defect models within bridgmanite. Examples of potential Al defect structures in bridgmanite for the charge-coupled mechanisms (a, b) and the oxygen vacancy mechanism (c-f).

Grüninger, H., Liu, Z., Siegel, R., Boffa Ballaran, T., Katsura, T., Senker, J. & Frost, D. J. (2019) Oxygen vacancy ordering in aluminous bridgmanite in the Earth’s lower mantle. Geophysical Research Letters, 46: doi.org/10.1029/2019GL083613