Calorimetric studies of the energetics of order-disorder in the system Mg1-xFexCa(CO3)(2)

Calorimetric studies by Chai and Navrotsky (1996) on dolomite-ankerite ener getics have been extended by including two additional types of samples: a v ery disordered stoichiometric MgCa(CO3)(2) prepared from low-temperature aq ueous solution and three largely ordered natural samples of intermediate ir on content. Combining these data with previous work, we see three distinct energetic tr ends. These represent samples with nearly complete order, nearly complete d isorder, and intermediate order. From these trends, the enthalpy of complet e disordering is estimated to be 33 +/- 6 kJ/mol for MgCa(CO3)(2) and 18 +/ - 5 kJ/mol for FeCa(CO3)(2).