A new modeling of the X-ray diffraction by disordered lamellar structures, such as phyllosilicates.

The “classical” modeling of powder X-ray diffraction (XRD) patterns of lamellar structures, such as phyllosilicates, assumes that the samples are composed of “crystals” having various thickness and well-defined translations between layers. This model is able to describe the high-angle domain of XRD patterns but sometimes fails in the low-angle region. The new model proposed here considers the samples to be composed of “particles” that have larger sizes than crystals and contain defects such as cracks, inner-porosity, bent layers, edge dislocations, etc. These defects induce variations in the d-spacings, introduced in the calculation by distributions of the d-spacings. For phyllosilicates, this model is consistent not only with XRD, but also with small-angle X-ray scattering (SAXS) data, transmission electron microscopy (TEM) results, and high-resolution transmission electron microscopy (HRTEM) observations

Influence of the Outer Surface Layers of Crystals on the X-Ray Diffraction Intensity of Basal Reflections

International audienceThis study presents a mathematical formalism describing diffraction effects from periodic and mixed-layer minerals in which the outer surface layers of crystals differ from layers forming the core of the crystals. XRD patterns calculated for structure models of chlorite and irregular chlorite-smectites terminated on both sides of the crystals by either brucite-like or 2:1 layers show the strong influence that different outer surface layers make on the distribution of basal reflection intensities. Simulation of the experimental XRD patterns from two chlorite samples having different Fe-content shows that in these two samples the chlorite crystals were terminated by brucite-like layers on both sides. In contrast, crystals in a corrensite sample were terminated by water molecules and exchangeable cations. The nature of diffraction effects due to outer surface layers is discussed

Structure of Synthetic K-rich Birnessites Obtained by HighTemperature Decomposition of KMnO4. 2 Phase and Structural Heterogeneities

International audienceSynthetic K-rich birnessites (KBi) were prepared from the thermal decomposition of a fine-grained KMnO4 powder heated in air atmosphere at temperatures ranging from 2001000°C. The qualitative analysis of powder X-ray diffraction (XRD) patterns reveals a complex range of structural transformations from one metastable phase to the other, often through intermediate mixed-layer structures (MLSs). Phase and structural heterogeneities of KBi samples synthesized at 700°C, 800°C and 1000°C (referred to as KBi7, KBi8h and KBi10h) have been studied in details by chemical and thermal analysis and by simulation of the experimental powder XRD patterns. Two-layer orthogonal (2O), and hexagonal (2H) as well as three-layer rhombohedral (3R) polytypes were identified in these samples. The 2O structure consists of vacancy-free layers and their orthogonal symmetry is linked to the high content of layer Mn 3+ cations and to the unique azimuthal orientation of Mn3+ octahedra which are elongated because of Jahn-Teller distortion. In the 2H and 3R polytypes, the layers have a hexagonal symmetry as they contain only Mn 4+- and vacant octahedra. As a result, their interlayers have a heterogeneous cation composition, because of the migration of Mn 3+ from the layers to the interlayers. In addition to the periodic KBi polytypes, KBi7 and KBi8h contain MLSs in which layer pairs of the 2H polytype are interstratified at random with those of the 3R or of the 2O polytype. Interstratification of incommensurate 2O and 2H structural fragments leads to peculiar diffraction effects and represents a new type of structural disorder in birnessites. The increase of temperature from 700°C to 1000°C is associated with the replacement of 3R/2H, 2H, and 2O/2H mixed-layered structures by the more stable 2O polytype. KBi10h consists of a mixture of a minor 2H phase with three 2O varieties having slightly different layer unit-cell parameters. This phase heterogeneity results from the partial disorder in the orientation of Mn 3+ octahedra. The average structural formulae, K + 0.265Mn 3+ 0.145(Mn 4+ 0.82

Structure of heavy metal sorbed birnessite. Part 1: Results from X-ray diffraction

