Structure of H 2O-saturated peralkaline aluminosilicate melt and coexisting aluminosilicate-saturated aqueous fluid determined in-situ to 800 °C and ∼800 MPa

Abstract

The structure of H 2O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800 °C) and pressure (up to 796 MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na 2O•4SiO 2–Na 2O•4(NaAl)O 2–H 2O (5 and 10 mol% Al 2O 3, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H 2O at nominal densities (from PVT properties of pure H 2O) of 0.85 g/cm 3 (NA10 experiments) and 0.86 g/cm 3 (NA5 experiments) with the aluminosilicate + H 2O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell. Molecular H 2O (H 2O°) and OH groups that form bonds with cations exist in all three phases. The OH/H 2O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H 2O, f H2O) with (OH/H 2O°) melt > (OH/H 2O°) fluid at all pressures and temperatures. Structural units of Q 3, Q 2, Q 1, and Q 0 type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of Q 0 and Q 1 increases and Q 2 and Q 3 decrease with f H2O. Therefore, the NBO/ T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of f H2O. The NBO/ T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to f H2O. The melt structural data are used to describe relationships between activity of H 2O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Q n -species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H 2O content.