[SIT23-P03] Effect of water on the structure of sodium silicate melts under pressure
Keywords:hydrous silicate melt, liquid structure, synchrotron X-ray diffraction, high pressure and temperature
Here, structures of dry Na2O-8/3SiO2 (NS8/3) melts and hydrous ones with 5 wt% (NS8/3-H5) and with 9 wt% H2O (NS8/3-H9) were determined under high pressure and temperature by means of the in-situ X-ray diffraction (XRD). The in-situ XRD study was carried out using a cubic-type multi-anvil apparatus MAX80, which is installed at the AR-NE5C beamline of Photon Factory, KEK at Tsukuba, Japan. The experimental P-T conditions were about 1.4–6.7 GPa and 900–1060 °C, respectively.
The obtained total structure factors, S(Q) of dry NS8/3 and NS8/3-H5 melts show that the position of the first sharp diffraction peaks (FSDP) monotonically shift to higher-Q with pressure. In contrast, the pressure dependence of the FSDP for NS8/3-H9 melt has a minimum around 4 GPa. This minimum indicates the intermediate-range order structure of the melt extends up to 4 GPa and shrinks at higher pressure. The reduced radial distribution functions, G(r) of NS8/3-H9 melt indicate different pressure evolutions in the average Na-O bond lengths, showing a sharp elongation between 3 and 4 GPa. This implies an enlargement of the -Na-O-Na- domain (Greaves 1985; Noritake 2017), and so this elongation can explain the decrease of the FSDP position up to 4 GPa. With increasing pressure,the O-O and Si-Si lengths in NS8/3-H9 melt remains constant, whereas these lengths in dry NS8/3 melt monotonically decrease. The difference in this behavior to the pressure increase can be explained that the Si-O-Si angle dominantly bends in polymerized melts with increasing pressure, while modifier cations mainly work as a cushion and the Si-O-Si angle does not shrink accordingly in depolymerized melts (Wang et al. 2014).