A new (Mg (sub 0.5) Fe (super 3+) (sub 0.53) )(Si (sub 0.5) Al (super 3+) (sub 0.5) )O (sub 3) LiNbO (sub 3) -type phase synthesized at lower mantle conditions

A new (Mg (sub 0.5) Fe (sub 0.5) (super 3+) )(Si (sub 0.5) Al (sub 0.5) (super 3+) )O (sub 3) LiNbO (sub 3) -type phase was synthesized at 27 GPa and 2000 K under highly oxidized conditions using an advanced multi-anvil apparatus. Single crystals for this phase are 0.2-0.3 mm in dimension and maroon in color. They crystallize in a noncentrosymmetric structure with space group R3c and lattice parameters of a = b = 4.8720(6) Aa, c = 12.898(2) Aa, and V = 265.14(8) Aa (super 3) . Fe (super 3+) and Al (super 3+) cations substitute into A (Mg (super 2+) ) and B (Si (super 4+) ) sites through charge-coupled substitution mechanism, respectively. The distortion of BO (sub 6) (B = Si (sub 0.5) Al (sub 0.5) (super 3+) ) octahedra is 1.6 times higher than that of AO (sub 6) (A = Mg (sub 0.5) Fe (sub 0.5) (super 3+) ) octahedra. This phase is probably recovered from bridgmanite at lower-mantle conditions by a diffusionless transition because of the displacement of A cations and distortion of BO6 octahedra on releasing pressure. Bridgmanite can thus contain the FeAlO (sub 3) component (50 mol%) beyond previously reported solubility limit (37 mol%). The present study shows that the Earth's most abundant elements form a new Fe (super 3+) - and Al (super 3+) -rich LiNbO (sub 3) -type compound from bridgmanite at lower mantle conditions. This new compound provides a new insight into the complicated crystal chemistry of LiNbO (sub 3) -type phase/bridgmanite and constrains the pressure and temperature conditions for shocked meteorites.