Effect of temperature on hydrothermal synthesis of analcime and viséite

The zeolite minerals analcime (Na₁₆(Al₁₆Si₃₂O₉₆) · 16H₂O) and viséite (Na₂Ca₁₀(Al₂₀Si₆P₁₀O₆₀(OH)₃₆) · 16H₂O) were synthesized by hydrothermal treatment of artificial glasses of the respective same composition in a temperature range between 80 and 630°C at 100 MPa H₂O pressure. The crystal symmetry of both zeolites varied systematically with temperature under the given experimental conditions, starting from orthorhombic symmetry at low synthesis temperatures, tetragonal at medium leading to cubic symmetry at highest zeolite synthesis temperatures. The crystal sizes varied between 500 nm and 100 μm. In case of analcime water free tectosilicates instead of zeolites were formed at T≥500°C while viséite was formed up to 630°C. The investigations showed the direct correlation between chemical composition of the starting materials and the formed zeolite phase under the given p–T conditions.