Molecular dynamics simulations of seismic discontinuities and phase transitions of MgSi[O.sub.3] from 4 to 6-coordinated silicate via a novel 5-coordinated phase

This paper reports detailed molecular dynamics simulations over a large pressure4emperature range (0-140 GPa and 300-2000 K) in the Earth's mantle starting from the upper mantle phase enstatite MgSi[O.sub.3]. The simulated seismic velocities show several discontinuities corresponding to the phase transitions of enstatite. With increasing pressure, enstatite MgSi[O.sub.3] transforms first to a new novel five-coordinated silicon phase, and then to the lower-mantle perovskite phase involving six-coordinated silicon atoms. The new intermediate phase is crystalline but orientafionally disordered. The calculated seismic velocities and densities across the phase transitions for a pure MgSi[O.sub.3] 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 silicon coordination.