Cation Disorder and Elasticity in MgSiO3 Akimotoite

The earth's transition zone, which extends from 410 and 660 km depth, is characterized by a complex series of polymorphic phase transformations. Akimotoite (MgSiO₃-ilmenite) is a high pressure polymorph of enstatite that is thought to be important in cold portions of the deep transition zone. The crystal structure of this mineral has not been determined at mantle conditions. Studies of analog materials indicate that akimotoite may exhibit cation disorder in the mantle, a behavior that would be expected to have an important influence on its stability and physical properties. In order to investigate this issue we performed first principle calculations of the R\={3} structure of akimotoite, as observed experimentally at ambient conditions, and a novel structure in which each (001) layer contains equal amounts of Mg and Si. Our studies agree with the experimental finding that the R\={3} structure has the lowest energy at low pressure. However, the energy of one of the cation-exchanged structures approaches that of R\={3} as pressure is increased and becomes more stable at pressure greater than 40 GPa and zero temperature. The novel structure is 4.6% denser, has a 13.6% higher bulk modulus and a 18.5% larger shear moduls at zero pressure as compared to the R\={3} structure. These preliminary results suggest that order-disorder transitions in akimotoite may occur at mantle temperatures and that a disordered structure (R\={3}c) or partially disordered structure may be the stable state within the stability field of akimotoite. This behavior may effect the location of the ilmenite/majorite and ilmenite/perovskite phase transitions and may help to reconcile recent first principles computations of the akimotoite to perovskite phase transformation with experiment.