Compressibility of hydrated and anhydrous sodium silicate-based liquids and glasses, as analogues for natural silicate melts, by Brillouin scattering spectroscopy

A mathematical formalism was tested on compressibility studies of water, before applying it to the high pressure-temperature compressibility studies of hydrated and anhydrous sodium silicate-based liquids and glasses. The hypersonic sound velocity, refractive index and attenuation coefficient obtained using Brillouin light scattering spectroscopy technique were in agreement with literature data. From the measured sound velocities, the pressure dependence of the bulk moduli and density of liquid water were calculated, using Vinet equation of state. The formalism was extended to the Brillouin scattering studies of the elastic properties of alkaline-calcium silica hydrogels and float glass, which exhibits a dramatic increase in the pressure dependence of longitudinal velocity and a discontinuity in the compressibility at about 6 GPa. It is demonstrated that an apparent second-order transition to a new amorphous phase can form via the abrupt onset of a new compressional mechanism, which may be triggered by a shift in polymerization of the glass or an onset of a change in coordination of silicon.; Brillouin scattering measurements were carried out on an aqueous solution of Na2O-2SiO2 and anhydrous Na2O-2SiO 2 glass and liquid at high P-T conditions. The "modified" platelet scattering geometry has allowed a determination of the longitudinal velocity independently from refractive index, and hence the adiabatic compressibility and density of liquids as a function of pressure and temperature. The observed increase in density of the melt and glass phases formed at high P-T conditions is likely associated with structural effects. The large values of KS' of the liquid phase illustrate that the means of compaction of the liquid differs substantially from that of the glass, and that the liquid is able to access a wider range of compaction mechanisms. The measured bulk modulus of Na2O-2SiO2 aqueous solution is closer to values of silicate melts than to that of end-member water at high pressures. Thus, water-rich silica-bearing solutions present at depth are likely to be difficult to seismically distinguish from anhydrous silicate melts based solely upon their sound velocities. The data on water-bearing compounds allow the pressure dependence of the partial molar volume of water to be assessed to upper mantle depths.