Abstract Clay minerals are key indicators of soil evolution and the building blocks of soil behavior. Long-term agronomic trials, in which soil samples have been collected and archived over several decades, provide a valuable opportunity to track the transformation of clay minerals over centennial timescales. To achieve this, the clay mineralogy of the archived soil samples must be characterized in great detail. In this context, the initial clay mineralogy of the <2 μm fraction of one of the world’s oldest experimental sites, the 42 plots of Versailles (France, 1929), was determined using an approach that coupled sequential fractionation with X-ray diffraction profile modeling of 00ℓ reflections. This analysis of the original clay mineralogy confirmed the great complexity of temperate soil clay mineralogy. Seven clay phases were identified, including two quasi-discrete kaolinite and illite phases, and five randomly mixed-layered minerals (MLMs), three of which had three different 2:1 layer types. Results showed that MLMs accounted for >70% of the clay phases, with their proportions increasing as particle sizes decreased. The widespread occurrence of kaolinite-illite MLMs and three 2:1 component MLMs in temperate loess soils was also assessed. Comparisons with previous descriptions of clay mineralogy in temperate soils demonstrated that despite their complex crystal structures, the three 2:1 MLMs exhibited similar relative contributions from their constituent layers, particularly the illite layers. The high proportions of such illitic clay phases quantified in the study suggested that they could serve as key markers in the study of pedogenesis in soils that have developed from sedimentary rocks or deposits under temperate climatic conditions.

Abstract In Brazil, the largest bentonite deposits, located in the Boa Vista region of Paraíba State, have been studied extensively. However, with the near depletion of high-quality varieties from this region, new deposits are being explored elsewhere in the Brazilian northeast. Among these, the Formosa deposit, in Maranhão State, stands out. In this study, the <0.1 μm clay fraction highly enriched in smectite from the Formosa deposit was characterized and compared with three commercial smectites (Bofe, Chocolate, and Verde Lodo) from the Boa Vista region. Analytical techniques included X-ray diffraction, infrared spectroscopy, transmission electron microscopy, and atomic absorption spectroscopy. Although previously described as montmorillonites, their crystal chemistry has not yet been elucidated fully. The present study therefore aimed to clarify the crystal chemistry of these smectites by resolving the distribution of layer charge between tetrahedral and octahedral sheets and evaluating its implications for swelling behavior. The results revealed that these Al-rich smectites also exhibit a significant tetrahedral layer charge. Indeed, they possess sufficient tetrahedral charge (~0.15 per half unit cell) to induce interlayer swelling after the Hofmann-Klemen effect, a value lower than that commonly reported for natural smectites. The structural and chemical similarities between Formosa and the three commercial smectites demonstrate that the Formosa deposit is a promising alternative source to the smectites from Boa Vista.

