X-ray Diffraction Profile Modeling Research Articles

Revealing Transition States during the Hydration of Clay Minerals.

A molecular-scale understanding of the transition between hydration states in clay minerals remains a challenging problem because of the very fast stepwise swelling process observed from X-ray diffraction (XRD) experiments. XRD profile modeling assumes the coexistence of multiple hydration states in a clay sample to fit the experimental XRD pattern obtained under humid conditions. While XRD profile modeling provides a macroscopic understanding of the heterogeneous hydration structure of clay minerals, a microscopic model of the transition between hydration states is still missing. Here, for the first time, we use molecular dynamics simulation to investigate the transition states between a dry interlayer, one-layer hydrate, and two-layer hydrate. We find that the hydrogen bonds that form across the interlayer at the clay particle edge make an important contribution to the energy barrier to interlayer hydration, especially for initial hydration.

Experimental evidence of the contrasting reactivity of external vs. interlayer adsorption sites on swelling clay minerals: The case of Sr2 +-for-Ca2 + exchange in vermiculite

The potential contrasting reactivity of external (edge and basal) and interlayer adsorption sites is studied by investigating the Sr2+-for-Ca2+ exchange in three different size fractions (0.1–0.2, 1–2, and 10–20μm) of vermiculite. For a given equivalent fraction of Sr2+ in solution, the obtained experimental adsorption isotherms showed that the equivalent fraction of adsorbed Sr2+ on the solid decreased with decreasing particle size.Using a combination of bulk chemistry, X-ray diffraction profile modeling of 00ℓ reflections and morphological data, the interlayer site compositions were found to be independent of the vermiculite size fraction for a given chemical composition of the solution and without any preference between Ca2+ and Sr2+ cations. The lack of selectivity of both of these cations for interlayer adsorption sites [log Kint.(Ca2+/Sr2+)=0] is supported by molecular simulation results indicating that Ca2+ and Sr2+ have similar interactions with the clay surface and water molecules in the interlayer space of vermiculite. In contrast, external adsorption sites exhibited a clear preference for Ca2+ cations [−2.5<log Kext.(Ca2+/Sr2+)<−1.0]. The two-site model (i.e., interlayer and external adsorption sites), which is constrained by the relative amounts of both types of sites derived from morphological analysis, is found to well interpret experimental ion-exchange experiments.The two-site ion-exchange model derived from this study represents a morphology-constrained model with clear evidence of contrasting reactivity between external and interlayer adsorption sites. This model can be used to improve our understanding and prediction of the behavior of Sr2+ cations in contaminated soils with a wide range of particle sizes.

Experimental aluminization of vermiculite interlayers: An X-ray diffraction perspective on crystal chemistry and structural mechanisms

Natural aluminization of swelling clay minerals is ubiquitous in acidic soils leading to the formation of hydroxy-interlayered (HI) minerals. This process has drawn special attention over the last 4–5 decades owing to the negative impact on soil fertility of the induced reduction of cation exchange capacity. Combination of chemical analyses and of X-ray diffraction profile modeling on a series of experimentally self-aluminized samples aimed at an improved description of HI minerals and more especially of their interlayer crystal chemistry. Both exchangeable alkali cations and hydroxy (Al, Fe) cations coexist within HI vermiculite interlayers. The latter form discontinuous gibbsite-like interlayer sheets with ~15% completeness, despite layer-to-layer distances similar to that of chlorite. From charge compensation considerations, the isolated hydroxy (Al, Fe) cation clusters bear ~1.5 positive charges per cation. Analysis of X-ray diffraction data indicates that aluminization of initially swelling interlayers is a layer-by-layer process leading to mixed layers composed of randomly interstratified swelling and aluminized layers, all aluminized layers likely hosting a similar number of hydroxy (Al, Fe) cations along the aluminization process. This model contradicts the widely accepted description of HI minerals as a solid solution between expandable 2:1 clay and aluminous chlorite end-members. As a consequence, the proportion of HI layers in the mixed layer is a robust estimate of aluminization progress and both the amount of extractible (Al, Fe) and the extent of CEC decrease are positively correlated to this essential parameter.

