The system: Two alkali feldspars-KCl-NaCl-H2O at moderate to high temperatures and low pressures

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Because of frequent discrepancies between the available experimental data and the measured composition of alkali chloride aqueous solutions coexisting with two alkali feldspars in high temperatures-low pressures natural systems, a systematic investigation of the system KAlSi3O8-NaAlSi3O8-KCl-NaCl-H2O has been undertaken. Experiments have been carried out at temperatures from 300 °C to 660 °C, pressures from 0.2 to 2 kbar and total chloride concentrations ranging from 0.05 to 14 moles/kg H2O. No effect of pressure on the feldspars solvus could be detected. Smoothing the experimental data on the basis of the regular assymetric solid solution model yields a critical temperature of 661°C and a critical composition of Or0.36Ab0.64. The “equilibrium constant” C = m KCl/m NaCl does not depend on total chloride molality, as long as the aqueous solution is homogeneous. But, in the miscibility gap (liquid+vapour) of the fluid, C is always lower in the vapour than in the liquid. The higher the temperature and the lower the pressure, the more striking this effect. For instance, at 500 ° C∶ C vaqour/C liquid = 1 above 1 kb, 0.9 at 600 bars, 0.8 at 500 bars, 0.7 at 400–450 bars. The effect of pressure can be neglected in homogeneous fluids and in the liquid phase of unmixed fluids, but it is very important in the vapour phase (dilute solutions at low pressure). The selected values of C max are (±0.01)∶ 300 ° C∶0.083; 400 ° C∶0.139; 500 ° C∶0.200; 600 ° C∶0.264; 650 ° C∶0.298 Such a behaviour of the fluid at low pressures explains the abnormally low values of m KCl/m NaCl measured in many natural hydrothermal systems. A new mechanism of “alkali metasomatism” (especially potassic alterations) is also proposed, taking into account the unmixing of alkali chloride aqueous solutions. This model seems particularly interesting in late magmatic hydrothermal processes, such as those occuring in porphyry type deposits.