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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 Å at pH 10 to b = 2.834 Å 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. [ABSTRACT FROM AUTHOR]

Layer charge deficit is a key crystal-chemical parameter for the classification and nomenclature of 2:1 phyllosilicates that also controls some of their fundamental properties such as cation exchange capacity, expandability, water content, or rheology. The clay community has thus devoted significant efforts to determine this parameter either from the smectite/vermiculite chemical composition (the structural formula method) or experimentally using the alkylammonium method, that theoretically allows the determination of the mean layer charge density and of its distribution. In the present study density functional theory (DFT) and molecular dynamics (MD) simulations are compared to experimental X-ray diffraction data for a series of synthetic saponites with layer charge ranging from medium-charge smectite to medium-charge vermiculite. Consistency of computed and measured layer-to-layer distances confirms the ability of computational approaches to accurately predict the organization of alkylammonium cations in smectite interlayers. Thermalization of DFT-optimized structure models through MD simulations allows probing possible alternative interlayer configurations such as the electrostatically-favored location of ammonium heads above/below Al-substituted tetrahedra. X-ray diffraction results showed that layer-to-layer distances intermediate between those corresponding to the stable h1, h2, etc. interlayer configurations described in the literature result from the interstratification of different stable configurations. The overall consistency of computational and experimental results confirms also the ability of the alkylammonium method to accurately determine a mean value of layer charge density consistent with smectite structural formula when using the revised equations proposed by Laird et al. (1989). The validity of the method appears however limited to smectite-group minerals [layer charge density ranging from ~0.5 to ≤1.2 per O 20 (OH) 4 ] most likely owing to the coexistence of both stable "layered" and paraffin-like interlayer configurations for higher values of layer charge. In addition, this consistency challenges the ability of the method to quantitatively describe distributions of layer charge in expandable phyllosilicates even when using the complete series of alkylammonium cations. • Layer charge of synthetic saponites is assessed using the alkyl ammonium method (AAM). • Variation of layer-to-layer distances with alkyl chain length is compared with simulations. • Consistency of experimental and computational data confirms the robustness of the AAM. • The AAM appears unable to determine distribution of layer charge. • The AAM appears unable to differenciate high values of layer charge (vermiculites). [ABSTRACT FROM AUTHOR]

Abstract: In surficial environments, the fate of many trace metals 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, synthetic vernadite (δ-MnO2) was equilibrated at pH 5 or 7, reacted with dissolved Zn to produce Zn-sorbed δ-MnO2 with Zn/Mn atomic ratios from 0.003 to 0.156, and characterized structurally. The octahedral layers in the Zn-free vernadite contain on average ∼0.15 vacancies, ∼0.13–0.06 Mn3+ and ∼0.72–0.79 Mn4+. The layer charge deficit is compensated in the interlayer by Mn3+ bonded over Mn vacancy sites and Na+ located in the interlayer mid-plane. The average lateral dimension of coherent scattering domains (CSDs) deduced from X-ray diffraction (XRD) modeling is ∼5nm, consistent with that observed by transmission electron microscopy for individual crystals, indicating that the amounts of edge sites can be estimated by XRD. The average vertical dimension of CSDs is ∼1nm, equivalent to 1.5 layers and less than the observed 3–4 layers in the particles. Zinc sorption at pH 5 and 7 on pre-equilibrated vernadite induced crystal dissolution reducing the lateral CSD size ∼15–20%. Zinc K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and XRD show that Zn occurs in the interlayer above vacancies as a triple-corner-sharing surface complex, which is fully tetrahedral at low Zn/Mn ratios and increasingly octahedral at higher ratios. As Zn/Mn increases, the site density of layer Mn3+ decreases from 0.13±0.01 to 0.03±0.01 at pH 5 and from 0.06±0.01 to 0.01±0.01 at pH 7, and that of layer vacancies correspondingly increases from ∼0.15 to 0.24 and 0.21 at pH 5 and 7, respectively. These changes likely occur because of the preference of Zn2+ for regular coordination structures owing to its completely filled third electron shell (3d10 configuration). Thus, sorption of Zn into the interlayer causes the departure of layer Mn3+, subsequent formation of new reactive layer vacancies, and an increase in surface area through a reduction in particle size, all of which dynamically enhance the sorbent reactivity. These results shed new light on the true complexity of the reactive vernadite surface, and pose greater challenges for surface-complexation modeling of its sorption isotherms. [Copyright &y& Elsevier]

Smectite hydration impacts dynamical properties of interlayer cations and thus the transfer and fate of H 2 O, contaminants, and nutriments in surficial environments where this ubiquitous clay mineral is often one of the main mineral components. The influence of key crystal-chemical parameters, such as the amount of charge or the presence of fluorine, rather than hydroxyl groups, in smectite anionic framework, on hydration, organization of interlayer species, and related properties has been described for tetrahedrally substituted trioctahedral smectites (saponites). Despite the ubiquitous character of octahedrally substituted smectites, that make most of the world bentonite deposits, the influence of charge location on smectite hydration properties has not received similar attention. A set of octahedrally substituted trioctahedral smectites (hectorites) with a common structural formula Na x Mg 6-x Li x Si 8.0 O 20 (OH) 4 and a layer charge (x) varying from 0.8 to 1.6 was thus synthesized hydrothermally. The distribution of charge-compensating Na+ cations and of associated H 2 O molecules was determined experimentally from the modeling of X-ray diffraction data obtained along water vapor desorption isotherms. Consistent distributions of charge-compensating cations and of associated H 2 O molecules were also computed from GCMC simulations as a function of layer charge. Interlayer H 2 O contents [2.5–5.5 and 8.0–10.0 H 2 O molecules per O 20 (OH) 4 for 1W and 2W hydrates, respectively] are similar in all Na-saturated smectite samples, independent of the location and amount of their layer charge. In contrast to synthetic saponite, for which stability of most hydrated layers was increased by increasing layer charge, the stability of synthetic hectorite hydrates is only marginally affected by layer charge. Consistently, the layer-to-layer distance of Na-saturated hectorite 2W (and 1W) layers is independent of layer charge (15.10–15.65 Å and 12.0–12.6 Å, respectively). The contrasting hydration behavior of synthetic Na-saturated saponite and hectorite is likely due to different electrostatic attraction between the 2:1 layer and interlayer cation, the charge undersaturation of O atoms at the surface of hectorite 2:1 layer being more diffuse compared to saponite. Combined with previous results on saponites, the present data and sample set provides key constraints to assess the validity of force fields simulating clay-water interactions for an unmatched variety of smectite with contrasting locations and amounts of layer charge deficits. Unlabelled Image • Hectorites with a layer charge varying from 0.8 to 1.6 were synthesized hydrothermally. • H 2 O contents of 1W and 2W layers are independent of layer charge amount and location. • Hectorite hydration is marginally affected by the amount of layer charge. • Saponite & hectorite different hydration behaviors due to undersaturation of O atoms. [ABSTRACT FROM AUTHOR]