Structural Analysis of Silicate Glasses and Melts under High Pressure and High Temperature Conditions Based on Synchrotron X-ray and Neutron Diffraction Experiments

Understanding the structural response of silicate glasses and melts to pressure and composition is crucial for discussing deep magmatism, which plays a central role in geochemical processes throughout Earth’s evolution. To investigate these responses, high pressure X-ray and neutron diffraction experiments were conducted on two samples：one is a basaltic glass, and the other is a Na6Si8O19 melt. In the basaltic glass, compression up to 9 GPa results in an increase in the mean O-O coordination number（CNOO）, while the mean O-O distance（rOO）remains nearly constant. The onset of the increase in CNOO around 2 GPa is interpreted because of elastic softening of the fourfold-coordinated silicate network. Above ～9 GPa, the CNOO reaches a plateau, and rOO begins to decrease. This shift is interpreted as a transition in the compression mechanism from the bending of tetrahedral networks to densification through increased oxygen packing. For the hydrous Na6Si8O19 melts, in-situ neutron experiments revealed that the D-O distance increases with pressure, attributed to the formation of -O-D-O- bridging species. This pressure-induced formation indicates a more rigid incorporation of hydrogen into the melt structure. In addition to the shrinkage of modifier domains, this structural change governs the compression behavior of the hydrous silicate melt. In contrast, the compression of the dry Na6Si8O19 melt is primarily controlled by the bending of Si-O-Si angles.