Changes in the local environment surrounding magnesium ions in fragile MgO-SiO2 liquids

X-ray diffraction data have been collected in situ for a series of magnesium silicate liquids between and including the two mineral compositions enstatite and forsterite (MgSiO3 and Mg2SiO4). The pair distribution functions obtained from these data show that the local oxygen environment around the magnesium ions contrasts with that found in the equivalent glasses. We identify a transition window between 42 and 38 mole %SiO2 where the average magnesium coordination number changes on cooling from a Mg-O coordination number of ~5.1±0.2 in the liquid state to a value of 4.5±0.1 in the glass. This change in structure reflects a competition between a sparsely polymerised silicate network and a disordered magnesium framework in the liquid, while the silicate network dominates in the glass.