Thermodynamic properties of saponite, nontronite, and vermiculite derived from calorimetric measurements

Abstract

The stability of clay minerals is an important factor in assessing the durability of containment barriers for deep waste storage. In that context, the complete thermodynamic data set of three 2:1 ferro-magnesian clay minerals have been determined at 1 bar and from 2 to 520 K, using calorimetric methods. The studied clay samples were, respectively, the Na-saturated saponite Sap-Ca-1, Na 0.394 K 0.021 Ca 0.038 (Si 3.569 Al 0.397 Fe 3+ 0.034 )(Mg 2.948 Fe 2+ 0.021 Mn 0.001 )O 10 (OH) 2 , the Ca-saturated nontronite NAu-1, Ca 0.247 K 0.020 (Si 3.458 Al 0.542 )(Mg 0.066 Fe 3+ 1.688 Al 0.268 Ti 0.007 )O 10 (OH) 2 , and the Ca-saturated Santa Olalla vermiculite, Ca 0.445 (Si 2.778 Al 1.222 )(Al 0.192 Mg 2.468 Fe 3+ 0.226 Fe 2+ 0.028 Ti 0.018 Mn 0.007 )O 10 (OH) 2 . The standard enthalpies of formation were obtained by solution-reaction calorimetry at 298.15 K. The heat capacities were measured between 2 and 520 K, using low-temperature adiabatic calorimetry, heat-pulse calorimetry, and differential scanning calorimetry. The standard entropies and the Gibbs free energies of formation at 298.15 K have been calculated from these values. Finally, the equilibrium constants at 298.15 K have been determined. A comparison between these experimental data and estimated values obtained from prediction models available in the literature enabled the most usual calculation methods available to date to be assessed for each thermodynamic property.