The effect of pressure on the elastic properties and seismic anisotropy of diopside and jadeite from atomic scale simulation

The full elastic constants tensors of diopside (CaMgSi2O6) and jadeite (NaAlSi2O6) have been calculated using a planewave and pseudopotentials based implementation of density functional theory within the generalised gradient approximation at pressures between 0 and 20 GPa. Both minerals stiffen over this pressure range with the isotropic average bulk moduli increasing by ~50% and the shear moduli by ~20%. However, in detail the behaviour of the individual elastic constants varies and this drives changes in the anisotropy.Overall, and in contrast to predictions based on the extrapolation of calculations based on inter-atomic potential models, the elastic anisotropy of diopside decreases with increasing pressure. The elastic anisotropy of jadeite increases slightly at low pressure, exhibits a maximum at around 10 GPa and then begins to slowly decrease. Despite the small changes in the total and maximum anisotropy, the shear-wave anisotropy for certain propagation directions vary dramatically with pressure. For example, the anisotropy experienced by a shear-wave propagating in the [010] direction in diopside doubles between 5 and 15 GPa.