Experimental investigation of the reaction: An-rich plagioclase + quartz + fluid = analcime-wairakitess

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^oC. 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 activites led to the formation of mixed analcime-wairakite crystals or analcime crystals. The mechanisms 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