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 Tienrichments 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.
Acknowledgments
Thanks are due to Fabrizio Nestola, Ian Swainson, and three anonymous referees for their insightful comments. The research was supported by “progetto di Ateneo 2014, University of Firenze” to L.B., by CNR, Istituto di Geoscienze e Georisorse sezione di Firenze, Italy, and by the Russian Foundation for Basic Research (project nos. 16-05-00419 and 15-05-50033) to E.S. and A.B. E.S. thanks Geodynamics Research Center, Ehime University, Matsuyama, Japan, for support of her visit in 2016.
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