Coordination changes in magnesium silicate glasses

Glasses made from the magnesium silicate minerals enstatite (MgSiO 3 ) and forsterite (Mg 2 SiO 4 ) 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% SiO 2 to 33% SiO 2 (Mg 2 SiO 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. The change in structure is associated with a discontinuous change in the rheological properties of these glass-forming liquids close to the forsterite composition.