Structural control of the Rb distribution between K-micas and fluid in Brazil pegmatites

A geochemical study of pegmatitic micas from Minas Gerais State in Brazil was performed with an electron microprobe, in order to examine the variations of Rb, K, Al and F contents. It is observed a linear decreasing relationship between the [Rb/K] ratio of the micas and their contents in AlVI. The interpretation is based on the hypothesis that the partition coefficient CRb/Kmica/fl between fluid and mineral does not vary significantly as a function of temperature and pressure in the narrow conditions of crystallization of pegmatites. It is suggested that the relation: CRb/Kmica/fl=0.55X(5-[AlVI]) is of crystallochemical order. Micas with low contents in Al take higher contents in Rb because the potassic sites where Rb enters are larger. This relation gives another way of calculating the values [Rb/K] of the fluids knowing Rb, K, Al and Si in the micas. This crystallochemistry also allows us to foresee a direct correlation between Rb and F in the pegmatitic micas.