Dimitri Prêt

Spatial distribution of porosity and minerals in clay rocks from the Callovo-Oxfordian formation (Meuse/Haute-Marne, Eastern France)—implications on ionic species diffusion and rock sorption capability

Imaging techniques, adapted to clay rocks, combined with mathematical correlations of petrophysical parameters bring a new petrographic description of this clay rock integrating multi-scale data. Rock heterogeneities (mineral and porosity distributions) are studied at the mineral pore, the core and the borehole scale for interpreting diffusion tests. Although the infra-micrometric pores are associated with the clay particles and larger pores are associated with bioclasts and detrital minerals, especially tectosilicates, relationships between minerals and pores are quite complex at the borehole scale. They are mainly controlled by mineral spatial arrangements and contents, since micro- to macrostructures of centimetric size are encountered in each lithofacies. Effects of surface interactions and pore-constriction electrochemical controls were underscored for a large proportion of the effective porosity. Although the effective porosity is physically interconnected in the rock volume, diffusion and sorption are discussed in terms of structure and accessibility to sorption sites. These data could be the basis for the construction of a quantitative transport model by diffussion taking into account the sample heterogeneities from the mineral pore size to the centimetric core size ranges. The conceptualisation of the organization of the porosity in such low permeability clayey medium, considered together with the analysis of interface-derived effects constitutes an important step forward in understanding the mechanisms affecting the migration of single ionic species. This understanding should help in extrapolating transport parameters obtained at the centimetric scale from laboratory experiments to the geological formation scale.

Anodic Activation of Iron Corrosion in Clay Media under Water-Saturated Conditions at 90 °C: Characterization of the Corrosion Interface

To understand the process governing iron corrosion in clay over centuries, the chemical and mineralogical properties of solids formed by free or anodically activated corrosion of iron in water-saturated clay at 90 degrees C over 4 months were probed using microscopic and spectroscopic techniques. Free corrosion led to the formation of an internal discontinuous thin (<3 microm thick) magnetite layer, an external layer of Fe-rich phyllosilicate, and a clay transformation layer containing Ca-doped siderite (Ca(0.2)Fe(0.8)CO(3)). The thickness of corroded iron equaled approximately 5-7 microm, consistent with previous studies. Anodic polarization resulted in unequally distributed corrosion, with some areas corrosion-free and others heavily corroded. Activated corrosion led to the formation of an inner magnetite layer, an intermediate Fe(2)CO(3)(OH)(2) (chukanovite) layer, an outer layer of Fe-rich 7 A-phyllosilicate, and a transformed matrix layer containing siderite (FeCO(3)). The corroded thickness was estimated to 85 microm, less than 30% of the value expected from the supplied anodic charge. The difference was accounted for by reoxidation at the anodically polarized surface of cathodically produced H(2)(g). Thus, free or anodically activated corroding conditions led to structurally similar interfaces, indicating that anodic polarization can be used to probe the long-term corrosion of iron in clay. Finally, corrosion products retained only half of Fe oxidized by anodic activation. Missing Fe probably migrated in the clay, where it could interact with radionuclides released by alteration of nuclear glass.

The petrography of weathering processes: facts and outlooks

Abstract Rock weathering has been investigated from atomic to global scales through the different but complementary approaches of mineralogy, petrography, geomorphology and geochemistry. The sequences of mineral reactions involved in the alteration process are now well known. They explain the global trend of weathering phenomena but do not account for the actual rock transformation dynamics. In particular, they ignore the intimate relation of the mineral reaction progress with the increase in connected porosity. At the hand specimen scale, heterogeneity is the rule: mineral reactions are controlled by local physicochemical conditions. Alteration processes depend largely on the rock microstructure properties. They proceed through nearly-closed, semi- and completely open microsystems which are interconnected by fractures or pores. Before being leached out by the solutions which flow in the large fractures (flux), the soluble elements migrate inside the connected porosity through chemical diffusion. The dissolution of the primary minerals is mediated through local gradients of chemical potential. With increasing alteration, the rock porosity increases, as does the length of the fluid passageways and their constrictivity and tortuosity. Consequently, the apparent diffusion coefficient for the most soluble elements decreases. The amplitude of the chemical potential gradients for the most soluble elements is reduced by the progressive coating of the reactive surfaces by clays and Fe oxyhydroxides. The residence time of these elements inside the weathered rock increases as alteration progresses; an effect enhanced by their temporary adsorption on the exchangeable sites of clays and Fe oxyhydroxides. Consequently, the weathering rate decreases with time. A possible new way to calculate weathering rates could be to measure the residence time of soluble elements inside the different microsystems during their migration towards the diluted solution which occurs in the large fractures.

