Dissolution enthalpies of magnesian calcites

A series of synthetic and biogenic magnesian calcites was dissolved in weak acetic acid solutions to measure the enthalpies of dissolution at 25 degrees C. For the synthetic phases, heat released was 33.5 kJ/mol for calcite, decreasing to 33 kJ/mol for a phase of 2 mol% MgCO3, and increasing to 35 kJ/mol for a phase of 15 mol%. Values of excess enthalpies, Delta H-xs, calculated using calcite and magnesite end-members, average about 1 kJ/mol for the synthetic phases. Total entropies of solid solution formation, Delta S-SS, also were calculated using available data on Gibbs free energies of formation and these excess enthalpies. Values of Delta S-SS range from -2 J/(mol-K) at 2 mol% to -5 J/(mol-K) at 15 mol% MgCO3. These negative values of Delta H-xs and Delta S-SS suggest that some form of ordering (cation?) is obtained in the synthetic phases, and that vibrational entropies of the solid solution are diminished in comparison to the end-members. In contrast, biogenic samples generally have positive values of Delta H-xs, increasing from +1 kJ/mol at 5 mol% to +3 kJ/mol at 20 mol% MgCO3. Most values of Delta S-SS are equal (within errors) to values expected from configurational enthalpy alone. Thus, in most biogenic materials cation ordering probably is not obtained, and most phases are more typical of equivalent-site solid solutions.