CRYSTAL-CHEMISTRY OF FORSTERITE - A FIRST-PRINCIPLES STUDY

We present a first-principles study (local density approximation) of t he structural properties of forsterite under pressure. This highly ani sotropic magnesium orthosilicate is the most abundant phase of the Ear th's upper mantle, and its elastic properties determine the rheology o f this region. We perform full structural optimizations and investigat e its compressive behavior up to 25 GPa. We obtain a pressure dependen ce of lattice parameters that agrees well with experiments to 17.2 GPa . We predict that the coordination polyhedra compress essentially isot ropically, and we explain the anisotropy of forsterite in terms of the nonuniform distribution of coordination polyhedra having different bu t nearly uniform compressibilities. In agreement with Brodholt et al. (1996), we do not find theoretical evidence for sudden changes in comp ression mechanisms in this mineral as had been suggested from experime nts. Our results support the hypothesis that such compressive anomalie s are caused by solidification of the pressure medium.