Bridgmanite-like crystal structure in the novel Ti-rich phase synthesized at transition zone condition

Abstract

A new Ti-bearing bridgmanite-like phase with a threefold commensurate superstructure of the ideal MgSiO 3 -perovskite structure was observed in a [Mg 5/6 Al 1/6 ][Si 1/2 Ti 1/3 Al 1/6 ]O 3 crystal synthesized in the model system Mg 3 Al 2 Si 3 O 12 –MgTiO 3 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) A, V = 494.4(2) A 3 , which represents a 3 a × 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 MgSiO 3 . 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.