Stabilities of calcite and aragonite

Abstract

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 CaHCO 3 + in the aqueous phase. The value of the dissociation constant of CaHCO 3 + 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 CO2 values and the bearing of these errors on calculations of K values are discussed.