Preparation and properties of layered silica and layered alumino-silica hydrate from natural apophyllite

Crystalline layered silica hydrates were prepared from a natural apophyllite pretreated with 0.1 mol·dm^-3 hydrochloric acid at 5° or 25°C. In these treatments, a product with higher crystallinity was obtained by acidic treatment at a lower temperature. The two silica hydrates had the different local silica structure, and were found to correspond to the silicas reported as an H-apophyllite and a Silica-AP, respectively. The silica hydrates have a fundamental silica sheet structure of apophyllite, and the structure is destroyed by heating above 200°C due to condensation of silanol groups in the interlayer spaces. On the other hand, a crystalline layered alumino-silica hydrate was also obtained from a natural apophyllite pretreated with an aluminum chloride solution. Formation of four different hydrate phases depended mainly upon pH of the reacting solution and/or temperature in the drying process. The introduced aluminum ion was coordinated tetrahedrally and occupied mainly at the interlayer of the silica sheets. On heating, these alumino-silicates maintained their crystal structures up to ca. 400°C. These results suggested that the introduced aluminum ion should prevent the basal silica sheet distance from narrowing and the silanol groups from condensing. In consequence, the sheet structure of these alumino-silicates were more stabilized thermally than that of the silica hydrate.