An associated solution model for albite-water melts

An associated solution model linking microscopic and macroscopic data is developed to describe the thermodynamic properties of NaAlSi₃O₈-H₂O melt mixture. The model is based upon the following two homogeneous melt speciation reactions: Na_0.75Al_0.75Si_2.25U₆ ⇔ 0.3Na₂Al₂SiO₆ + 0.15NaAlSi₂O₆ + 0.55Si₃O₆; Na₂Al₂SiO₆ + H₂O ⇔ Na₂Al₂(OH)₂SiO₅, where the first reaction represents the formation of three-membered rings in albite melt and the second one represents hydrolysis of Al-O-Al linkages. The equilibrium constants of these two reactions are calibrated on the basis of available water speciation data, phase equilibrium data, thermodynamic data, solubility data, and P-V-T data. The calculated thermodynamic mixing properties show that relative to our standard states, albite and H₂O melt components exhibit strong negative deviation from ideality. The value for G^ex for any given P and Treaches a minimum at approximately 33 mol% water, which coincides with the composition at which there is a significant change in many microscopic and macroscopic properties in this system. For all P, T conditions, it is calculated that hydroxyl-bearing species dominate over molecular water at low water contents but the latter becomes dominant after the total water content exceeds about 33 mol%. When the total water content in the albite melt increases, the abundance of molecular water increases and that of albite species decreases, while the abundances of both three-membered rings and hydroxyl-bearing species (Na₂Al₂(OH)₂SiO₅) first increase, then decrease. The calculated water solubility is positively correlated with P and negatively correlated with T for pressures lower than approximately 5 kb but positively correlated with T for pressures higher than this, consistent with the behavior suggested by previous workers. The water solubility has been readily modeled by hydrolysis of Al-O-Al linkages in spite of their very small abundance in these melts. The calculated hypersolidus phase relations are consistent with available experimental data. In addition, critical phenomena are predicted within the system, in agreement with the implications of available experimental data.