Le système feldspathique ternaire Na-K-Rb: modélisation et expérimentation

Abstract

It is possible to model the excess molar Gibbs free energy at any point of the ternary system albite-sanidine-rubidium feldspar from a knowledge of the three equilibria between hydrothermal chloride solutions and binary feldspars. The domain of immiscibility determined at 600 °C, 150 MPa by ternary exchange experiments corresponds to an excess energy larger than 3 kJ·mol −1. The composition of the fluid in equilibrium with feldspars is richer in Na than the one-phase feldspar or 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 which seem to correspond to a solvus. Some hypotheses are presented for future studies.