THE TERNARY NA-K-RB FELDSPAR SYSTEM - MODELIZATION AND EXPERIMENTATION

It is possible to model the excess molar Gibbs free energy at any poin t of the ternary system albite-sanidine-rubidium feldspar from a knowl edge of the three equilibria between hydrothermal chloride solutions a nd binary feldspars. The domain of immiscibility determined at 600 deg rees C, 150 MPa by ternary exchange experiments corresponds to an exce ss energy larger than 3 kJ.mol(-1). The composition of the fluid in eq uilibrium with feldspars is richer in Na than the one-phase feldspar o r the mechanical mixture of two feldspars, except near the albite end member, and its K/Rb ratio is always similar to that of the solid. The experiments cannot explain the compositions of natural Rb feldspars w hich seem to correspond to a solvus. Some hypotheses are presented for Future studies. ((C) Academie des sciences / Elsevier, Paris.)