Showing papers by "Bruno Lanson published in 2014"

Solid-state transformation of nanocrystalline phyllomanganate into tectomanganate: influence of initial layer and interlayer structure.

Abstract: In surficial environments, the fate of many elements is influenced by their interactions with the phyllomanganate vernadite, a nano-sized and turbostratic variety of birnessite. To advance our understanding of the surface reactivity of vernadite as a function of pH, synthetic vernadite (δ-MnO2) was equilibrated at pH ranging from 3 to 10 and characterized structurally using chemical methods, thermogravimetry and modelling of powder X-ray diffraction (XRD) patterns. With decreasing pH, the number of vacant layer sites increases in the octahedral layers of δ-MnO2 (from 0.14 per layer octahedron at pH 10 to 0.17 at pH 3), whereas the number of layer Mn3+ is, within errors, equal to 0.12 per layer octahedron over the whole pH range. Vacant layer sites are capped by interlayer Mn3+ sorbed as triple corner-sharing surface complexes (TC sites). The increasing number of interlayer Mn3+ with decreasing pH (from 0.075 per layer octahedron at pH 10 to 0.175 at pH 3) results in the decrease of the average Mn oxidation degree (from 3.80 ± 0.01 at pH 10 to 3.70 ± 0.01 at pH 3) and in the lowering of the Na/Mn ratio (from 27.66 ± 0.20 at pH 10 to 6.99 ± 0.16 at pH 3). In addition, in-plane unit-cell parameters are negatively correlated to the number of interlayer Mn at TC sites and decrease with decreasing pH (from b = 2.842 A at pH 10 to b = 2.834 A at pH 3), layer symmetry being systematically hexagonal with a = b × 31/2. Finally, modelling of X-ray diffraction (XRD) patterns indicates that crystallite size in the ab plane and along the c* axis decreases with decreasing pH, ranging respectively from 7 nm to 6 nm, and from 1.2 nm to 1.0 nm (pH 10 and 3, respectively). Following their characterization, dry samples were sealed in polystyrene vials, kept in the dark, and re-analysed 4 and 8 years later. With ageing time and despite the dry state, layer Mn3+ extensively migrates to the interlayer most likely to minimize steric strains resulting from the Jahn–Teller distortion of Mn3+ octahedra. When the number of interlayer Mn3+ at TC sites resulting from this migration reaches the maximum value of ∼ 1/3 per layer octahedron, interlayer species from adjacent layers share their coordination sphere, resulting in cryptomelane-like tunnel structure fragments (with a 2 × 2 tunnel size) with a significantly improved layer stacking order.

Interlayer structure model of tri-hydrated low-charge smectite by X-ray diffraction and Monte Carlo modeling in the Grand Canonical ensemble

Abstract: The present study aims primarily at refining a structure model for interlayer cations and H2O molecules in tri-hydrated (3W) smectite (d(001) = 18-19 angstrom). The <2 mu m fraction of the SWy-2 source clay (low-charge montmorillonite) was saturated by Mg2+, Ca2+, Ba2+, or Na cations, before collection of X-ray diffraction (XRD) patterns at 98% relative humidity. Experimental d(001) values derived for the essentially homogeneous 3W hydrates provided volume constraints for Grand Canonical Monte Carlo (GCMC) simulations. Computed atomic density distribution of interlayer species were used in turn to calculate XRD intensities of 00l reflections. The agreement between calculated and experimental 00l intensities allowed validating the GCMC results of both interlayer H2O content and distribution of interlayer species (cations and H2O molecules). Computed atomic density profiles do not correspond to the usual model of three discrete planes of H2O molecules but rather exhibit two sharp planes of H2O molecules wetting the clay surfaces (at similar to 2.7 angstrom from the clay layer surface). Additional H2O molecules belong to cation hydration shells or define a poorly organized ensemble filling internal voids. This alternative model suggests that the stability of the 3W hydrate results from the dual interaction of some H2O molecules with interlayer cation, through their second hydration shell; and with the 2:1 clay surface. Computed atomic density profiles were approximated to propose an interlayer structure model for 3W smectite. This simplified model includes two sets of two planes (symmetrical relative to the interlayer mid-plane) for H2O molecules and one set for interlayer cations. This model allows reproducing experimental XRD patterns for the different samples investigated and thus represents a valid set of parameters for routine quantitative analysis of XRD profiles in an effort to determine smectite reactivity close to water-saturated conditions. Implications of such studies are crucial to provide experimental constraints on the behavior of the main vector of element transfer under conditions common in surficial environments and prevailing in waste repositories. In addition, the present study provides an experimental validation of structure models derived from the widely used ClayFF model, and thus allows its use to predict the fate of water in clayey systems close to water-saturated conditions.

Structure of nanocrystalline calcium silicate hydrates

Abstract: Nanocrystalline calcium silicate hydrates (C-S-H) are the main hydration products and the main binging phases in many types of cement, including ordinary Portland cement. As a result of its ubiquity is these engineered systems, C-S-H controls main cement physical and chemical properties. These properties depend on C-S-H calcium to silicon atomic ratio (Ca/Si), which is commonly described as ranging between 0.6 and 2.3. This means that C-S-H structure dictates cement macroscopic properties. As a consequence, its crystal-chemistry must be understood to be able to understand (and thus predict) cement properties. However, despites decades of study, its crystal structure is still a matter of debate. Depending on the authors and on the calcium to silicon ratio, the structure is described similar to one or more of the following minerals: tobermorite, jennite and possibly portlandite. Such inaccuracy largely results from its X-ray diffraction patterns, which exhibits only a few weak and mostly asymmetrical diffraction maxima and thus cannot be refined using classical methods. By using a specific formalism for the analysis of X-ray diffraction patterns, previously applied to phyllosilicates and phyllomanganates, it is here proposed that C-S-H can be described as a lamellar structure similar to nanocrystalline and turbostratic tobermorite, turbostratism meaning that there is the systematic presence, between adjacent layers, of a random rotation about the normal to the layers and/or a random translation in the layer plane. This model was validated using complementary methods (including transmission electron microscopy, synchrotron X-ray absorption spectroscopy and synchrotron high-energy X-ray scattering). From analysis of literature data, it is proposed that the evolution of C-S-H structure as a function of Ca/Si can be described as interstratification of two different types of layers having calcium to silicon ratios of 0.6 and 1.25, plus discrete Ca(OH)2 at higher ratios.