A ternary expansion of the generalized point approximation is the simplest model that can treat the order-disorder relations, phase equilibria, and solution enthalpy systematics for (Ca,Mg,Fe)CO3 solutions. A minimum of eight interaction parameters are needed to fit experimentally determined phase relations: two for each binary join, plus one each for asymmetry in the Ca-Fe and Ca-Mg joins. The model places limits on both the stable and metastable occurrence of dolomite-ankerite solid solution. Calculated phase relations show that the stability of ordered CaFe(CO3)2 is limited to T < ~450 °C by the breakdown to calcite + siderite solid solutions. This maximum temperature rises with Mg content. Between 1000 and 1200 K, there is an isobaric singularity where tricritical calcite-dolomite-ankerite solid solution coexists with magnesite-siderite solid solution. For still higher temperatures, the stability of ordered dolomite-ankerite solid solution is limited to high Mg contents by the intersection of the order-disorder transition with the two-phase field boundaries for calcite + dolomite-ankerite solid solutions and magnesite-siderite + dolomite-ankerite solid solutions, as predicted by Goldsmith et al. (1962). The maximum temperature for metastability of ordered CaFe(CO3)2 is predicted to be 767 °C, rising more than 300 °C with complete Mg for Fe substitution.

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