Investigation of clay mineralogy in a temperate acidic soil of a forest using X-ray diffraction profile modeling: Beyond the HIS and HIV description

The crystal structures as well as the amount of hydroxy-interlayered clay minerals resulting from the Al-hydroxylation process in temperate acidic soil developed in a forest were studied by modeling experimental X-ray diffraction (XRD) patterns. The <2μm clay fraction of this soil was separated into four sub-fractions of different particle sizes (<0.05, 0.05–0.1, 0.1–0.2, and 0.2–2μm), and XRD patterns were obtained after different treatments of the samples (i.e., calcium saturation and air-drying, ethylene-glycol solvation, or potassium saturation and heating at 110°C). The qualitative analysis of the experimental XRD patterns revealed the presence of hydroxy-interlayered-smectite (HIS) and hydroxy-interlayered-vermiculite (HIV) phases commonly considered in the description of clay mineralogy in acidic soil environments. Quantitative information derived from the fitting procedure for the different sub-fractions allowed for the determination of the complex mineralogy of the <2μm clay fraction through the identification and quantification of ten clay phases. The results revealed that the HIS and HIV clay phases considered so far in the literature actually consist of three-component randomly interstratified mixed-layer minerals (MLMs) with contrasting relative proportions of smectite, hydroxy-interlayers (HI) and illite layers. Moreover, the quantitative results derived from this study showed that these two MLMs account for less than 50% of the total HI layer abundance. In that regard, most HI layers are present in illite-rich MLMs with poorly resolved XRD reflections. The contrasting behaviors of smectite and HI layers in the coarsest 0.2–2μm sub-fraction compared to those in the finer sub-fraction indicated the possible increase of the layer charge of the expandable layers in the coarsest sub-fraction. Finally, it was shown that the detailed structural analysis derived from the methodology used in the present study is conducive for estimating the degree of expandable layer transformation in natural soil samples while reconciling the specific behaviors of expandable and HI layers from natural soil with those described by laboratory experiments performed on monomineralic samples.

Impact of swelling clays on the spalling decay of building limestones: insights from X-ray diffraction profile modeling

Abstract:IntheProvenceregion(south-easternFrance),alargepartofthebuildingheritagewaserectedusingabioclasticlimestonecalled ‘‘Pierre du Midi’’. Under this common name, the heterogeneity of the material and, specifically, its stability upon climaticexposure depend on the actual extraction location. In some cases, building stones are affected by spalling decay which is commonlyrelated to the presence of clay minerals although without quantitative support. The present study aims at characterizing andquantifying the clay mineralogy of eight ‘‘Pierre du Midi’’ samples showing various spalling degrees.The combination of transmission electron microscopy (TEM) coupled to energy-dispersive X-ray spectrometer (EDX) and of X-ray diffraction (XRD) full-profile modeling of 00‘ reflections patterns is used for quantitative phase analysis. Identification of clayminerals present in the , 4 mm fractions is performed on the basis of qualitative analysis of experimental XRD patterns recorded inbothair-dried(AD)andglycolated(EG)states.Thecrystal-chemistryofclaymineralsisthenrefinedfromTEM-EDXanalyseswhichare used as an essential constraint for the modeling of both AD and EG XRD patterns.Theresultshighlightthatthecomplexclaymineralogyoflimestonescanbeunraveledandquantifiedwhenusingthepresentcoupledapproach.Finally,thecontrastingsensitivityofsamplestospallingdecaycanbeexplainedbytheoverallproportionofexpandablelayers,essentiallypresent inFe-rich mixed-layers,inthe clayfraction.A content ofexpandable layers higherthan0.80 wt. % resultsina highsensitivitytospallingdecay,whereas rocks withlessthan0.20wt. % ofexpandablelayersremainessentially unaffected.Key-words: clay minerals; building stone; spalling decay; XRD profile modeling; TEM-EDX; mixed layer minerals; intra-crystalswelling; Fe-montmorillonite.

