Abstract

A new Ti-bearing bridgmanite-like phase with a threefold commensurate superstructure of the ideal MgSiO3-perovskite structure was observed in a [Mg5/6Al1/6][Si1/2Ti1/3Al1/6]O3 crystal synthesized in the model system Mg3Al2Si3O12–MgTiO3 at 20 GPa and 1600 °C. The compound was found to be orthorhombic, space group Pnma, with lattice parameters a = 14.767(3), b = 6.958(1), c = 4.812(1) Å, V = 494.4(2) Å3, which represents a 3a × b × c superstructure of the typical Pnma perovskite structure. The structure was refined to R = 0.024 using 846 independent reflections. The superstructure mainly arises from the ordering of titanium in one of the octahedral positions. Crystal-chemical details of the different polyhedra in the superstructure are discussed in comparison to pure MgSiO3. This is the first documented superstructure of a bridgmanite phase, and Ti-rich bridgmanite in the lower mantle arising from local Ti-enrichments may exhibit different physical properties and elemental partitioning behavior from Ti-poor, peridotitic bridgmanite. The study also shows that large amounts of Ti can stabilize bridgmanite-like compounds at considerably lower pressure than lower mantle conditions.

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