Redox reactions involving Fe-bearing clay minerals play a significant role in the cycling of elements such as carbon, and for the mobility of inorganic and organic contaminants in subsurface environments. The layered alumo-silicate structure of clay minerals can accommodate up to 30 wt% Fe in both octahedral and tetrahedral coordination. Tetrahedral Fe is naturally present in Fe-rich clay minerals like nontronite, yet the behavior and fate of tetrahedral Fe during redox reactions remains poorly constrained. Here, we synthesized a series of nontronites with 0 to 27% of the total Fe present as tetrahedral Fe(III) that we subjected to chemical reduction, re-oxidation, and re-reduction with dithionite and hydrogen peroxide, respectively. We used FT-IR and Mössbauer spectroscopies, SEM-EDS, HRTEM and XRD to monitor changes in crystal chemistry and the fate of tetrahedral Fe during the redox manipulation. Reduction of all nontronites resulted in almost all structural Fe becoming reduced (77-100% Fe(II)/Fe total ), including a portion of the initially present tetrahedral Fe, and re-oxidation restored all Fe to its ferric form. The initial 2:1 smectite structure was conserved throughout all redox manipulations, with the typical structural changes during Fe reduction, mainly trioctahedral domain formation and structural OH-group loss, being largely reverted by re-oxidation. However, the initial reduction led to significant loss of tetrahedral Fe from the nontronite structure, and the tetrahedral Fe content in all nontronites converged to the same value of ~0.2 per half formula unit. The concomitant decrease in particle size and number of stacked layers in a significant portion of each nontronite led to the presence of two distinct populations of particles and suggests partial reductive dissolution as a plausible mechanism for tetrahedral Fe release. While some of the Fe was present in the aqueous phase, increased interlayer distances in reduced and re-oxidized nontronites implies that 48-63% of the released Fe became intercalated, presumably in the form of hydroxylated pillars. Overall, highly similar structures, crystal chemistry, and tetrahedral Fe content resulted for all nontronites irrespective of their initial tetrahedral Fe content and remained stable in subsequent redox manipulations, indicating the reversible reduction and re-oxidation of both octahedral and tetrahedral Fe in the initially altered clay mineral structure. Our results provide compelling evidence for why naturally occurring clay minerals contain only low amounts of tetrahedral Fe and that both octahedral and tetrahedral Fe in clay minerals may contribute to electron transfer reactions in natural environments.

Abstract In order to better understand the wettability of mica minerals, we combined molecular simulations with near-ambient pressure XPS measurements of the early hydration stages of phlogopite mica containing either structural OH atoms or structural F atoms, for various monovalent compensating cations. The evolution of the environment of the atoms constituting the system (structural atoms, counterions, and water molecules) was studied as a function of increasing relative humidity. The evolution with relative humidity of the NAP-XPS component of O assigned to liquid water was considered as representative of a water adsorption isotherm. At low RH (≤0.2), fluorinated samples display significantly lower water adsorption than their hydroxylated counterparts, which agrees with a delay in hydration of surface cation for the fluorinated samples, evidenced by NAP-XPS. In parallel, alchemical transformations at different surface water contents were performed using molecular dynamics. This allowed calculating for two different micas the ratio between relative humidities for a given surface water amount. In the case of mica–OH, simulations showed that the surface with Na+ is always more hydrophilic than that with Cs+. For mica-F, the Cs+ sample appears to be more hydrophilic at low hydration states (≤≈0.5 monolayer of adsorbed water), while the opposite is observed for higher amounts of adsorbed water. It may be that at low surface coverages, the interactions between fluorine and small Na cations are stronger than with Cs+. Despite the uncertainties associated with both the NAP-XPS experiment and the force field used in simulations, the comparison between experiments and simulations yields good agreement at high relative humidity and minor discrepancies at low relative humidity. The hydrophilic/hydrophobic nature of the surface appears to strongly depend on the very first hydration stages, i.e., on the balance between cation/surface and cation/water interactions.

Abstract The hydration of smectite clay minerals is a multiscale process where macroscopic properties emerge from the molecular organization of water within nanometric interlayer galleries. While the combination of X-ray diffraction (XRD) and Grand Canonical Monte Carlo (GCMC) simulations has become a standard for describing these systems, vibrational spectroscopy has long suffered from a lack of quantitative integration due to optical distortions inherent in Attenuated Total Reflection (ATR) geometry. In this work, we introduce a rigorous physical framework to transform polarized ATR-FTIR into a quantitative metrological probe, fully integrated with the classical XRD-GCMC workflow. We go beyond standard data processing by developing a methodology based on electromagnetic theory and Effective Medium Approximations (EMA) to extract the intrinsic anisotropic extinction coefficients (kxy and kz) of hydrating clay films. The model explicitly accounts for the evolving composition of the film’s porosity, where the relative proportions of water and air dictate the evolution of the anisotropic effective refractive index. This approach allows us to decouple the extrinsic optical response from the molecular absorption, effectively transforming the ATR probe into a quantitative tool. The application of Multivariate Curve Resolution-Alternative Least Squares (MCR-ALS) to these optical constants reveals a distribution of water populations (0W, 1W, and 2W) that matches the molar proportions derived from XRD profile modeling with remarkable accuracy. Furthermore, the experimental order parameters () extracted from the dichroic response provide a direct validation of the molecular orientations predicted by GCMC simulations using ClayFF and SPC/E force fields. By reconciling molecular-scale orientation (IR) with crystal-scale periodicity (XRD) and thermodynamic predictions (GCMC), this self-consistent multiscale framework provides a comprehensive depiction of interlayer environments and offers a robust pathway for investigating 2D-confined fluids in complex mineral systems.

