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_rH₉₇₇(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_fH°₂₉₈· (ZrSiO₄) =−2034.2 (±3.1) kJ/mol and Δ_fG°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.