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: Δ_r H ₉₇₇(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 Δ_f H °₂₉₈· (ZrSiO₄) =?2034.2 (±3.1) kJ/mol and Δ_f G °₂₉₈ (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.