Nonisothermal crystallization kinetics and stability of leucite and kalsilite from K2O-Al2O3-SiO2 glasses

The crystallization mechanisms and elemental stability of leucite and kalsilite formed from K₂O-Al₂O₃-SiO₂ glasses were investigated by X-ray powder diffraction (XRD), X-ray fluorescence (XRF), Raman spectroscopy and differential scanning calorimetry (DSC). Glass samples with compositions along the leucite-kalsilite tie-line were produced by melt processing and were then heat-treated at 850, 950, and 1250°C for times ranging from 5 minutes to 1000 hours. Kalsilite is an unstable phase that behaves as an intermediate precursor to leucite. Crystalline materials in which kalsilite is the major phase lose potassium upon prolonged heat treatment (1000 hours at 1250°C), in contrast to those with leucite, in which little or no compositional alteration is detected. The formation of leucite from stoichiometric kalsilite is accompanied by the formation of potassium-doped alumina. The activation energies for leucite and kalsilite crystallization, determined via application of the Kissinger equation to thermal analysis data, were 579 and 548 kJ/mol, respectively. Finally, production of pure leucite can be achieved with more favorable crystallization kinetics when starting with off-stoichiometric compositions.