Oxygen vacancy ordering in aluminous bridgmanite in the Earth’s lower mantle


In this study, the MgAlO2.5 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). Five-fold coordinated Al is formed by single-oxygen vacancy sites but four-fold coordination must result from short range ordering of oxygen vacancies, producing oxygen-vacancy clusters (Fig. 2). Characterizing the occurrence of such oxygen-vacancy structures is an important 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).

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