Stabilities of calcite and aragonite

A revaluation of the 25° C activity-product constants of calcite (K_C) and aragonite (K_A) was made on the basis of the known solubilities of these phases for which the activity of total dissolved calcium was corrected for the presence of the ion pair CaHCO₃⁺ in the aqueous phase. The value of the dissociation constant of CaHCO₃⁺ was taken to be 10^-1.225±0.02. This value, combined with values of the analytical concentrations in solutions with partial pressure P_CO2 =0.97 atmosphere, leads to K_C=l0^-8.52±0.04 and K_A= 10^-8.36±0.04. Based on these K values, standard free energies of formation of calcite and aragonite were calculated to be -270,144±375 and -269,926±375 calories mole^-1, (-1,130,282±1,569 and -1,129,370±1,569 joules mole^-1), respectively. From the 25°C K values, using appropriate entropy and heat capacity data, values of K_C and K_A were calculated over the temperature range 0° to 200°C. Possible errors in interpretation of measured pH values and inferred P_CO2values and the bearing of these errors on calculations of K values are discussed.