Diffusive transport of water in porous feldspars from granitic saprolites: In situ experiments using FTIR spectroscopy

A novel experimental cell was developed for in situ measurements of transport phenomena in porous media using Fourier-Transform Infrared (FTIR) Spectroscopy. The technique was employed at ambient pressure in the temperatures range of 11–44 °C to study the H₂O → D₂O exchange between water-saturated weathered feldspars (bulk porosity of 5–19 vol% for feldspar) from granitic saprolites and a surrounding aqueous liquid. Such measurements are an important step for understanding internal weathering reactions of feldspars in soils and aquifers. Effective diffusion coefficients D_eff for water in water-saturated porous feldspars were determined assuming one-dimensional diffusion in a quasi-homogeneous medium. The values of D_eff vary from 7.2 × 10⁻¹⁰ to 1.9 × 10⁻¹¹ m²/s and are 1–2 orders of magnitude lower than the diffusion coefficients (D) of protons and molecular H₂O in liquid water. The activation energy for the H₂O → D₂O exchange process in porous feldspars ranges from 7.8 to 18.8 kJ/mol.The results imply that the effective diffusivity of water is mainly controlled by physical properties of the feldspars like porosity, pore connectivity, pore geometry and distribution. Perthitic feldspars with homogeneous pore distribution in the albitic lamellas have diffusional tortuosity factors X = D/D_eff between 3 and 10 while alkali feldspars with inhomogeneously distributed and disconnected pores have much higher X values up to 129. Diffusion anisotropy has been verified for a vein perthite with diffusion perpendicular to the lamellas being faster by 0.3–0.5 log units than within the lamellas. It has to be emphasized that the study is based only on few selected feldspars, including perthitic feldspar, and additional work on samples with different weathering stages is needed to test the importance of the different parameters controlling diffusive transport in the pore system.