We present LamelODF, a MATLAB-based software platform for automated extraction and mapping of axisymmetric orientation distribution functions (ODFs) from 2D X-ray diffraction patterns of lamellar minerals. Building on a maximum-entropy-method-derived ODF specifically for clay mineral systems, the software provides a streamlined workflow for texture analysis under the assumption of transverse isotropy. The program features dedicated file converters for both laboratory and synchrotron data formats, batch processing capabilities, and flexible background-correction algorithms. The analytical pipeline performs azimuthal intensity integration with background subtraction, followed by non-linear fitting to extract quantitative orientation parameters including the 〈P 2〉 order parameter, the deviation angle δ between the main orientation of lamellar particles and the detector reference, and integrated intensities, focusing specifically on basal 001 reflections to enable rapid processing of thousands of diffraction patterns for spatial mapping applications. Mapping of a laboratory-prepared porous clay medium and a natural soil sample demonstrates the software's ability to detect density stratification, sedimentation discontinuities and complex geological structures such as relict topsoil crusts. The software successfully discriminates between different clay mineral phases, providing a practical complementary tool for researchers investigating the organization of lamellar minerals in natural and engineered materials.

The time evolution of the water diffusion front in illite, an analogous clay model for clay-rich formations considered in the context of nuclear waste repositories, was imaged using through-diffusion experiments coupled to neutron tomography. Analysis of neutron tomographies under fully and partially water-saturated conditions revealed the absence of disconnected regions due to partial saturation of the pore space. The mobility of water was decreased in partially saturated sample following a reduction in the overall water content. The influence of the diffusion cell components on the obtained water diffusion profiles was investigated in detail using two-dimensional through-diffusion calculations with the CrunchClay reactive transport code. The modeling results confirmed that the presence of polyetheretherketone grids and rubber O-rings can introduce some biases when determining the effective diffusion coefficients. The position of the circulation canals next to the sample was an additional obstacle to water diffusion. All these effects related to the experimental setup were quantified, allowing an accurate determination of the effective coefficients of water and rock capacity factors. Neglecting these effects using a simple one-dimensional diffusion equation resulted in an underestimation of these values of approximately 20%. A correction method is proposed to overcome these biases for the cell geometry considered in this study. The differences reported in the literature between experiments and calculated diffusive transport parameters of water in partially water-saturated conditions must then be attributed to changes in the physical properties of the sample and water transport mechanisms.

Clay minerals are well documented to facilitate the retention of water and organic matter in terrestrial soils, Martian regolith, and meteorites. Yet, the mechanisms underlying water trapping within these mineral-organic matter associations are poorly understood. Here, we investigate these mechanisms with montmorillonite, a smectite clay, populated with carbohydrates of different structures. By capturing relative proportion of bound versus freely exchangeable waters by mass spectrometry during thermogravimetric analysis, we observe up to a 2.3-fold increase in bound waters in samples with adsorbed carbohydrates. Temperature-dependent carbon loss from adsorbed 13C-labeled carbohydrate determines increase in carbohydrate trapping at low moisture. We determine that the amount of trapped organic carbon is correlated positively with the population of bound waters. Molecular dynamics simulations of a carbohydrate-populated clay nanopore identify different interfacial waters, involving direct single or multiple hydrogen bonds on the clay surface without or with simultaneous hydrogen bonding with adsorbed carbohydrates. Quantum mechanics-based computations highlight up to 5-fold greater binding energy for bound waters associated with adsorbed carbohydrates on the clay surface, compared to bound waters in the absence of carbohydrates. Thus, our experimental and theoretical results collectively reveal that interfacial waters bridging hydrated organic matter to the clay surface facilitate water trapping within mineral-organic associations.