Influence of the ionic strength and solid/solution ratio on Ca(II)-for-Na+ exchange on montmorillonite. Part 1: Chemical measurements, thermodynamic modeling and potential implications for trace elements geochemistry

Na/Ca ion-exchange isotherms were performed on a Na-saturated montmorillonite for different constant normalities of the aqueous chloride solution and at two solid/solution ratios. The experimental data suggest that the affinity of Na(+) for the solid increases with total normality and m/V ratio and that a significant proportion of CaCl(+) is also sorbed. A thermodynamic modeling procedure with one sorption site and three sorbed cations (Na(+), Ca(2+) and CaCl(+)) was applied to interpret the data. We show that by accounting for the activities of aqueous species, the experimental data obtained at different total normalities for a given solid/solution ratio can be fitted using a unique set of selectivity coefficient values. However, when the m/V ratio is decreased from 25 to 2.5 g/L, an increase in the log Kc(Na(+)/Ca(2+)) of up to 0.4 ± 0.05 was required to interpret the data with a constant log Kc(Na(+)/CaCl(+)) value. The same behavior concerning the increase in the log Kc(Na(+)/Ca(2+)) with a m/V ratio decrease was observed when using a multi-site model taking into account the specific sorption of H(+). The results clearly indicate that the Na(+)/Ca(2+) selectivity coefficients strongly depend on the solid/solution ratio but are independent of the exchanger composition and the total normality of the solution. Such findings provide possible pitfalls when using selectivity coefficients obtained in diluted suspension to assess the sorption in compacted clayrocks and allow a better prediction of the sorption of trace elements in competition with major cations.

Untangling the environmental from the dietary: dust does not matter.

Both dust and silica phytoliths have been shown to contribute to reducing tooth volume during chewing. However, the way and the extent to which they individually contribute to tooth wear in natural conditions is unknown. There is still debate as to whether dental microwear represents a dietary or an environmental signal, with far-reaching implications on evolutionary mechanisms that promote dental phenotypes, such as molar hypsodonty in ruminants, molar lengthening in suids or enamel thickening in human ancestors. By combining controlled-food trials simulating natural conditions and dental microwear textural analysis on sheep, we show that the presence of dust on food items does not overwhelm the dietary signal. Our dataset explores variations in dental microwear textures between ewes fed on dust-free and dust-laden grass or browse fodders. Browsing diets with a dust supplement simulating Harmattan windswept environments contain more silica than dust-free grazing diets. Yet browsers given a dust supplement differ from dust-free grazers. Regardless of the presence or the absence of dust, sheep with different diets yield significantly different dental microwear textures. Dust appears a less significant determinant of dental microwear signatures than the intrinsic properties of ingested foods, implying that diet plays a critical role in driving the natural selection of dental innovations.

Influence of the ionic strength and solid/solution ratio on Ca(II)-for-Na+ exchange on montmorillonite. Part 2: Understanding the effect of the m/V ratio. Implications for pore water composition and element transport in natural media.

The aim of the present paper is to clarify previous results showing that selectivity coefficients determined for the exchange of Na(+) for Ca(2+) in montmorillonite were dependent on the solid/solution ratio. The organization of montmorillonite suspensions upon Na(+)/Ca(II) exchange was analyzed by combining optical microscopy, small-angle X-ray scattering and X-ray diffraction. All samples displayed flocculated characteristics, eliminating the possibility of contrasting accessibility of sorption sites with the solid/solution ratio. Modeling of experimental X-ray diffraction patterns was used to quantify the relative proportions of interlayer Ca(2+) and Na(+) cations along the exchange isotherm. The results further confirmed the influence of the solid/solution ratio on the degree of interlayer Ca(II)-for-Na(+) exchange, and specific selectivity coefficients for interlayer sites were determined. The effect of the solid/solution ratio was finally interpreted by the resulting local changes in the solution chemistry. We demonstrated that by accounting for the Donnan effect, the different data can be interpreted using a single selectivity coefficient. The obtained Kc constant was successfully applied to interpret existing hydrogeochemical data on a natural aquitard. This most likely represents a more constrained and valid approach for the modeling of reactive element transport in natural media than does the poorly defined Kd parameter.

In-situ interaction of cement paste and shotcrete with claystones in a deep disposal context

In-situ sampling was performed in the Andra Meuse/Haute Marne (France) Underground Research Laboratory (URL) allowing the study of two cement based materials/claystone interfaces that have undergone 4 to 5 years of interaction. The first interface concerned a shotcrete that was sprayed on the wall of an access drift at the laboratory level and the second one, a class G cement paste that was injected in a borehole filled from the surface and was intercepted during the excavation of a new gallery. In the first case, the hydrodynamic conditions were controlled by the ventilation of the drift; while in the second, the cement paste and claystone materials were considered saturated and far from any mechanical perturbation. A multi scale investigation was carried out to identify any evidence of alkaline perturbation in the cement based materials and the claystone. Chemical, mineralogical, and textural measurements were thus performed on the different materials in contact at the nanometer to a centimeter scale. Results showed that all the perturbations resulting from the geochemical contrast between the cement materials and the claystone were limited to a μm scale on each side of the interfaces. Carbonation was observed in the cement materials leading to an opening or a clogging of the porosity according to the hydrodynamic conditions and the formulation of the cement material. The distribution of the cation exchange population was also modified in the claystone in contact with the cement paste where a potassium saturation of the exchangeable fraction was identified. The originality of the present work is that realistic field controlled samples from the Andra URL were studied, hereby allowing the evaluation of the impact of natural heterogeneities of the in-situ experimental conditions (that is, hydrodynamic conditions, engineered damage zone, mineralogical variations) on the perturbations at the cement paste/claystone interfaces. An important result is that the clogging of porosity was not homogenous along the interfaces.

