Enthalpy of Formation of Zircon

Using high‐temperature solution calorimetry in molten 2PbO. B 2 O 3 , the enthalpy of reaction of the formation of zircon, ZrSiO 4 , from its constituent oxides has been determined: Δ r H 977 (ZrSiO 4 ) =−27.9 (± 1.9) kJ/mol. With previously reported data for the heat contents of ZrO 2 , SiO 2 , and ZrSiO 4 and standard‐state enthalpies of formation of ZrO 2 and SiO 2 , we obtain Δ f H ° 298 · (ZrSiO 4 ) =−2034.2 (±3.1) kJ/mol and Δ f G ° 298 (ZrSiO 4 ) =−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.