The swelling behavior of smectite clay minerals is known to have two distinct regimes. The crystalline swelling involves the incorporation of a few water sheets between clay layers while the osmotic swelling corresponds to the complete delamination of clay layers in a solvent. Although a linear transition between osmotic and crystalline swellings is proposed, a complete description of smectite swelling has never been achieved. Here, acoustic levitation coupled with Small Angle X-ray Scattering (LevSAXS) is proposed to follow the evolution of the interlayer space of a smectite in a single levitated droplet. The advantage is to track fast non-equilibrium phenomena over a wide concentration range, while avoiding anchoring effects during drying. The results reveal a gradual shift from osmotic to crystalline swelling, marked by a transition from a pure nematic glass to a coexistence zone where the nematic phase contracts and a saturated crystalline phase emerges. This transition occurs through a continuous process, forming interstratified structures ultimately progressing to an unsaturated crystalline state. Applied to the emblematic case of clay swelling, LevSAXS opens new perspectives to investigate or reconsider the swelling mechanisms of other low-dimensional 2D materials.

Through-diffusion experiments were carried out under partially water saturated conditions and using uncharged tracers (HTO and HDO; water molecule with one tritium and deuterium atom, respectively), negatively charged tracers (125I- and 36Cl- ) and positively charged tracer (22Na+) in two reference clayey materials: (i) illite, a negatively-charged clay mineral with only interparticle porosity and (ii) vermiculite, a negatively-charged clay mineral with both inter-particle and interlayer porosities. For both types of porous media, a sharp decrease in diffusive flux was observed for all three types of tracer when water saturation (Sw) was reduced from Sw = 0.95 to 0.88. The data were compared to those obtained previously with porous media made of kaolinite, a weaklycharged clay mineral having only inter-particle porosity, to quantify the relative influences of surface charge and pore geometry/pore size distribution on diffusion properties as a function of water saturation. The latter parameter had a prominent role on the diffusional properties in the investigated range of Sw, regardless of the type of clayey material. Then, a relationship predicting diffusivities of cation under partially water saturated conditions as a function of diffusivities of anion at the same level of water saturation is proposed for the reference clayey materials investigated.

Clay minerals wettability is a key property for predicting water distribution and pollutant migration in natural or artificial materials. This study attempted to understand the difference in hydrophilicity due to the exchange of structural hydroxyl groups by fluorine atoms, observed in a number of clays. To this end, contact angles of water droplets on hydroxylated and fluorinated talcs were calculated from molecular simulations with two different force fields (one non-polarizable and one polarizable). In parallel, careful measurements of contact angles on a hydroxylated talc monocrystal were undertaken in order to assess the ability of the force fields to reproduce experiments. As expected, fluorinated talc was slightly more hydrophobic than hydroxylated talc for both force fields. Moreover, although the two force fields lead to fluid properties at the interface that were significantly different, the associated contact angles (and related works of adhesion) remained quite close to each other and to contact angles obtained on similar silica-type surfaces. These contact angles overestimated the experimental ones. This could be tentatively assigned to the presence of steps on monocrystal surfaces that could slightly increase the hydrophilicity of the surface, resulting in slightly lower contact angles. When analyzing more carefully the differences between both force fields, it appeared that the use of a polarizable force field resulted in a higher depletion of the fluid close to the surface. This could indicate less attraction between the fluid and the solid and a lesser constraint for the fluid. The combination of these two effects leads to a lower entropy loss and consequently to slightly higher work of adhesion.

