Thermodynamics and kinetics of the zircon (ZrSiO_4) decomposition and the effect of alkaline - and alkaline earth - oxides

摘要

Based on annealing experiments with single crystals and natural raw materials with known impurities and grain size distribution the temperature of thermal dissociation of ZrSiO_4 has been fixed to 1673 ± 10℃ (T_(eutectic): 1683+ 10℃). ZrSiO_4 decomposes by a solid state reaction segregating SiO_2 forming discrete metastable intermediate phases with increased Si-content. Based on these results the thermodynamic dataset for the ZrO_2-SiO_2 system has been optimized and the phase diagram has been re-calculated. In combination with commercial databases in FACTSage this dataset has been used to calculate natural zircon compositions and, in combination with annealing experiments, to clarify the influence of single impurities in natural zircon raw materials such as rutile, illmenite, magnetite, kyanite, corundum, quartz etc. on the ZrSiO_4-decomposition. In addition the influence of alkaline and alkaline earth oxides on the thermal stability of zircon and the occurring corrosion reactions and mechanisms has been examined. Thus the present investigation provides information on the thermal stability and the application limits of zircon based materials in particular in contact with alkaline and alkaline earth containing environments.