Bridgmanite-like crystal structure in the novel Ti-rich phase synthesized at transition zone condition
Bridgmanite-like crystal structure in the novel Ti-rich phase synthesized at transition zone condition
American Mineralogist (2017) 102 (1): 227-230
- chemical composition
- crystal structure
- electron microscopy data
- electron probe data
- experimental studies
- high pressure
- lower mantle
- mantle
- metals
- partitioning
- perovskite structure
- phase transitions
- polyhedra
- pressure
- refinement
- SEM data
- silicates
- space groups
- synthesis
- synthetic materials
- titanium
- transition zones
- X-ray diffraction data
- bridgmanite
A new Ti-bearing bridgmanite-like phase with a threefold commensurate superstructure of the ideal MgSiO (sub 3) -perovskite structure was observed in a [Mg (sub 5/6) Al (sub 1/6) ][Si (sub 1/2) Ti (sub 1/3) Al (sub 1/6) ]O (sub 3) crystal synthesized in the model system Mg (sub 3) Al (sub 2) Si (sub 3) O (sub 12) -MgTiO (sub 3) at 20 GPa and 1600 degrees 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) Aa, V=494.4(2) Aa (super 3) , which represents a 3aXbXc 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 (sub 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.