Characteristics of the Thermally Decomposed Products from SiO2-Al2O3-H2O Compounds

OKADA Kiyoshi, OTSUKA Nozomu, OSSAKA Joyo

doi:10.2109/jcersj1950.93.1083_671

The evolved phases of spinel-type phase, mullite and cristobalite formed by calcining allophane, a clay mineral mainly composed of SiO₂, Al₂O₃ and H₂O, were characterized by various methods. Spinel-type phase was assigned to be γ-Al₂O₃ from the results of the lattice constants, infrared spectra, specific gravities and alkali extraction. Its chemical composition was considered to be nearly pure Al₂O₃ with little substitution of SiO₂. Mullite formed by calcining at 975°-1050°C contained an excess amount of Al₂O₃ over the composition 3 Al₂O₃⋅2SiO₂. On the other hand, mullite obtained at 1075°-1500°C developed prismatic shaped crystals which grew with increasing calcination temperature, and the composition was nearly 3Al₂O₃⋅2SiO₂. The lattice constants of cristobalite formed in calcined specimens were larger than those of pure ones, suggesting a possibility of substitutions of Al₂O₃ and Fe₂O₃ for cristobalite. More than 2% of Al₂O₃ and more than 1% of Fe₂O₃ are detected on cristobalite particles by an analytical transmission electron microscopy. Allophane was concluded to decompose firstly into γ-Al₂O₃ and amorphous SiO₂ with a little amount of mullite by calcination, which react to form mullite and that the remaining amorphous SiO₂ crystallizes into cristobalite at higher temperature.

Characteristics of the Thermally Decomposed Products from SiO<sub>2</sub>-Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O Compounds

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