SYNTHESIS, CHARACTERIZATION, AND ENERGETICS OF SOLID-SOLUTION ALONG THE DOLOMITE-ANKERITE JOIN, AND IMPLICATIONS FOR THE STABILITY OF ORDERED CAFE(CO3)(2)

Samples along the dolomite-ankerite join were synthesized using a pist on-cylinder apparatus and the double-capsule method. Some of the anker ite samples may be disordered. Thermal analysis and X-ray diffraction showed that all samples can be completely decomposed to uniquely defin ed products under calorimetric conditions (770 degrees C, O-2), and a well-constrained thermodynamic cycle was developed to determine the en thalpy of formation. The energetics of ordered and disordered ankerite solid solutions were estimated using data from calorimetry, lattice-e nergy calculations, and phase equilibria. The enthalpies of formation of ordered dolomite and disordered end-member ankerite from binary car bonates, determined by calorimetry, are -9.29 +/- 1.97 and 6.98 +/- 2. 08 kJ/mol, respectively. The enthalpy of formation of ordered ankerite appears to become more endothermic with increasing Fe content, wherea s the enthalpy of formation of disordered ankerite becomes more exothe rmic with increasing Fe content. The enthalpy of disordering in dolomi te (approximately 25 kJ/mol) is much larger than that in pure ankerite , CaFe(CO3)(2) (approximately 10 kJ/mol), which may explain the nonexi stence of ordered CaFe(CO3)(2).