Coordination changes in magnesium silicate glasses

Glasses made from the magnesium silicate minerals enstatite (MgSiO3) and forsterite (Mg2SiO4) and three intermediate compositions can be considered as analogues of quenched melts from the Earth and Lunar mantle. Combined neutron and X-ray diffraction data show an abrupt change in glass structure in the narrow compositional range 38% SiO2 to 33% SiO2 (Mg2SiO4). These structural changes reflect a change from a glass characterized by corner-shared SiO4 tetrahedra and an approximately equal mixture of MgO4 and MgO5 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. The change in structure is associated with a discontinuous change in the rheological properties of these glass-forming liquids close to the forsterite composition.