Crystallization behaviour and stability of β-cristobalite in the system CaO-Al2O3-SiO2

The temperature stability range of high- and low-temperature phases of cristobalite were investigated by a combinatory study, where X-ray diffraction and microstructural investigations via electron microscopy are compared. Two glass compositions, one within the cristobalite region of the ternary phase diagram CaO-Al<inf>2</inf>O<inf>3</inf>-SiO<inf>2</inf> and one within the mullite region of the same system were synthesized and afterwards heat treated at different temperatures. Both samples show very different microstructures and in addition to cristobalite, also other secondary phases (anorthite and mullite) as well as different crystallization mechanisms. The mullite containing sample shows volume crystallization with tiny mullite and larger cristobalite crystals, whereas the other sample crystallizes from the surface. Dependent from the matrix (and from secondary phases) surrounding the cristobalite crystals, a threshold temperature of 1350 to 1375 °C can be measured. Heat treatments above this temperature lead to a stabilization of the low-temperature phase of cristobalite during cooling to room temperature. Lower temperatures stabilize the high-temperature phase. Scanning as well as transmission electron microscopy in combination with elemental analyses via electron dispersive X-ray spectroscopy show that the compositions of the cristobalite crystals as well as the microstructures change above the threshold temperature, which explains the stabilization of different cristobalite phases in dependence of the temperature of heat treatment.