Cation exchange equilibria between plagioclase and aqueous chloride solution at 600 to 700 degree C and 2 to 5 kbar

Equilibrium conditions for the cation exchange reaction 2NaAlSi (sub 3) O (sub 8) + CaCl (sub 2) = CaAl (sub 2) Si (sub 2) O (sub 8) + 4SiO (sub 2) + 2NaCl between synthetic plagioclase and aqueous chloride solution have been experimentally investigated at temperatures of 600 degrees C and 700 degrees C and pressures of 2 and 5 kbar. Solid reaction products were examined by X-ray diffraction, scanning electron microscopy and microprobe techniques; solution compositions were analysed by atomic absorption spectrometry. Cation distributions do not vary significantly in the P-T range investigated. The plagioclase solid solution is continuous showing positive deviation from ideality. Assuming ideally dilute behaviour of the 2 normal chloride solution, activity-composition relations of the plagioclase crystalline solution have been calculated according to the procedure of Blencoe et al. (1982). The resulting activity coefficients of albite and anorthite endmembers are higher than values calculated from previous cation exchange experiments and from high-temperature calorimetric measurements.