Solubility product of siderite (FeCO3) as a function of temperature (25–250 °C)

The solubility of natural siderite was investigated from 25 to 250 °C in 0.1 mol kg− 1 NaCl aqueous solutions using a hydrogen-electrode concentration cell, which provided continuous in situ measurement of hydrogen ion molality. Iron(II) was analyzed by a revised Ferrozine-spectrophotometric method. The obtained apparent solubility products (Qsp-siderite) were extrapolated to infinite dilution to generate the solubility products (Ksp°-siderite), allowing calculation of the thermodynamic properties of siderite. Of all the temperature functions tested, the equation giving a reliable fit of our data in the investigated temperature range (25–250 °C) has the following form: log10Ksp°-siderite = a + b ∙ (T / K) + c ∙ (T / K)− 1 + d ∙ log10(T / K) with: a = 175.568, b = 0.0139, c = − 6738.483 and d = − 67.898. Based on this equation and its first and second derivatives with respect to T, we were able to derive values for the Gibbs energy of formation: ΔfG298.15o = (− 680.71 ± 2) kJ mol− 1, enthalpy of formation: ΔfH298.15o = (− 749.59 ± 2) kJ mol− 1, entropy: S298.15o = (109.54 ± 2) J mol− 1 K− 1 and heat capacity: Cp298.15o = (83.26 ± 2) J mol− 1 K− 1 of siderite (uncertainties are 3σ). The values of ΔfG298.15o and ΔfH298.15o are in very good agreement with the values reported by Robie et al. [Robie, R.A., Haselton, H.T. Jr., Hemingway, B.S., 1984. Heat capacities and entropies of rhodochrosite (MnCO3) and siderite (FeCO3) between 5 and 600 K. Am. Mineral. 69, 349–357] and Chai and Navrotsky [Chai, L., Navrotsky, A., 1994. Enthalpy of formation of siderite and its application in phase equilibrium calculation. Am. Mineral. 79, 921–929], respectively. The density model [Anderson, G.M., Castet, S., Schott, J., Mesmer, R.E., 1991. The density model for estimation of thermodynamic parameters of reactions at high temperatures and pressures. Geochim. Cosmochim. Acta 55, 1769–1779] reproduced correctly our experimental data and allowed the extrapolation of the siderite solubility product up to 350 °C by using our values of the Gibbs energy and enthalpy of formation of siderite combined with its entropy and the heat capacity equation given by Robie et al. [Robie, R.A., Haselton, H.T. Jr., Hemingway, B.S., 1984. Heat capacities and entropies of rhodochrosite (MnCO3) and siderite (FeCO3) between 5 and 600 K. Am. Mineral. 69, 349–357].