AB-INITIO STUDY OF MGSIO3 C2 E ENSTATITE

We have used a newly developed ab initio constant-pressure molecular d ynamics with variable cell shape technique to investigate the zero tem perature behaviour of high pressure clinoenstatite (MgSiO3-C2/c) from 0 up to 30 GPa. The optimum structure at 8 GPa, as well as structural trends under pressure, compare very well with experimental data. At th is pressure, we find noticeable ''fluctuations'' in the chain configur ation which suggests the structure is on the verge of a mechanical ins tability. Two distinct compressive behaviours then appear: one below a nd another above 8 GPa. This phenomenon may be related to the observed transition to a lower symmetry P2(1)/c phase which involves a reconfi guration of the silicate chains, and suggests that the C2/c structure at low pressures found here, may be an artifact of the dynamical algor ithm which preserves space group in the absence of symmetry breaking f luctuations. Comparison with calculations in other magnesium silicate phases, indicates that the size and shape of the silicate units (isola ted and/or linked tetrahedra and octahedra) are generally well describ ed by the local density approximation; however, the weaker linkages pr ovided by the O-Mg-O bonds, are not as well described. This trend sugg ests that, as in the recently studied case of H2O-ice, the structural properties of more inhomogeneous systems, like enstatite, may be impro ved by using gradient-corrected density functionals.