Nucleation and crystal growth of analcime from clear aluminosilicate solutions

Hydrothermal syntheses of the zeolitic mineral analcime have been carried out in clear aluminosilicate solutions with batch composition Al2O3:84SiO(2):87Na(2)O:2560H(2)O in the temperature range 130-160 degrees C and under autogenous pressures. Observation of the changes of the crystal sizes with time indicated that the analcime crystals grew at a constant rate, dependent on the synthesis temperature, and that the rate decreased when the particles settled to the bottom of the autoclave. At that point in the synthesis, a second population of nuclei was observed to form and grow in the solution above the settled crystals. It was demonstrated that when pure forms of silica were used in these syntheses, fewer crystals were formed. These results supported the concept that nucleus formation is promoted by impurities in the silica source.