In this paper, we address the formation of highly organized clay tactoids intercalated with a charged polymer (ionene) in an aqueous environment. We report on an original route to achieve such tactoids by starting with preformed clay tactoids, held together by multivalent inorganic atomic ions, as is the case in clay suspensions exchanged with Ca2+ or La3+ ions. Contrary to previously evoked mechanisms of disaggregation-aggregation or successive delamination of individual platelets ("peeling"), we observe clearly a reversible transition between the two types of clay tactoids at an almost constant number of clay nanoplatelets per tactoid. Our observations are based on small-angle X-ray scattering, where a strong correlation peak, called the stacking peak, is an indisputable signature of the stack/tactoid formation and is distinct for the multivalent-ion clay stacks and charged-polymer clay stacks. The initial state of multivalent-ion clay tactoids is essential for the formation of highly organized final charged-polymer clay tactoids. It is as if the preformed multivalent-ion clay tactoids provided a template into which the charged polymer chains get incorporated. Starting from individual delaminated clay layers (Na+ exchanged clays) does not lead to the same final charged-polymer clay stacks. The linear charge density of the polymer chains is another key parameter for the tactoid transition. For weakly charged polymer chains, the transition simply does not take place, even under conditions of a strong excess of the charged polymer. Overall, the final structure of charged-polymer clay stacks strongly depends on the initial structural state of the host clay platelets and the linear charge density of the guest polymer chains.

The swelling behavior of smectite clay minerals has been extensively investigated due to their critical role in construction stability. Two distinct swelling regimes have long been recognized: crystalline swelling, involving the incorporation of a few water sheets between clay layers, and osmotic swelling, characterized by the complete delamination of individual clay layers. Crystalline swelling mechanisms have been well established through hydration techniques applied to dry clay in a humid environment, while osmotic swelling has been observed through the gradual concentration of a diluted clay suspension into a more compact, glassy state. Although a linear transition between these two regimes has been proposed, it has yet to be experimentally demonstrated. In this study, we investigate the complete dehydration process of a levitated smectite suspension droplet using in-situ Small Angle X-ray Scattering (SAXS). The results reveal a gradual shift from osmotic swelling to crystalline swelling, marked by a transition from a pure nematic glass to a coexistence zone where the nematic phase contracts and a saturated crystalline phase emerges, ultimately progressing to an unsaturated crystalline state. This transition occurs through a continuous process in which the interlayer space decreases from four water layers to zero water layers, forming stratified interlayers along the way.

Water dynamics impacts many phenomena from geosciences to biology, especiallly in confined environments. In the presence of charged interfaces, there are some ions the role of which with regards to the water dynamics is unclear. Here a synthetic saponite clay, which is oriented in a film, is used as confining medium in the bilayer state. It confines two water layers between negatively charged planes, the charge of which is compensated by sodium cations. Water dynamics is determined both parallel and perpendicular to the charged clay layers with Neutron Spin Echo (NSE). This technique gives access to long enough times and directly provides the intermediate scattering function that is calculated on the other hand by Molecular Dynamics (MD) simulations. These latter also enable the study of cations dynamics, not experimentally accessible on this time scale. The results point towards a huge role of these cations on the water dynamics, mainly through their local structure and localization between the charged confining planes.

Most of the available data on diffusion in natural clayey rocks consider tracer diffusion in the absence of a salinity gradient despite the fact that such gradients are frequently found in natural and engineered subsurface environments. To assess the role of such gradients on the diffusion properties of clayey materials, throughdiffusion experiments were carried out in the presence and absence of a salinity gradient using salt-diffusion and radioisotope tracer techniques. The experiments were carried out with vermiculite samples that contained equal proportions of interparticle and interlayer porosities so as to assess also the role played by the two types of porosities on the diffusion of water and ions. Data were interpreted using both a classical Fickian diffusion model and with a reactive transport code, CrunchClay that can handle multi-porosity diffusion processes in the presence of charged surfaces. By combining experimental and simulated data, we demonstrated that (i) the flux of water diffusing through vermiculite interlayer porosity was minor compared to that diffusing through the interparticle porosity, and (ii) a model considering at least three types of porous volumes (interlayer, interparticle diffuse layer, and bulk interparticle) was necessary to reproduce consistently the variations of neutral and charged species diffusion as a function of salinity gradient conditions.

