The effect of cation-ordering on the elastic properties of majorite: An ab initio study

The effects of cation disorder and pressure on the structural and elastic properties of MgSiO3 majorite (Mj100) and MgSiO3–Mg3Al2Si3O12 solid solution (Py50Mj50) are modelled using the first principle simulations. Our results are consistent with the tetragonal phase as the stable structure for both compositions up to 25 GPa. Both pressure and disorder decrease the differences between c11 and c33, and between c44 and c66, indicating that the elastic properties move closer to cubic. The calculated bulk modulus and shear modulus are comparable with the reported experimental data. The bulk modulus of Mj100 varies little while the shear modulus decreases slightly with increasing cation disorder. The elastic properties of an ordered Py50Mj50 are nearly cubic in symmetry. Mg–Si disorder has much lower energy impact on Py50Mj50 than Mj100. In the Earth's mantle, variation of the Al content in the garnet will more significantly affect the seismic velocities than will the order/disorder of Si and Mg.