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
Q3,
Q2,
Q1, and
Q0 type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of
Q0 and
Q1 increases and
Q2 and
Q3 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
Qn-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H
2O content.
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