Molecular dynamics simulations of seismic discontinuities and phase transitions of MgSiO3 from 4 to 6-coordinated silicate via a novel 5-coordinated phase

This paper reports detailed molecular dynamics simulations over a large pre ssure-temperature range (0-140 GPa and 300-2000 K) in the Earth's mantle st arting from the upper mantle phase enstatite MgSiO3. The simulated seismic velocities show several discontinuities corresponding to the phase transiti ons of enstatite. With increasing pressure, enstatite: MgSiO3 transforms fi rst to a new novel five-coordinated silicon phase, and then to the lower-ma ntle perovskite phase involving six-coordinated silicon atoms. The new inte rmediate phase is crystalline but orientationally disordered. The calculate d seismic velocities and densities across the phase transitions for a pure MgSiO3 mantle are consistent with previous estimates. These studies suggest that the major discontinuities between the upper mantle, transition zone, and the lower mantle could arise partially due to the changes in the silico n coordination.