Thermal stability of zircon (ZrSiO4)

Based on annealing experiments with natural and synthetic raw materials of known grain size and impurity level as well as single crystals the temperature of thermal dissociation of ZrSiO4 was assessed at 1673 ± 10 °C. ZrSiO4 decomposes by a solid-state reaction releasing SiO2 in form of discrete metastable intermediate phases with superstoichiometric Si-content. The eutectic temperature in the ZrO2–SiO2 system was set to 1687 ± 10 °C. The thermodynamic dataset for the ZrO2–SiO2 system was optimised and the quasi-binary phase diagram was re-calculated. The significant influence of even small amounts of impurities on the decomposition temperature and kinetics is explained by the formation of low melting liquid phases resulting in the favoured dissolution of solid silica immediately after release from the zircon structure.