A thermodynamic model for the prediction of pore water composition of clayey rock at 25 and 80 °C — Comparison with results from hydrothermal alteration experiments

This study proposes a thermodynamic model that is able to predict the pore water chemical composition of clayey rocks at temperatures up to 80°C. The model is based on equilibrium with a quartz/kaolinite/calcite/dolomite/Mg-chlorite mineralogical buffer and accounts for the sorption properties of both the montmorillonite and illite phases.Hydrothermal alteration experiments were also performed at 80°C on centimeter-sized samples from the Callovo-Oxfordian clay-rich formation (Paris Basin, France) to assess the validity of the proposed model at this temperature. The experiments were performed in both open and closed systems, and the initial CO2(g) partial pressure (pCO2(g)) used in each system was fixed to log pCO2(g)=−0.4. After 15months of alteration, the aqueous and gas phases were extracted at 80°C and analyzed. The evolution of the clay mineralogy was also characterized using X-ray diffraction profile modeling of experimental patterns and transmission electron microscopy (TEM).The composition of the Callovo-Oxfordian pore water predicted by our model at 25°C is in good agreement with that reported in the literature and measured in the geological formation. At 80°C, experiments in open systems were used to assess the kinetics of alteration process and the time needed to reach equilibrium for several dissolution–precipitation mineral reactions. For the closed system, it is shown that the proposed model well predicts the chemical composition of the fluid extracted at 80°C, considering the existing variability in the solubility constants for both Mg-chlorite and kaolinite minerals. Despite the need for additional alteration experiments to better constrain the temperature-dependence of the predicted pCO2(g) values, the proposed model provides reliable predictions of pore fluid composition at 80°C, a temperature expected in the framework of radioactive waste storage.

Structural phase transitions during high-energy ball milling of BaTiO3

Microstructural characterization of ball milled perovskite BaTiO3 powders has been done by the modeling of X-ray diffraction profiles. The study reveals that on size reduction, BaTiO3 powders undergo a continuous, displacive, and diffusionless dynamic phase transitions involving tetragonal (T), monoclinic (M), and orthorhombic (O) symmetry via the second-order type [T → (T + M) → (M + O) → O] when stimulated by a high-power pulse of pressure in a planetary mill. The order parameter, a phenomenological quantity to describe the general behavior of a system going through phase transitions has been estimated using spontaneous strain calculated from lattice parameters or physical distortions derived from atomic coordinates or both. At room temperature, BaTiO3 nanoparticle achieved an orthorhombic phase when a critical size (<15 nm) has been reached at later stage of milling (≥70 h). Raman's study reveals similar structural phase transitions sequence on size reduction and TEM study reveals the corresponding particle diameter.

Unraveling complex &lt;2 m clay mineralogy from soils using X-ray diffraction profile modeling on particle-size sub-fractions: Implications for soil pedogenesis and reactivity

A specific methodology was developed to refine the complex clay mineralogy commonly encountered in soil environments. The soil examined was a Cambisol developed into a ferralitic paleosol. The sample was split into four sub-fractions of different particle sizes (&lt;0.05, 0.05-0.1, 0.1-0.2, and 0.2-2 μm), and their respective mass contributions to the overall &lt;2 μm clay fraction were determined. For each sub-fraction, X-ray diffraction (XRD) patterns were modeled using a trial-and-error approach based on the direct comparison of experimental and calculated profiles. Quantitative information derived from the fitting procedure for the different sub-fractions allowed for the determination of the complex mineralogy of the &lt;2 μm clay fraction through the identification and quantification of eight clay phases. The results show that the finest and most reactive clay fraction (&lt;0.05 μm) was totally hidden in the XRD pattern of the &lt;2 μm fraction, the fraction commonly considered in soil mineralogical analyses. Similarly, this procedure revealed the presence of illite-smectite-chlorite and kaolinite-illite mixed-layer minerals seldom described in soil literature using classical methods. The use of this methodology improved our understanding of the pedogenesis of this soil through the identification and quantification of clay phases structural properties. The analysis of the evolution of structural parameters with particle size allowed for the detection of local modifications in the interlayer composition of expandable and hydroxy-interlayered vermiculite layers. Following this approach, key information can be derived to determine subtle changes in clay mineralogical composition that are related to microorganism and/or plant activity.

Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 1. Results from X-ray Diffraction Profile Modeling

The dehydration of two Na-saturated synthetic saponites with contrasting layer charge was studied by modeling the X-ray diffraction (XRD) patterns recorded along a water vapor desorption isotherm. The interlayer configurations used to reproduce the XRD data over a large angular range include Na+ cations located in the interlayer midplane and H2O molecules normally distributed about one or two main positions for mono- and bihydrated layers, respectively. Although strongly reduced in comparison to natural smectites, hydration heterogeneity was systematically observed for these synthetic saponites, especially along the transition between two hydration states. Using improved models for the description of the interlayer organization, the influence of layer charge on the structure of interlayer water can be precisely assessed. In addition, the comparison with water contents obtained from water vapor gravimetry experiments allows discriminating the relative contributions of H2O molecules from 1W and 2W interlayers (crystalline water) and from the pore space network.

Early clay diagenesis in Gulf Coast sediments: New insights from XRD profile modeling

AbstractSamples from different depths in the Oligocene Frio formation (offshore Gulf of Mexico) were studied by X-ray diffraction (XRD), thermal analyses, and scanning electron microscopy. The experimental XRD patterns recorded from oriented and ethylene glycol (EG) solvated clay fractions of the samples were similar to those typical of random, mixed-layered illite-smectite (R0 I-S). The experimental XRD patterns recorded in air-dried (AD) and EG states were simulated using three different models. One of them corresponds to R0 I-S for which thickness and content of the interstratified layers were determined by the Środoń technique. The second model is represented by a single homogeneous I-S in which illite and smectite layers are interstratified with a tendency to segregation. The expandability of the segregated I-S model varies from 48% to 75% without any rational relationship between the smectite layer content and depth.The third model assumes that the clay fraction is a physical mixture of smectite and an R0 I-S. In this model the I-S contains 65% illite and 35% smectite layers independent of depth, whereas the smectite content varies from 28% to 63%. This model has consistently smaller profile factors, Rp, for both EG and AD XRD scans compared with the Rp values determined for the other two models.The mineralogical association, volcanic origin, narrow stratigraphic interval (427 m), and low maximum temperature (42°C) of the studied Frio Formation are considered. These features are completely consistent with the two-phase model and so the segregation model must be rejected. An authigenic origin of the pure smectite and an alternative detrital or authigenic origin of the R0 I-S are discussed.

Investigation of dioctahedral smectite hydration properties by modeling of X-ray diffraction profiles: Influence of layer charge and charge location

Hydration of the &lt;1 μm size fraction of a high-charge montmorillonite (Clay Minerals Society Source Clay SAz-1), and of low- and high-charge beidellites (Source Clays SbId-1 and SbCa-1, respectively) was studied by modeling of X-ray diffraction patterns recorded under controlled relative humidity (RH) for Sr- and/or Ca-saturated specimens. The influence of layer charge and charge location on smectite hydration was studied. Distribution of layers with different hydration states (dehydrated-0W, monohydrated-1W, bihydrated-2W, or tri-hydrated-3W) within smectite crystals often leads to two distinct contributions to the X-ray diffraction pattern, each contribution having different layer types randomly interstratified. Structure models are more heterogeneous for beidellite than for montmorillonite. For beidellite, two distinct populations of particles with different coherent scattering domain sizes account for the heterogeneity. Increased hydration heterogeneity in beidellite originates also from the presence of 0W (non-expandable) and of 1W layers under high relative humidity (RH) conditions. Similarly, after ethylene-glycol (EG) solvation, some beidellite layers incorporate only one plane of EG molecules whereas homogeneous swelling was observed for montmorillonite with the systematic presence of two planes of EG molecules. For montmorillonite and beidellite, the increase of layer charge shifts the 2W-to-1W and the 1Wto- 0W transitions toward lower RH values. For all samples, layer thickness of 0, 1, and 2W layer types was similar to that determined for low-charge SWy-1 montmorillonite (Source Clay SWy-1), and no change of layer thickness was observed as a function of the amount or of the location of layer charge. However, layer thickness increased with increasing RH conditions.

