Enthalpy of Formation of Zircon

Using high-temperature solution calorimetry in molt 2PbO {center dot} B{sub 2}O{sub 3}, the enthalpy of reaction of the formation of zircon, ZrSiO{sub 4}, from its constituent oxides has been determined: {Delta}{sub 4}H{sub 977}(ZrSiO{sub 4}) = {minus}27.9 ({plus minus}1.9) kJ/mol. With previously reported data for the heat contents of ZrO{sub 2} SiO{sub 2} and ZrSiO{sub 4} and standard-state enthalpies of formation of ZrO{sub 2} and SiO{sub 2}, we obtain {Delta}{sub f}H{sub 298}{degrees}. (ZrSiO{sub 4}) = {minus}2034.2 ({plus minus}3.1) kJ/mol and {Delta}{sub t}G{sub 298}{degrees} (ZrSiO{sub 4}) = {minus}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. In this paper the limitations of the data for predicting the breakdown temperature of zircon into its constituent oxides are discussed.