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Akimotoite, a MgSiO 3  polymorph present in the lower transition zone within ultramafic portions of subducting slabs and potentially also in the ambient mantle, will partition some amount of Al, raising the question of how this will afect its crystal structure and properties. In this study, a series of samples along the MgSiO 3 -Al 2 O 3  (akimotoite-corundum) solid solution have been investigated by means of single-crystal X‑ray difraction to examine their crystal chemistry. Results show a strong nonlinear behavior of the  a - and  c -axes as a function of Al content, which arises from fundamentally different accommodation mechanisms in the akimotoite and corundum structures. Furthermore, two Al 2 O 3 - bearing akimotoite samples were investigated at high pressure to determine the different compression mechanisms associated with Al substitution. Al 2 O 3 -bearing akimotoite becomes more compressible at least up to 20 mol% Al 2 O 3 , due likely to an increase in compressibility as the Al cation is incorporated into the SiO 6  octahedron. This observation is in strong contrast to the stifer corundum end-member having a  K  T  = 250 GPa, which is larger than that of the akimotoite end-member [ K  T  = 205(1) GPa]. These findings have implications for mineral physics models of elastic properties, which have in the past assumed linear mixing behavior between the MgSiO 3  akimotoite and Al 2 O 3  corundum end-members to calculate sound wave velocities for Al-bearing akimotoite at high pressure and temperature.

The influence of Al 2 O 3 on the structural properties of MgSiO 3 akimotoite