UNIFIED DESCRIPTION OF INCONGRUENT REACTIONS AND MINERAL SOLUBILITIES AS A FUNCTION OF BULK COMPOSITION AND SOLUTION PH IN HYDROTHERMAL SYSTEMS

Depiction of mineral solubilities on isobaric isothermal bulk composition - pH diagrams facilitates considerably the correlation of the consequences of irreversible reactions with the solubilities of incongruent reaction products. The reaction-progress variable (xi) is related to the bulk composition variable (eta(c), which has units of moles (kg H2O)-1) by the stoichiometries of the irreversible reactions and the relative rates of reaction of the reactant minerals. To illustrate the advantages of using diagrams of this kind to describe simultaneously mineral solubilities, phase relations, and mass transfer resulting from reaction of minerals with hydrothermal fluids in both open and closed systems, equilibrium solubilities of stable minerals in the system K2O-Na2O-Al2O3-SiO2-H2O were computed as a function of pH in NaCl-HCl-NaOH-H2O solutions in which m(NaCl) = 0.01 or 1.0 at various pressures and temperatures of geological interest. These solubilities were then plotted together with representative reaction paths on eta(Al2Si2O5(OH)4) - pH, eta(KAl3Si3O10(OH)2) - pH, eta(KAlSi3O8) - pH, and eta(NaAlSi3O8) - pH diagrams. Consideration of these diagrams indicates that kaolinite, muscovite, and K-feldspar dissolve congruently only over relatively narrow ranges of solution pH. Above and below the pH limits of these ranges, the minerals dissolve incongruently to form quartz, diaspore, or aluminosilicates. Albite dissolves incongruently over the entire range of solution pH from zero to 12. Changes in solution compositon and the identities and number of moles of minerals that precipitate and dissolve along incongruent reaction paths can be assessed directly from bulk composition - pH diagrams, which can also be used to interpret phase relations in terms of mineral solubilities over wide ranges of pressure, temperature, and fluid composition.