The unraveling of the hydrophobicity/hydrophilicity molecular signature of nanometric bidimensional confined systems represents a challenging task with repercussions in environmental transport processes. Swelling clay minerals represent an ideal model system, as hydrophobicity can be modified during material synthesis by substituting hydroxyls by fluorine in the structure, without additional surface treatment. This following work presents a combined approach, integrating experimental inelastic neutron scattering spectroscopy and ab initio molecular dynamics simulations, with the objective of advancing our understanding of the role of surface hydroxylation/fluorination and the extent of confinement on water properties. From computed structures, the analysis of molecular hydrophobicity/hydrophilicity signature was investigated in detail through water-cation-surface interactions. The results elucidate the influence of fluorination on interlayer species, thereby tracing the impact of the surface on the diminished number of water molecules in such a sample. It is notable that the strong cation-water interaction can overcome the disruptive influence of fluorine, thereby maintaining comparable water hydration shells around cations and resulting in an almost identical bidimensional confinement geometry for both hydroxylated and fluorinated specimens. The analysis of the hydrogen-bond network revealed a significant reorganization of the water molecules due to fluorination. Our results suggest that a quantitative molecular signature of hydrophobicity/hydrophilicity can be derived from the analysis of the formation of cavities in the confined fluid. This new finding represents a robust approach for generalizing the hydrophobicity/hydrophilicity character for a wide variety of bidimensional systems while proposing a framework for the design of new materials with controlled water properties.

Clay minerals are used in a wide number of natural or artificial materials for municipal or nuclear waste management in which water diffusion is the principal transfer process. However, a quantitative assessment of the impact of the preferred orientation of lamellar clay particles on water diffusion is still lacking. Using 3D Brownian dynamics simulation on representative virtual clay porous media, a systematic study of water diffusion for single-porosity (illite or kaolinite) and dual-porosity (vermiculite) systems was conducted. The simulated water diffusion coefficients were validated through comparison with experiments and were used to build an Archie model including the degree of anisotropy in particle orientation. The results showed that water diffusion can be predicted based on a correct description of the solid phase organization and that clay particle orientation, such as interparticle porosity, is a primary parameter governing water mobility. Moreover, the anisotropy of water diffusion can be linked to the degree of particle preferred orientation, irrespective of the porosity value. The modified version of the Archie model for water diffusion in clay systems proposed here has many potential applications where decoupling of porosity and preferred orientation is needed, including better prediction of water transfers or improved designs of clay liners with sustainable use of natural mineral resources.

Swelling capacity of smectite was studied over decades regarding its application as barrier in disposal of nuclear wastes in geological repositories as well as the induced volume change potential in soils according to moisture. In order to improve our knowledge in the swelling capacity of smectite, a miniaturized oedometer was developed to combine swelling pressure measurement with wide angle X-ray scattering (WAXS) characterization in real time during hydration of smectite. This coupled set up allowed studying hydration of smectite up to saturation under confined condions and linking crystalline swelling to pressure at various densities. The modeling of the WAXS patterns gave also quantitative information about the relative proportion of the different interlayer water types at saturation. In situ and operando data were acquired for homo-ionic Na+- and Ca2+-exchanged smectite at two different densities (1.5 and 1.8 g/cm3). The results showed that the swelling pressure rise was correlated to a sequence of water layer type with the transition from 0W to interstratification of 2W/3W layers, depending on the density. The cation valency controlled the rate of hydration with faster hydration in the case of divalent exchanged smectite. At saturation, with increasing density, the amount of 3W layers decreased to the gain of 1W and 0W layers. Results also confirmed that at saturation and a density of 1.8 g/cm3, the interlayer porosity represented the total one. Finally, this development provided opportunity to improve our knowledge in the swelling mechanism of compacted swelling clay materials upon hydration.

