K-Na exchange between alkali feldspars and aqueous KCl and NaCl is an important process in many natural hydrothermal systems and proceeds according to the reaction: K A l S i 3 O 8 s + N a C l ( a q ) ↔ N a A l S i 3 O 8 s + K C l ( a q ) Existing experimental calibrations of the equilibrium constant display inconsistencies and in some cases have relied on ambiguous assumptions such as deriving it from experiments with concentrated solutions without accounting for activity coefficients. In order to test the validity of these previous approaches, we conducted an equilibrium experiment at 450 °C and 427 bars (equivalent to a pure water density of 0.31 g cm −3 ) for 16 different alkali chloride molalities ranging from 13 m down to 10 -4 m, each with three different starting compositions to approach equilibrium from different sides. At the lowest concentrations, the aqueous electrolyte activity coefficients appear to approach unity such that we can derive a revised aqueous equilibrium K/Na mole ratio of 0.039 ± 0.001 at infinite dilution and an equilibrium constant of 0.044 ± 0.002, much lower than the previous literature value of 0.170–0.173. Using the revised value, we calculate how the KCl/NaCl activity coefficient ratio changes with chloride concentration. The bulk activity coefficient of KCl is up to 5 times lower than that of NaCl, in contrast to the commonly made assumption of equality to NaCl as the dominant solute. An analysis in terms of the predictive b γ activity model shows that for the most commonly used parameters and assumptions, the Debye-Hückel contribution to the KCl/NaCl activity coefficient ratio predicts the wrong sign. Using such an analysis, the empirical b γ term would no longer be a minor correction or, alternatively, non-unity activity of the neutral ion pairs would have to account for a significant fraction of the needed correction. Another possibly important reason for the dominance of the correction term(s) is that at our nominally supercritical experimental conditions the standard state would refer to the electrolyte in water of vapor-like density while the more concentrated solutions display liquid-like behavior. As additional result, we provide two lines of evidence that for certain concentrations the alkali feldspar-equilibrated fluid is in the two-phase vapor + liquid state at the experimental conditions, slightly below the critical pressure. The vapor phase has a bulk alkali chloride molality of ∽ 1 m with a K/Na mole ratio of 0.15–0.16 while the liquid phase has ∽ 3.6 m with K/Na of ∽ 0.2 ± 0.01. As the experimental trend of K/Na ratios with total chloride concentration displays a non-linear, curved shape, albite can form from K-feldspar by either isothermal dilution of a high-salinity, feldspar-equilibrated brine or by mixing a brine and a vapor phase. These new mechanisms can naturally explain albitization in diverse geological settings, such as Au-rich porphyry Cu, iron oxide copper–gold deposits, and sub-solidus albitization in granitic intrusions.
Roodpeyma et al. (Thu,) studied this question.