Abstract:
The reaction An-rich plagioclase + quartz + fluid = analcime (NaAlSi4O12 · 2H2O) - wairakite (CaAl2Si4O12 2H2O ss (solid solution series) has been studied experimentally in the temperature range 350-200°C. Distilled water or 0.1 M NaHCO3 solution was used as fluid. All experiments have been performed under low silica activity. Due to the short duration of the experiments the reaction had not approached equilibrium and partly metastable phases were formed. Wairakite nucleated in the laumontite stability field in the system plagioclase-quartz-H2O. Higher sodium activities led to the formation of mixed analcime-wairakite crystals or analcime crystals. The mechanism of zeolite formation seems to comprise topotaxial growth on the plagioclase surface caused by a dissolution-precipitation process. Some variations in the nucleation and growth kinetics were noticed which appear to be due to different physico-chemical conditions of zeolite formation.