Akimotoite, a MgSiO3 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 affect its crystal structure and properties. In this study, a series of samples along the MgSiO3 akimotoite -Al2O3 corundum solid solution have been investigated by means of single-crystal X-ray diffraction in order to examine their crystal chemistry. Results show a strong non-linear behavior of the aand c-axes as a function of Al content, which arises from fundamentally different accommodation mechanisms in the akimotoite and corundum structures. Furthermore, two Al2O3-bearing akimotoite samples were investigated at high pressure in order to determine the different compression mechanisms associated with Al substitution. Al2O3-bearing akimotoite becomes more compressible at least up to a content of 20 mol% Al2O3, due likely to an increase in compressibility as the Al cation is incorporated into the SiO6 octahedron. This observation is in strong contrast to the stiffer corundum end-member having a KT = 250 GPa larger than that of the akimotoite end-member (KT = 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 MgSiO3 akimotoite and Al2O3 corundum end-members in order to calculate sound wave velocities for Al-bearing akimotoite at high pressure and temperature.

Bridgmanite is the most abundant mineral in the Earth's lower mantle. Consequently, it should govern the structure, dynamics, and evolution of the lower mantle. Accordingly, it is important to have the best possible knowledge concerning its chemistry. The major component of bridgmanite is MgSiO 3 . However, it can incorporate Al 3+ , Fe 2+ , and Fe 3+ as secondary elements. Hence, the chemistry of bridgmanite needs to be investigated in the Mg-Fe-Al-Si-O system. Because it too challenging to determine the effect of each individual element on the bridgmanite chemistry when investigating this five-component system, we need to start by investigating bridgmanite in simple systems and then expand to more complex systems. Another essential strategy in our series of investigations is to uniquely control the bridgmanite chemistry via the coexisting phases based on the phase rule. Using these strategies, the phase relations of the MgO-Al 2 O 3 -SiO 2 , MgO-Fe 2 O 3 -SiO 2 , and MgO-FeAlO 3 -SiO 2 systems with specific coexisting phases as a function of pressure and temperature have been investigated in recent studies (