Experimental study of Li-rich granitic pegmatites: Part III. Thermodynamic implications of the experiments in the Na-Li-Cs system: Consequences for the properties of solutes

Previous experimental studies in the systems albite-petalite-pollucite-quartz-H₂O or albite-spodumene-pollucite-quartz-H₂O, both with NaCl, LiCl, and CsCl in solution, have shown that the minerals form restricted solid solutions and that the composition of the fluid is buffered by the coexisting mineral assemblages. New experiments on the assemblage petalite + pollucite + quartz + chloride solution confirm the internal consistency of the experiments in the whole system of albite-petalite-pollucite-quartz fluid at 450 and 600 °C, 1.5 kbar. The variation in the composition of the fluids with temperature allows us to calculate the difference in entropy terms for the solutes in the fluid: NaCl, LiCl, CsCl. Comparisons among the experiments show that (1) pressure has no influence upon the properties of the fluids, (2) there is no measurable effect of mixing entropies in the fluid, and (3) the standard Gibbs energy of formation of the aqueous species, which is certainly related to the hydration of the species, changes with temperature, in a different way for the different species.