The structure of MgO-SiO ₂ glasses at elevated pressure

The magnesium silicate system is an important geophysical analogue and neutron diffraction data from glasses formed in this system may also provide an initial framework for understanding the structure-dependent properties of related liquids that are important during planetary formation. Neutron diffraction data collected in situ for a single composition (38mol% SiO ₂) magnesium silicate glass sample shows local changes in structure as pressure is increased from ambient conditions to 8.6GPa at ambient temperature. A method for obtaining the fully corrected, total structure factor, S(Q), has been developed that allows accurate structural characterization as this weakly scattering glass sample is compressed. The measured S(Q) data indicate changes in chemical ordering with pressure and the real-space transforms show an increase in MgO coordination number and a distortion of the local environment around magnesium ions. We have used reverse Monte Carlo methods to compare the high pressure and ambient pressure structures and also compare the high pressure form with a more silica-poor glass (Mg ₂SiO ₄) that represents the approach to a more dense, void-free and topologically ordered structure. The MgO coordination number increases with pressure and we also find that the degree of continuous connectivity of SiO bonds increases via a collapse of interstices.