Experimental study in the system albite-andalusite-quartz-NaCl-HCl-H 2O at 600°C and 400 to 2000 bars

Abstract

The composition of aqueous fluid equilibrated with a mineral assemblage of quartz, albite, and andalusite was experimentally obtained at 600°C, with pressure ranging from 400 to 2000 bars and NaCl concentrations ranging from 0.05 to almost 100 m. This allowed us to determine 1. (1) the pressure dependence of the apparent equilibrium constant (Ka) of the following reaction under critical and subcritical conditions, 1 2 Al 2SiO 5 + 5 2 SiO 2 + 1 2 H 2O + NaCl = NaAlSi 3O 8 + HCl 2. (2) the composition of coexisting vapor and liquid phases in the NaCl-HCl-H 2O system. The Ka was obtained as the HCl NaCl concentration ratio in dilute fluid (〈 2 m). The equilibrium constant ( K) of the reaction, which is equivalent to the Ka with an assumption of a unit activity coefficient ratio of HCl and NaCl, varies from 0.013 at 2000 bars to 1.7 at 400 bars with ( ∂ lnK/ ∂P) being greater at lower pressure. The large pressure dependence of the equilibrium constant is attributed to the large difference in partial molar volumes of HCl and NaCl in the aqueous phase at high temperature and low pressure. The compositions of the coexisting vapor and liquid phases are graphically estimated on a diagram showing concentrations of NaCl and HCl by regarding the above reaction as a buffer reaction. The concentration ratio of HCl in vapor to liquid phase ( m HCl vapor m HCl liquid ) ranges from 1.5 at 1000 bars to 7 at 400 bars. The addition of HCl into the NaCl-H 2O system increases the critical pressure and also increases the concentration of NaCl in the vapor phase at low pressure. These results indicate that the effects of pressure are comparable but inverse to temperature effects and must be considered in modeling the geochemical structure of hydrothermal systems.