The solution behavior of H2O in peralkaline aluminosilicate melts at high pressure with implications for properties of hydrous melts

Solubility and solution mechanisms of H2O in depolymerized melts in the system Na2O-Al2O3-SiO2 were deduced from spectroscopic data of glasses quenched from melts at 1100 degrees C at 0.8-2.0 GPa. Data were obtained along a join with fixed nominal NBO/T = 0.5 of the anhydrous materials [Na2Si4O9-Na-2(NaAl)(4)O-9] with Al/(Al+Si) = 0.00-0.25. The H2O solubility was fitted to the expression, X-H20 = 0.20 + 0.0020f(H2O) - 0.7X(Al) + 0.9(X-Al)(2), where X-H2O is the mole fraction of H2O (calculated with O = 1), f(H2O) the fugacity of H2O, and X-Al = Al/(Al+Si). Partial molar volume of H2O in the melts, V-H2O(melt), calculated from the H2O-solulbility data assuming ideal mixing of melt-H2O solutions, is 12.5 cm(3)/mol for Al-free melts and decreases linearly to 8.9 cm(3)/mol for melts with Al/(Al+Si) similar to 0.25. However, if recent suggestion that V-H2O(melt) is composition-independent is applied to constrain activity-composition relations of the hydrous melts, the activity coefficient of H2O, gamma(melt)(H2O), increases with Al/(Al+Si).