High-temperature transformation in kaolinite: the role of the crystallinity and of the firing atmosphere

Two samples of kaolinite of differing crystallinity have been fired at three temperatures (1260°C, 1500°C and 1600°C) in an air, argon or nitrogen atmosphere. XRD analysis of these calcined samples indicates that the content and the cristallinity of cristobalite and orthorhombic mullite depend on two parameters: the “crystallinity” of the raw material and, at a lesser extent, the “firing atmosphere”. Cristobalite, independent of the firing atmosphere, develops at a temperature as low as 1260°C with the poorly crystallized kaolinite. Its formation probably results from the greater thermal stability range of precursors (γ-alumina and amorphous aluminosilicate phase) of high-temperature phases with sudden breakdown of this stability. This phenomenon is more tenuous with the well crystallized kaolinite sample. Then, the disappearance of XRD lines of cristobalite on behalf of an intense background around 2ƒ = 20–22° is probably due to thermal amorphization. It occurs at lower temperature during firing of a well crystallized kaolinite. On the other hand, for both kaolinite samples, mullite crystallizes as an alumina overstoichiometric phase when firing temperature increases and a sensitive decrease of XRD lines of mullite occurs with sample fired at 1600°C in a nitrogen or argon atmosphere.