Effet de la composition des micas trioctaedriques sur les distributions de Rb et Cs a l'etat de traces

An experimental study is reported on the distributions of trace amounts of Rb and Cs between some trioctahedral micas and a hydrothermal solution at 600°C and 1000 bars. The equilibrium constant K_X^min = (X/K)_aq/(X/K)_min varies considerably with the composition of micas. K_Cs^min is equal to 4.2 for phlogopite of theoretical composition, whereas it becomes equal to 8 for a mica close to eastonite. It reaches up to 17 for a mica in which Mg is largely substituted by Al. In annite, Fe(II) analogue of phlogopite, K_Cs^min becomes equal to 1.8. With Rb, the variations of the equilibrium constant are smaller than with Cs. K_Rb^min is 0.65 for phlogopite and increases up to 1.9 for a more aluminous mica. It is 0.44 for annite.The distributions depend very much on the ratios Mg/Fe and Si/Al of the trioctahedral micas. Consequences in geothermometrical interpretation of alkali trace element distributions in natural rocks are considered.