Estimation of the standard molal heat capacities, entropies and volumes of 2:1 clay minerals

Abstract The dearth of accurate values of the thermodynamic properties of 2:1 clay minerals severely hampers interpretation of their phase relations, the design of critical laboratory experiments and geologically realistic computer calculations of mass transfer in weathering, diagenetic and hydrothermal systems. Algorithms and strategies are described below for estimating to within 2% the standard molal heat capacities, entropies, and volumes of illites, smectites and other 2:1 clay minerals. These techniques can also be used to estimate standard molal thermodynamic properties of fictive endmembers of clay mineral solid solutions. Because 2:1 clay minerals like smectite and vermiculite are always hydrated to some extent in nature, contribution of interlayer H2O to their thermodynamic properties is considered explicitly in the estimation of the standard molal heat capacities, entropies, and volumes of these minerals. Owing to the lack of accurate calorimetric data from which reliable values of the standard molal heat capacity and entropy of interlayer H2O can be retrieved, these properties were taken in a first approximation to be equal to those of zeolitic H2O in analcite. The resulting thermodynamic contributions per mole of interlayer H2O to the standard molal heat capacity, entropy, and volume of hydrous clay minerals at 1 bar and 25°C are 11.46 cal mol−1, 13.15 cal mol−1 K−1 and 17.22 cm3 mol, respectively. Estimated standard molal heat capacities, entropies and volumes are given for a suite of smectites and illites commonly used in models of clay mineral and shale diagenesis.

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