Distribution des éléments Na, K, Rb et Cs à l'état de trace entre feldspaths alcalins et solutions hydrothermales à 650° C, 1 kbar: données expérimentales et interpretation thermodynamique

Abstract

New experimental data on alkali metals (Na, K, Rb and Cs) distribution between alkali feldspars and aqueous vapor phases are presented. All experiments were conducted at 650°C, 1 kbar, in the presence of solutions of 2 molar total alkali chloride concentration. Three types of experiments have been performed: 1. 1. Exchange reactions, involving two elements i and j, between a feldspar solid solution ƒ(ij) and an aqueous vapor phase. Three systems (Na-K, K-Rb and Na-Rb) have been investigated. 2. 2. Distribution of a trace element i (Na, K, Rb or Cs) between a feldspar ƒ(j) (Na, K or Rb) and an aqueous vapor phase. Nine equilibria have been studied. 3. 3. Distribution of a trace element i (Na, Rb or Cs) between a feldspar ƒ(jk) (Na-K or K-Rb) and an aqueous vapor phase. Four equilibria have been studied. A thermodynamic interpretation of the experimental results has been developed. In the binary equilibria of type 2, the trace element distribution depends on a constant related to the thermodynamic properties of the pure phase ƒ(j) and on another term, related to the deviation of the solid solution ƒ(ij) from the ideality. In the ternary equilibria of type 3, theoretical considerations indicate that the trace element distribution cannot be evaluated by a simple extrapolation between the end members. When the excess molal Gibbs free energy of the solid solution ƒ(jk) is strongly positive, the trace element is more easily accepted in the intermediate members of the series than in the end members. The Rb and Cs distributions in the albite-sanidine solid solutions are consistent with this theory. Crystallochemical relationships are established between the thermodynamic properties of the minerals and the ionic radii of their alkali metals.