A generalized model for predicting the thermodynamic properties of clay minerals

A set of models for estimating the enthalpy of formation, the entropy, the heat capacity and the volume of dehydrated phyllosilicates is presented. The model for entropy and heat capacity estimation is essentially based on a method of decomposition into polyhedral units, similar to that published by Holland (1989). The model for predicting the enthalpy of formation is based on the electronegativity scale, as previously developed by Vieillard (1994a, 1994b). For the sake of consistency, the models are parameterized using the same critical selection of thermodynamic properties from the literature. This includes a set of direct measurements especially dedicated to clay minerals that had not been taken into account in previous calculation methods. The accuracy of the predictions is tested for each property. The verification tests are also carried out for minerals that include different chemical elements than the phases used to derive the model constants, especially lithium-bearing micas. Verification tests also concern the Gibbs energy function that combines contributions from both models. Finally, the models are used in order to propose a complete thermodynamic database for clay mineral end-members. The consistency of the stability domains calculated on the basis of these thermodynamic properties is investigated by drawing relevant predominance diagrams for some chemical systems of interest. The models proposed represent a significant improvement with respect to previous works as regards the global accuracy of the estimates and because the developments were realized and tested using the same set of minerals, whose properties had been collected through a critical selection of the literature.