Synthesis, characterization, and enthalpy of mixing of the (Fe,Mg)C0 3 solid solution

Solid solutions along the magnesite-siderite join have been synthesized from mechanical mixtures of iron oxalate and basic magnesium carbonate using hydrothermal synthesis techniques at 500°C and about 2 kilobars. The double capsule method was used to provide the atmosphere necessary to prevent decomposition and oxidation of the Fe 2+-bearing carbonate. The synthetic samples were characterized by x-ray diffraction (XRD) and thermogravimetric analysis (TGA). Unit cell parameters, determined from x-ray diffraction in step scan mode, show a linear relationship with composition. The compositions and homogeneity of the sample were confirmed by electron microprobe analysis. Enthalpy of mixing was measured using a Calvet-type calorimeter at 770°C. A two step calorimetric method was developed and confirmed to be successful in determining the energetics of the Fe 2+-bearing carbonates. The first step involved decomposition of the sample in an oxygen atmosphere, and then the decomposition of the sample in an oxygen atmosphere, and then the decomposition products (a mixture of a spinel phase, MgFe 2O 4, and MgO or hematite) were dissolved in lead borate solvent in air as the second step. The enthalpies of mixing are slightly positive, with a regular solution parameter of 4.44 ± 0.75 kJ/mol. Under the assumption of regular solution, the phase diagram of the FeC0 3MgCO 3 system has been calculated, and the critical temperature for exsolution is about −6°C. Thus, the (Fe,Mg) CO 3 solid solution is expected to be complete and nearly ideal in all geological environments.