[SIT20-P05] High-pressure structural investigations for hydrous sodium silicate melts
Keywords:hydrous silicate melt, effect of water, amorphous structure, high pressure and temperature
Here, structures of hydrous Na2O-8/3SiO2 melts with 5 wt% (NS8/3_H5) and with 9 wt% H2O (NS8/3_H9) were determined under high pressure and temperature by 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 PF-AR at Tsukuba, Japan. The experimental P-T conditions were about 1.5–3.0 GPa and 550–970 °C, respectively.
The obtained total structure factors, S(Q) of both liquids show that the position of the first sharp diffraction peaks (FSDP) shift to higher-Q with pressure. This positional shift indicates the intermediate-range order structure of present melts become compact. The reduced radial distribution functions, G(r) indicate different pressure evolutions in the average Si-O bond lengths (rSi-O) depending on their water contents. The rSi-O in the NS8/3_H9 melt shows the monotonic decrease with increasing pressure, however, the rSi-O of the NS8/3_H5 melt hardly changes after its contraction below 1.6 GPa. The structure of the NS8/3_H9 melt is assumed to be slightly more depolymerized than the NS8/3_H5 melt at ambient pressure (Zotov and Keppler, 1998). This difference might be due to a phenomenon that the number of strongly hydrogen-bonded SiOH groups, which are favored in sodium silicate glasses and melts (Xue and Kanzaki, 2004), increase more significantly during pressure-induced depolymerization in the NS8/3_H9 melt than in the NS8/3_H5 melt. This raises Si(-NBO)/Si(-BO) (NBO: non-bridging oxygen, BO: bridging oxygen) ratio, and hence, the rSi-O becomes shorter.