A new method for quantitative petrography based on image processing of chemical element maps: Part II. Semi-quantitative porosity maps superimposed on mineral maps

Abstract Visualizing and quantifying the spatial heterogeneity of rock textures (i.e., mineral and porosity distributions) is of great interest in petrology or for understanding petrophysical properties. Spatial heterogeneities are not accurately revealed by usual techniques based on microscopy or bulk physical measurements. Detailed mineral mapping is already available from processing of chemical element maps acquired using an electron probe microanalyzer (Part I). The present paper is devoted to developing a new, coupled method for obtaining porosity maps from the same initial data. According to the difference between measured and theoretical sums of oxide weight percentages, a mean porosity is semi-quantitatively estimated for each pixel of the map (i.e., not fully absolute or accurate). All pores, including nanometer-size ones, are taken into account, whereas a sample area of several square millimeters is analyzed (spatial resolution of a few micrometers). The textural heterogeneities are thus visualized from the complementary maps of solids and voids. By superimposing these two maps, both the mean porosity and a porosity histogram associated with each rock-forming mineral are obtained. Such porosity measurements integrate the pore amounts within mono-crystals larger than the X-ray emission volume or between nanometer-size crystals of a matrix. When porosity changes are associated with a given mineral (various crystal arrangements, dissolution, etc.), pluri-modal distributions appear on porosity histograms. Thresholding each histogram mode then allows these processes to be localized. We used the MX80 bentonite to test this methodology, which represents a useful tool to study the local deformations and alteration of each rock-forming mineral, as well as to model transport properties.

Distribution of Water in Synthetic Calcium Silicate Hydrates

Understanding calcium silicate hydrates (CSHs) is of paramount importance for understanding the behavior of cement materials because they control most of the properties of these man-made materials. The atomic scale water content and structure have a major influence on their properties, as is analogous with clay minerals, and we should assess these. Here, we used a multiple analytical approach to quantify water distribution in CSH samples and to determine the relative proportions of water sorbed on external and internal (interlayer) surfaces. Water vapor isotherms were used to explain the water distribution in the CSH microstructure. As with many layered compounds, CSHs have external and internal (interlayer) surfaces displaying multilayer adsorption of water molecules on external surfaces owing to the hydrophilic surfaces. Interlayer water was also quantified from water vapor isotherm, X-ray diffraction (XRD), and thermal gravimetric analyses (TGA) data, displaying nonreversible swelling/shrinkage behavior in response to drying/rewetting cycles. From this quantification and balance of water distribution, we were able to explain most of the widely dispersed data already published according to the various relative humidity (RH) conditions and measurement techniques. Stoichiometric formulas were proposed for the different CSH samples analyzed (0.6 < Ca/Si < 1.6), considering the interlayer water contribution.

INVESTIGATING THE ANISOTROPIC FEATURES OF PARTICLE ORIENTATION IN SYNTHETIC SWELLING CLAY POROUS MEDIA

The present study investigated the anisotropy in the orientation of particles in synthetic swelling clay media prepared from the sedimentation of particle-sized fractions of vermiculite. The different size fractions (>0.1, 0.1–0.2, 1–2, and 10–20 μm) were chosen because they represent the wide range of particle sizes of swelling clay minerals encountered in natural environments. Small-angle neutron scattering (SANS) and neutron diffraction measurements allowed the characteristic scattering/diffraction features to be derived and the quantitative information about the particle orientation distributions along two directions with respect to the sedimentation plane to be extracted. The results obtained confirmed that the increase in particle size was associated with the development of a random orientation for the particles, whereas the hydration state had a negligible impact on the organization of the porous media. For finer vermiculite particles, the rocking curves demonstrated an anisotropy of the systems that is similar to those reported previously on natural montmorillonite minerals. This result suggests that the location of the layer charge has little or no impact on the anisotropy features of particle orientation. For the coarsest fraction (10–20 μm), quantitative information about the particle orientation revealed that the relative proportion of the isotropic contribution represents up to 85% of the material. The anisotropy in the 2D SANS patterns revealed a pore-network anisotropy that was consistent with the particle size.