X-ray Studies on Mullite Formed in the Fired Kaolinite

Abstract

Mullite formed in the fired kaolinite was studied by X-ray powder method and was discussed from the structural viewpoints. The formation temperature of mullite is greatly influenced with the time of heat treatment. For the heat treatment of 100 hrs, mullite develops at temperature as low as 900°C. Accurate measurements of the lattice constant of mullites formed at temperatures beween 1100°C and 1480°C were made by slow scanning with silicon as an internal standard. The lattice parameter c decreases very slightly with the increase of firing temperature, and the parameter b increases up to 1200°C and finally obtains almost constant value at or above 1200°C. On the other hand, the parameter a decreases with the increase of temperature, and the largest change is observed in the parameter a among the three parameters a, b and c. These progressive changes of the lattice constant were discussed on the basis of the mechanism of kaolinite-mullite reaction series proposed by Brindley et al. The axial ratio (a/b) of mullites formed from kaolinite increases successively with the decrease of firing temperature. This phenomenon indicates that mullites formed at lower temperatures approach more closely to tetragonal symmetry as compares with mullites formed at higher temperatures. However, in the fired kaolinite, there was not such tetragonal mullite as reported to be formed from coprecipitated Al₂O₃-SiO₂ gels and natural allophanes. It was confirmed that the alumino-silicate formed in the fired kaolinites is not sillimanite but mullite by using the relationship between unit cell volume and cell edge c established by Agrell and Smith.