A new (Mg0.5Fe0.53+)(Si0.5Al0.53+)O3 LiNbO3-type phase synthesized at lower mantle conditions

Abstract

Abstract A new (Mg0.5Fe0.53+)(Si0.5Al0.53+)O3 LiNbO3-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) Å, c = 12.898(2) Å, and V = 265.14(8) Å3. Fe3+ and Al3+ cations substitute into A (Mg2+) and B (Si4+) sites through charge-coupled substitution mechanism, respectively. The distortion of BO6 (B = Si0.5Al0.53+) octahedra is 1.6 times higher than that of AO6 (A = Mg0.5Fe0.53+) 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 FeAlO3 component (50 mol%) beyond previously reported solubility limit (37 mol%). The present study shows that the Earth's most abundant elements form a new Fe3+- and Al3+-rich LiNbO3-type compound from bridgmanite at lower mantle conditions. This new compound provides a new insight into the complicated crystal chemistry of LiNbO3-type phase/bridgmanite and constrains the pressure and temperature conditions for shocked meteorites.