Experimental Investigation of Cation Exchange Equilibria of the Gallium Feldspar Solid Solution (K, Rb)GaSi3O8 with KCl–RbCl–H2O Fluids at 550°C and 1.5 kbar with Applications to the Description of the Properties of Solid Solutions with the Feldspar Structure

Feldspars of the KGaSi₃O₈–RbGaSi₃O₈ series were synthesized at 550°C and 1.5 kbar, and their exchange reactions with 1 M KCl + 1 M RbCl solution were investigated. Using the experimental data, Margules parameters were calculated for the description of the excess mixing energies of the (K,Rb)GaSi₃O₈ solid solution: W^G1 = W^G2 = 4.46 ± 0.11 kJ/mol. We refined the unit-cell parameters of the solid solutions; the excess mixing volume is described by the Margules model: W^V1 = 2.647 ± 0.05 and W^V2 = –0.883 ± 0.04 cm³/mol. The results were compared with feldspars studied previously. Empirical relations were proposed for the calculation of the energy Margules model and unit-cell volumes for various minerals with the feldspar structure.