Evidence of different structures in magnesium silicate liquids: coordination changes in forsterite- to enstatite-composition glasses

Abstract

Combined neutron and X-ray diffraction data show changes in the structure of magnesium silicate glasses, ranging in composition from enstatite (MgSiO 3) to forsterite (Mg 2SiO 4). The most abrupt change occurs in the narrow compositional range 38% SiO 2 to 33% SiO 2 (Mg 2SiO 4). These structural changes reflect a change from a glass characterized by corner shared SiO 4 tetrahedra and an approximately equal mixture of MgO 4 and MgO 5 polyhedra, to one in which the average coordination of magnesium by oxygen is increased from 4.5±0.1 to 5.0±0.1. Both these local environments are very different from that of their crystalline counterparts. Reverse Monte Carlo (RMC) simulation fits to the experimental neutron, and X-ray diffraction data have been performed to extract the partial radial distribution functions for the enstatite and forsterite glasses. The structural changes are interpreted as a change in the underlying configurational landscape and are considered to be a manifestation of similar structural changes in the stable liquid. These configurational changes are associated with a discontinuous change in the rheological properties of these glass-forming liquids close to the forsterite composition and suggest differences in the transport properties of magnesium silicate liquids at high pressure. Preliminary data on compressed magnesium silicate glasses indicates that higher coordinate Mg–O is expected to be more stable with increasing pressure.