ENTHALPY OF FORMATION OF ZIRCON

Using high-temperature solution calorimetry in molten 2PbO.B2O3, the enthalpy of reaction of the formation of zircon, ZrSiO4, from its constituent oxides has been determined: DELTA(r)H977(ZrSiO4) = -27.9 (+/-1.9) kJ/mol. With previously reported data for the heat contents of ZrO2, SiO2, and ZrSiO4 and standard-state enthalpies of formation of ZrO2 and SiO2, we obtain DELTA(f)H298-degrees.(ZrSiO4) = -2034.2 (+/-3.1) kJ/mol and DELTA(f)G298-degrees (ZrSiO4) = -1919.8 kJ/mol. The free energy value is in excellent agreement with a range previously estimated from solid-state reaction equilibria. At higher temperature also the data are in close agreement with existing data, though the data sets diverge somewhat with increasing T. The limitations of the data for predicting the breakdown temperature of zircon into its constituent oxides are discussed.