Crystal Structure of Mixed-Layer Minerals and Their X-ray Identification: New Insights from X-ray Diffraction Profile Modeling

Modeling of experimental X-ray diffraction (XRD) patterns represents the optimum approach to the structure determination of mixed-layer structures (MLSs) that are commonly found in natural clay-rich samples. This approach allows for a detailed structural characterization of both pure and mixed-layer clay phases and for a semi-quantitative phase analysis in complex mixtures. The two informations are essential to gain new insight into the actual nature of reactions taking place in geological environments. Significant new findings obtained at different scales (from that of the particle to that of the elementary layer) on the actual structure of MLSs by modeling XRD profiles are reported.

New Insights on the Distribution of Interlayer Water in Bi-Hydrated Smectite from X-ray Diffraction Profile Modeling of 00l Reflections

The interlayer configuration proposed by Moore and Reynolds and commonly used to reproduce the 00l reflections of bi-hydrated smectite is shown to be inconsistent with experimental X-ray diffraction data. 1 The alternative configuration of interlayer species with cations located in the mid-plane of the interlayer and one sheet of H2O molecules on each side of this plane is also shown to imperfectly describe the actual structure of bi-hydrated smectites. Specifically, the thermal fluctuation of atomic positions (Debye-Waller factor) used to describe the positional disorder of interlayer H2O molecules has to be increased to unrealistic values to satisfactorily reproduce experimental X-ray diffraction data when using this model. A new configuration is thus proposed for the interlayer structure of bi-hydrated smectite. Cations are located in the mid-plane of the interlayer whereas H2O molecules are scattered about two main positions according to Gaussian-shaped distributions. This configuration allows reproducing all 00l reflections with a high precision, with only one new variable parameter (width of the Gaussian function). The proposed configuration is consistent with those derived from Monte-Carlo calculations and allows matching more closely the amount of interlayer water that can be determined independently from water vapor adsorption/desorption isotherm experiments. In addition, the proposed configuration of interlayer species appears valid for both dioctahedral and trioctahedral smectites exhibiting octahedral and tetrahedral substitutions, respectively, thus not allowing to differentiate these expandable 2:1 phyllosilicates from their respective interlayer configuration.

Analysis of protein-labeling solutions ( 99mTc) by zonal electrophoresis

The potential contrasting reactivity of external (edge and basal) and interlayer adsorption sites is studied by investigating the Sr2 +-for-Ca2 + exchange in three different size fractions (0.1–0.2, 1–2, and 10–20 μm) of vermiculite. For a given equivalent fraction of Sr2 + in solution, the obtained experimental adsorption isotherms showed that the equivalent fraction of adsorbed Sr2 + on the solid decreased with decreasing particle size.Using a combination of bulk chemistry, X-ray diffraction profile modeling of 00ℓ reflections and morphological data, the interlayer site compositions were found to be independent of the vermiculite size fraction for a given chemical composition of the solution and without any preference between Ca2 + and Sr2 + cations. The lack of selectivity of both of these cations for interlayer adsorption sites [log Kint.(Ca2 +/Sr2 +) = 0] is supported by molecular simulation results indicating that Ca2 + and Sr2 + have similar interactions with the clay surface and water molecules in the interlayer space of vermiculite. In contrast, external adsorption sites exhibited a clear preference for Ca2 + cations [− 2.5 < log Kext.(Ca2 +/Sr2 +) < − 1.0]. The two-site model (i.e., interlayer and external adsorption sites), which is constrained by the relative amounts of both types of sites derived from morphological analysis, is found to well interpret experimental ion-exchange experiments.The two-site ion-exchange model derived from this study represents a morphology-constrained model with clear evidence of contrasting reactivity between external and interlayer adsorption sites. This model can be used to improve our understanding and prediction of the behavior of Sr2 + cations in contaminated soils with a wide range of particle sizes.