Thermodynamic properties of illite, smectite and beidellite by calorimetric methods: Enthalpies of formation, heat capacities, entropies and Gibbs free energies of formation

The thermodynamic properties of three aluminous 2:1 clay minerals were acquired at 1.013 bars and at temperatures between 5 and 500 K using various calorimetric methods. Calorimetric measurements were performed on hydrated and dehydrated <2 μm clay fractions of smectite MX-80 (Wyoming), illite IMt-2 (Silver Hill) and beidellite SBId-1 (Black Jack Mine). After purification, the mineralogical analyses gave the following structural formulae:Na0.409K0.024Ca0.009(Si3.738Al0.262)(Al1.598Mg0.214Fe0.1733+Fe0.0352+)O10(OH)2,K0.762Na0.044(Si3.387Al0.613)(Al1.427Mg0.241Fe0.2923+Fe0.0842+)O10(OH)2 and Ca0.185K0.104(Si3.574Al0.426)(Al1.812Mg0.09Fe0.1123+)O10(OH)2 for smectite MX-80, illite IMt-2 and beidellite SBId-1, respectively. Heat capacities were measured by low temperature adiabatic calorimetry and differential scanning calorimetry, from 5 to 500 K. Standard enthalpies of formation were obtained from solution–reaction calorimetry at 298.15 K. The standard Gibbs free energies of formation of the clay minerals were also calculated, together with the equilibrium constants at 25 °C, for anhydrous and hydrated minerals. A comparison between these experimental data and estimated values obtained from prediction models available in the literature, enabled the calculation method that appears to be the most relevant to be selected, at least for aluminous 2:1 clay minerals.