Experimental Study of Cation-Exchange Equilibria of Solid Solutions of Gallium Feldspars (Na,K)GaSi3O8 with Water-Salt Fluid (NaCl-KCl-H2O) at 550°C and 1.5 Kbar

Data on Na and K distribution between feldspar and solution at 550 degrees C and 1.5 kbar under hydrothermal conditions were obtained in experiments on cation-exchange equilibria of gallium feldspars according to the reaction NaGaSi3O8 + KCl aq = KGaSi3O8 + NaCl aq. The distribution of Na and K between feldspars and fluid is nonideal: in the sodium part of the system, potassium enriches the solution relative to feldspar, then an inversion occurs and, at X-K(Fsp) > 0.8, potassium is redistributed into feldspar relative to the fluid. The existence of a region of immiscibility of the solid solution is shown, the concentration dependences of the Na and K distribution coefficients between feldspar and fluid are determined, and the unit cell parameters of the (Na,K)GaSi3O8 solid solutions were refined. The excess volume of mixing of solid solutions of gallium alkali feldspars is estimated from X-ray diffraction (XRD) data. Correlations between the excess integral energies and mixing volumes with structural parameters were identified. The derived dependences make it possible to derive the values of the energy functions of mixing of solid solutions of framework aluminosilicates from structural parameters.