Over several decades, a wealth of literature has been devoted to correlations between the chemistries of phyllosilicates and their crystallographic unit-cell parameter values. The c parameter is currently used because of its relation to the layer-to-layer distance, characteristic of the various families of phyllosilicates. The b parameter is also of interest because it allows measurement of the layer lateral dimensions and inherent structural adjustments. This unit-cell distance can be extracted from X-ray diffraction traces from the (06l;33l) diffraction region and by attributing the main diffraction peak observed to a 060 reflection, leading to the relationship b = 6.d(060). The aim of this paper is to revisit the relationships between the b value (or equivalent) of the phyllosilicate (i.e. TO, TOT and TOTO) or hydroxide (i.e. hydroxide, oxyhydroxide and layered double hydroxide) families and the layer chemistry based on a mean ionic radius R of octahedral cations, calculated as R = Sigma(n)(i=1) (r(i).x(i)), where r(i) is the ionic radius of the octahedral cation i and x(i) is its molar fraction over n types of octahedral cations (Sigma(n)(i=1) (x(i)) = 1). The data were collected from the literature and involved both natural and synthetic samples with both dioctahedral and trioctahedral structures of the octahedral sheet. The results showed that b values can be linked strongly to R, leading to suitable linear regressions for all of the studied structures. All correlations were found to be applicable irrespective of the di- or trioctahedral nature of the octahedral sheet, and these are discussed in light of (1) the lateral dimension of the octahedral sheet and (2) the dimensional misfit between the tetrahedral and octahedral sheets. For hydroxide families, all data can be gathered on a single b vs R correlation line, and the dimensional properties of the octahedral sheet can be interpreted simply based on an oxygen-cation-oxygen mean distance. For TO structures, two general b vs R correlation trends were reported, and these were assigned to two adjustment mechanisms corresponding to distinct types of tetrahedral and octahedral distortions. For the mica TOT family, two main trends were also reported, whereas the use of the synthetic mica series allowed us to demonstrate that the obtained scattering of data was mainly driven by the presence of multiple limited solid solutions. Such chemical complexity was also noted for smectites, especially regarding the tetrahedral composition and associated variability in layer charge. This variability made it difficult to propose a general regression correlating b to R values for smectites, although the regression obtained for neutral TOT layers can apply as a first-order relation. Finally, a single general b vs R correlation was obtained for chlorites, and the observed slope of the regression was interpreted according to the role played by the isolated hydroxide sheet on the evolution of the lateral dimension of the structures.

Diffusion is the main transport process of water and solutes in clay-rich porous media owing to their very low permeability, so they are widely used as barriers against contaminant spreading. However, the prediction of contaminant mobility can be very complicated when these media are partially water-saturated. We conducted diffusion experiments for water (HTO and HDO) and ions (22Na+ and 125I−) through partially water saturated compacted kaolinite, a weakly charged clay material, to quantify the distinct diffusive behavior of these species. The osmosis method was used to set kaolinite samples at 67, 86 and 100% saturation. The results showed that desaturation led to a sharp decrease in diffusive rates by factors of 6.5, 18 and 35 for HTO, 125I− and 22Na+, respectively, from 100 to 67% of the degree of saturation. Thus, to interpret water diffusivities, we proposed a model taking into account the diffusion of water in both gas and liquid phases, using diffusion data obtained for ions, considered as inert species. This model was capable of properly predicting water diffusive flux, especially at a low degree of saturation (67% saturation), for which the assumption made for the occurrence of air phase continuity throughout the sample appears to be more relevant than at 86% saturation.