ENTHALPY OF FORMATION OF SIDERITE AND ITS APPLICATION IN PHASE-EQUILIBRIUM CALCULATION

The enthalpy of formation of siderite at 298 K has been determined fro m transposed temperature drop calorimetric measurements of enthalpies of decomposition of siderite at both 978 and 1075 K in an O2 atmospher e. Synthetic siderite was used for the calorimetric experiments. Entha lpy of formation values from the experimental enthalpies at 978 and 10 75 K are consistent, and the final results is DELTAH(f,298)0 = -750.6 +/- 1.1 kJ/mol. The heat contents of hematite, H978-H298 and H1075-H29 8, determined in the present work, are 1-2 kJ/mol lower than those rep orted in the literature, but the measured heat content of corundum usi ng the same experimental method is in good agreement with the tabulate d values. The consistency of the enthalpy of formation of siderite at 298 K derived from experiments performed at different temperatures ind icates that CO (a potential intermediate product of the decomposition of siderite) is completely oxidized. This study provides a new methodo logy for calorimetric measurements of Fe(II)-bearing carbonates. The e nthalpy of formation of siderite has been used to calculate the univar iant curve for the equilibrium: siderite + hematite = magnetite + CO2. This curve lies about 100-degrees-C lower than that calculated using thermodynamic data from literature and about 70-degrees-C lower than t he recent reversal at 10 kbar (between 863 and 873 K) of Koziol and Ne wton (1993). Some possible reasons for these discrepancies are discuss ed, and the thermodynamic data for phases in this system are reevaluat ed.