Formation of Quartz-Like Compounds on Firing Kaolinite-Magnesia Mixture

Kaolinite mixed with 5 to 15wt% magnesia was fired at 1100°-1250°C and the phase transformation during firing was studied by powder X-ray diffraction analysis. The addition of MgO markedly promoted formation of mullite and cristobalite from kaolinite, and lowered their crystallization temperatures. In the samples containing 10 and 15wt% MgO, a large amount of Mg-Al spinel appeared with increasing firing temperature, and mullite decreased. Formation of the spinel increased the amount of isolated amorphous silica and promoted its crystallization. A quartz-like compound was formed at 1100°-1150°C and the cristobalite formed at 1150°-1250°C.