Synthesis, characterization, and enthalpy of mixing of the (Fe,Mg)CO3 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 degrees C and about 2 kilobars. The double capsule method was used to provide the atmosphere necessary to prevent decomposition and oxidation of the Fe2+-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 degrees C. A two step calorimetric method was developed and confirmed to be successful in determining the energetics of the Fe2+ -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, MgFe2O4, and MgO or hematite) were dissolved in lead berate 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 FeCO3-MgCO3 system has been calculated, and the critical temperature for exsolution is about -6 degrees C. Thus, the (Fe,Mg)CO3 solid solution is expected to be complete and nearly ideal in all geological environments.