Assessment of feldspar solubility constants in water in the range 0° to 350°C at vapor saturation pressures

The equilibrium constants for endmember feldspar hydrolysis for the following reactions NaAlSi₃O₈ + 8H₂O = Na⁺ + Al(OH)^-₄ + 3H₄SiO⁰₄ KAlSi₃O₈ + 8H₂O = K⁺ + Al(OH)^-₄ + 3H₄SiO⁰₄ CaAl₂Si₂O₈ + 8H₂O = Ca⁺² + 2Al(OH)^-₄ + 2H₄SiO⁰₄ are accurately described by log K_low-albite-96.267+305,542/T²-3985.50/T-28.588 · 10^-6 · T²+35.790 · log T log K_high-albite-97.275+306,065/T²-3313.51/T-28.622 · 10^-6 · T²+35.851 · log T log K_microcline-78.594+311,970/T²-6068.38/T-27.776 · 10^-6 · T²+30.308 · log T log K_sanidine-77.837+316,431/T²-5719.10/T-27.712 · 10^-6 · T²+29.738 · log T log K_anorthite-88.591+326,546/T²-2720.61/T-40.100 · 10^-6 · T²+31.168 · log T where T is in K. They are valid in the range 0° to 350°C, at 1 bar below 100°C and at vapor saturation pressures (P_sat) at higher temperatures. The equations for low-albite and microcline are valid for Amelia albite and for microcline with the same degree of Al-Si order (Z = 0.95), respectively. The same kind of log K-temperature equations have also been retrieved for alkali-feldspar and plagioclase solid solutions (tables 4 and 6). The calculated feldspar equilibrium constants of this study indicate much higher solubility under weathering conditions than generally accepted to date, the difference being more than one and a half orders of magnitude at 0°C for both high- and low-albite, but somewhat less for microcline, sanidine, and anorthite. This higher solubility is largely a consequence of new data for the thermodynamic properties of H₄SiO⁰₄. The present results have a particularly important bearing on the stability of feldspars under weathering conditions and the study of interaction between water and rock/soil in this environment They are, however, also important for the study of equilibrium/dis-equilibrium conditions in active geothermal systems.