Enthalpy of formation of siderite and its application in phase equilibrium calculation

The enthalpy of formation of siderite at 298 K has been determined from transposed temperature drop calorimetric measurements of enthalpies of decomposition of siderite at both 978 and 1075 K in an O₂ atmosphere. Enthalpy of formation values from the experimental enthalpies at 978 and 1075 K are consistent, and the final result is ΔH⁰ _f,298 = -750.6 ± 1.1 kJ/mol. The enthalpy of formation of siderite has been used to calculate the univariant curve for the equilibrium: siderite + hematite = magnetite + CO₂. This curve lies about 100°C lower than that calculated using thermodynamic data from literature and about 70°C lower than the recent reversal at 10 kbar of Koziol and Newton (1993). Some possible reasons for these discrepancies are discussed, and the thermodynamic data for phases in this system are reevaluated. -from Authors