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Members of the siderite-magnesite series of carbonates have been investigated in nitrogen using differential scanning calorimetry. The mineral specimens contained between 0.3-0.95 mole fraction iron. Decomposition temperatures decreased markedly with increasing Fe substitution. Enthalpies of decomposition showed a linear dependence upon the degree of Fe and Fe + Mn substitution. The fit (R 2 = 0.995) in the case of Fe + Mn suggested that DSC can be used to distinguish members of the series. Decomposition products consisted of substituted iron oxides in most eases.Siderite naturally forms a complete solid solution series with magnesite. Further substitution by Mn 2+ or CaZ+also frequently occurs. The abundance and industrial significance of carbonates containing Fe and Mg has made them the subject of various studies. Different techniques have been applied including X-ray diffraction [1, 2], reflectivity measurements [3] and more extensively infrared spectroscopy [4][5][6][7].Thermal techniques, including thermagnetometry [8] have been used to study the decomposition of siderites in different atmospheres, and the subject has been reviewed [9]. So far differential scanning calorimetry (DSC) has been employed in the decomposition of members of the dolomiteankerite carbonate series [10] and carbonates occurring in coal [11]. The thermal decomposition of Mn-bearing carbonates have also been studied [12] by thermal techniques, including DSC, and compared with the decomposition of dolomite.

The dolomite-ferroan-dolomite-ankerite series of carbonate minerals has been investigated in flowing carbon dioxide using high-temperature DSC. Decomposition products were analysed by X-ray diffraction. The minerals studied included dolomite containing no iron, and members substituted by iron in the molar ratio range of 0.082 to 0.49. Complete resolution of the three main endothermic features was observed and enthalpy values AH, determined from each. The individual and total enthalpy values showed a linear dependence upon the Fe and Mg content of the members across the series. A decrease in the enthalpy of decomposition occurs with increasing Fe content. The effect of Fe substitution was readily observed and the estimated limit of detection is less than 1% FeO. Accuracy of the measurements was limited by the purity of the minerals themselves.

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