International audienceThe structure of heavy-metal sorbed synthetic birnessites (MeBi) was studied by powder X-ray diffraction (XRD) using a trial-and-error fitting procedure to improve our understanding of the interactions between buserite/birnessite and environmentally important heavy metals (Me) including Pb, Cd, and Zn. MeBi samples were prepared at different surface coverages by equilibrating at pH 4 a Na-rich buserite (NaBu) suspension in the presence of the desired aqueous metal. Two main types of experimental XRD patterns were obtained as a function of the nature of Me cations sorbed from solution which exerts a strong control on layer stacking sequence, as well as on location and coordination of Me: 1) CdBi and PbBi samples correspond to a one-layer hexagonal (1H) structure, AbCb'A' C'b'AbC..., and 2) ZnBi exhibits a one-layer monoclinic (1M) structure in which adjacent layers are shifted by +a/3, AbCb'A'c'BcAc'B'a'CaBa'C'b'AbC. Simulated XRD patterns shows that octahedral layers contain 0.833 Mn cations (Mn 4+ and Mn 3+) and 0.167 vacant octahedra; Mn3+ interlayer and adsorbed Meinterlayer compensate for the layer charge deficit. Mn 3+ interlayer is octahedrally coordinated in all samples and is located above or below vacant layer octahedra sharing three Olayer with neighboring Mnlayer octahedra to form a triple-corner surface complex ( VITC sites). In ZnBi and CdBi samples, Me interlayer is also located in TC sites; all Cd is octahedrally coordinated whereas about 30% of Zn is tetrahedrally coordinated ( IVTC sites). In PbBi samples, all Pb is octahedrally coordinated, most of these cations (~75%) being located in TC sites. Additional Pb is located above or below empty tridentate cavities, sharing three edges with neighboring Mnlayer octahedra ( VITE sites). Structural formulae calculated for each sample show that during the NaBu-to-MeBi structural transformation, interlayer Na + and Mn2+ are replaced by Me and H+ adsorbed from solution, whereas Mn 3+ interlayer resulting from the equilibration of NaBu at low pH is less affected. Sorption of divalent Me above and below vacant layer sites provides optimal conditions for local charge compensation in MeBi

Multi-scale characterization of monument limestones.

Among the parameters influencing stone deterioration, moisture and water movements through the pore network are essential. This communication presents differents methods to characterize stones and to determinate the water transfer properties. Results are analysed for two limestones having similar total porosity, but characterized by different pore networks. These different porous systems govern dissimilar water properties

New insights on the distribution of interlayer water in bi-hydrated smectite from X-ray diffraction profile modeling 00l reflections

International audienceThe 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

Experimental study of smectite interaction with metal iron at low temperature: 1. Smectite destabilization.

Interaction between metal Fe and a variety of natural and synthetic smectite samples with contrasting crystal chemistry was studied by scanning electron microscopy and X-ray diffraction from experiments conducted at 80°C. These experiments demonstrate an important reactivity contrast as a function of smectite crystal chemistry. An XRD method involving the use of an internal standard allowed quantification of the relative proportion of smectite destabilized as a function of initial pH conditions as well as of smectite structural parameters. In mildly acidic to neutral pH conditions, a significant proportion of metal Fe is corroded to form magnetite without smectite destabilization. Under basic pH conditions, smectite and metal Fe are partly destabilized to form magnetite and newly-formed 1:1 phyllosilicate phases (odinite and crondstedtite). More specifically, systematic destabilization of both metal Fe and smectite is observed for dioctahedral smectites while trioctahedral smectites are essentially unaffected under similar experimental conditions. In addition, smectite reactivity is enhanced with increasing Fe3+ content and with the presence of Na+ cations in smectite interlayers. A conceptual model for smectite destabilization is proposed. This model involves first the release of protons from smectite structure, MeFe3+OH groups being deprotonated preferentially and metal Fe acting as proton acceptor. Corrosion of metal Fe results from its interaction with these protons. The Fe2+ cations resulting from this corrosion process sorb on the edges of smectite particles to induce the reduction of structural Fe3+ and migrate into smectite interlayers to compensate for the increased layer-charge deficit. Interlayer Fe2+ cations subsequently migrate to the octahedral sheet of smectite because of the extremely large layer-charge deficit. At low temperature, this migration is favored by the reaction time and by the absence of protons within the ditrigonal cavity. Smectite destabilization results from the inability of the tetrahedral sheets to accommodate the larger dimensions of the newly formed trioctahedral domains resulting from the migration of Fe2+ cations