Confinement properties of zeolite-rich rocks with respect to water and various ionic tracers

The studied materials are zeolite-rich rocks (>70 wt% analcime, named analcimolite) forming a laterally continuous aquitard within the southern of the Tim Mersoï Basin (Niger). This unit, commonly referred to as the Abinky Formation, occurs as three mineralogical facies: a reduced facies containing Fe-chlorite, an oxidized facies with hematite, and an intermediate ‘transition’ facies. This analcimolite unit acts as an aquitard, separating the overlying and underlying sandstone aquifers, the latter locally providing potable water. 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
located in zeolite micropores). This confirms that water behaves as an inert tracer and that micropores contribute negligibly to water migration. For ionic tracers in reduced facies, ³⁶Cl^– is partially excluded from external pores (anionic exclusion) and slightly adsorbed, with an extent of about 0.1 meq/100 g, 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 disturbance.

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Fig. 1. Petrography and mineralogy of the three facies of interest, i.e., reduced, transition, and oxidized.
Macroscopic photographs of analcimolites samples from (A) reduced facies (B) transition facies, and (C) oxidized facies, showing polycrystalline analcimes (spherules).
SEM observations (BED mode) on thin sections: (D) the polycrystalline analcime of the reduced facies (filled with a mixture of Fe-chlorites and nanoanalcimes), (E) the polycrystalline analcime of the oxidized facies with an empty core.
SEM observation (SED mode) of rock fragment: (F) monocrystalline analcime.
X-ray diffractograms (G) of the three studied facies (from bottom to top: oxidized, transition, and reduced facies). Chl: chlorite, Anl: analcime, Hem: hematite, Ant: anatase. Note that amorphous phases are accounting for the background intensity upon the Rietveld pattern modelling.