SYNTHESIS, CHARACTERIZATION, AND ENTHALPY OF MIXING OF THE (FE,MG)CO3SOLID-SOLUTION

Solid solutions along the magnesite-siderite join have been synthesize d from mechanical mixtures of iron oxalate and basic magnesium carbona te using hydrothermal synthesis techniques at 500 degrees C and about 2 kilobars. The double capsule method was used to provide the atmosphe re necessary to prevent decomposition and oxidation of the Fe2+-bearin g carbonate. The synthetic samples were characterized by x-ray diffrac tion (XRD) and thermogravimetric analysis (TGA). Unit cell parameters, determined from x-ray diffraction in step scan mode, show a linear re lationship with composition. The compositions and homogeneity of the s ample were confirmed by electron microprobe analysis. Enthalpy of mixi ng was measured using a Calvet-type calorimeter at 770 degrees C. A tw o step calorimetric method was developed and confirmed to be successfu l in determining the energetics of the Fe2+ -bearing carbonates. The f irst step involved decomposition of the sample in an oxygen atmosphere , and then the decomposition of the sample in an oxygen atmosphere, an d then the decomposition products (a mixture of a spinel phase, MgFe2O 4, and MgO or hematite) were dissolved in lead berate solvent in air a s the second step. The enthalpies of mixing are slightly positive, wit h a regular solution parameter of 4.44 +/-0.75 kJ/mol. Under the assum ption of regular solution, the phase diagram of the FeCO3-MgCO3 system has been calculated, and the critical temperature for exsolution is a bout -6 degrees C. Thus, the (Fe,Mg)CO3 solid solution is expected to be complete and nearly ideal in all geological environments.