P. B. Hostetler
R. M. Siebert
C. L. Christ
1974
<p>A revaluation of the 25° C activity-product constants of calcite (<i>K<sub>C</sub></i>) and aragonite (<i>K<sub>A</sub></i>) 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<sub>3</sub><sup>+</sup> in the aqueous phase. The value of the dissociation constant of CaHCO<sub>3</sub><sup>+</sup> was taken to be 10<sup>-1.225±0.02</sup>. This value, combined with values of the analytical concentrations in solutions with partial pressure P<sub>CO2</sub> =0.97 atmosphere, leads to <i>K<sub>C</sub></i>=l0<sup>-8.52±0.04</sup> and <i>K<sub>A</sub></i>= 10<sup>-8.36±0.04</sup>. Based on these <i>K</i> values, standard free energies of formation of calcite and aragonite were calculated to be -270,144±375 and -269,926±375 calories mole<sup>-1</sup>, (-1,130,282±1,569 and -1,129,370±1,569 joules mole<sup>-1</sup>), respectively. From the 25°C <i>K</i> values, using appropriate entropy and heat capacity data, values of <i>K<sub>C</sub></i> and <i>K<sub>A</sub></i> were calculated over the temperature range 0° to 200°C. Possible errors in interpretation of measured pH values and inferred P<sub>CO2</sub>values and the bearing of these errors on calculations of <i>K</i> values are discussed.</p>
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U.S. Geological Survey
Stabilities of calcite and aragonite
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