Thermodynamics for clay minerals: Calculation tools and application to the case of illite/smectite interstratified minerals

We present two computing tools, ClayTherm and ISTherm, devoted to the estimation of the thermodynamic properties of both anhydrous and hydrated clay minerals (ClayTherm), and of illite/smectite (I/S) mineral series (ISTherm). The first computing tool, ClayTherm, is devoted to thermodynamic property estimates for clay minerals. It combines several previously published estimation models, including hydration aspects. Verification is provided, against a set of solubility data, selected from previous literature. A specific application ISTherm was subsequently developed based on the first tool. It focuses on the smectite-to-illite transformation, and is able to calculate the thermodynamic properties of a series of illite/smectite (I/S) interstratified minerals, starting from the composition of a single I/S sample. The thermodynamic functions have been completed for the mixing energies and the tool was then used in order to investigate the case of a natural I/S hydrothermal series from the Shinzan geothermal field (Japan). Activity diagrams have been calculated including illite/smectite and phase relations are found to be in agreement with previous mineralogical observations and solution chemical analyses. The I/S series from Shinzan is further investigated through reactive transport modelling by using a site-specific, augmented version of the geochemical database. Illitization through the formation of I/S is predicted over realistic reaction times, consistently with available mineralogical observations.