Parrotin et al., 2025

The Abinky formation, located in the southern part of the Tim Mersoï Basin (Niger) consists of three distinct facies, each characterized by specific mineralogical and grain organisation, but all containing analcime (~ > 70 wt%), an ANA-type zeolite. Reduced facies contains Fe-chlorite, while oxidized facies includes hematite, and the ‘transition’ facies is intermediate. Due to the low permeability of these analcime-rich rocks, diffusion is expected to be the dominant transport mechanism. However, no data on transport properties in zeolite-rich formations have been reported, and the mobility of water and ionic solutes remains unknown. This study aims to characterize diffusion in the three facies of the Abinky formation, identifying key controls on water and ion transport prior to any anthropogenic perturbation.
Through-diffusion experiments using water tracers (HDO and HTO) showed that water diffusion (effective diffusion coefficient from 1.3 to 2 × 10-11 m²/s) is primarily controlled by pore throat size rather than total porosity. A dual-porosity model was required, distinguishing a fast-transport network (open, low-tortuous zones) from a slow one (confined, tortuous zones). Despite variations in diffusion coefficients, all data are interpreted with capacity factors equal to total external porosity measured by water impregnation (excluding crystal water in zeolite micropores). This confirms that water behaves as an inert tracer and that micropores contribute negligibly to migration. For ionic tracers in reduced facies, 36Cl- is partially excluded from external pores (anionic exclusion) and slightly adsorbed with an extent of about 9 × 10-2 meq/100 g) that is a very small value compared to the cation exchange capacities of such rocks (up to ~40 meq/100 g for H+ and NH4+). In contrast, ²²Na⁺ exhibits strong adsorption due to isotopic exchange with 23Na+ in the analcime framework, leading to a diffusivity ~4 times higher than that of water. This dataset is a first step in constraining reactive transport models of water and solutes in zeolite-rich porous media under environmental conditions and prior to anthropogenic disturbances such as in-situ acid leaching.