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 2: Results from electron diffraction

International audienceSelected-area electron diffraction (SAED) and energy dispersive analysis were used to study the structure of synthetic heavy-metal sorbed birnessites (MeBi). Samples were prepared by equilibrating a suspension of Na-rich buserite (NaBu) at pH4 in the presence of various heavy metal cations (Me), including Pb, Cd, Zn, and Cu. Five main types of SAED patterns were observed. Types I and II were observed only for ZnBi micro-crystals, and they both consist of two super-cell reflection networks related by a mirror plane parallel to the a *c* plane. In direct space, these twinned networks correspond to the hexagonal supercells with AH = BH = 7b/ 3, and AH = BH = 7b, for ZnBi type I and II, respectively. In the two varieties, the supercells result from an ordered distribution of vacant layer octahedra capped by interlayer Zn in ZnBi layers. This distribution is described by a hexagonal cell with AH = 7b. In ZnBi micro-crystals of type I, interstratified twinned right- and left-handed fragments are similar to chalcophanite (ZnMn3O7-3H2O - Wadsley 1955; Post and Appleman 1988), and distributions of vacant layer octahedra from adjacent layers are regularly shifted with respect to each other by 1/3 of the long diagonal of the hexagonal layer unit cell. In ZnBi micro-crystals of type II, distributions of vacant layer octahedra are not regularly shifted from one layer to the adjacent one. SAED patterns of types III and IV occur for PbBi, ZnBi, and CdBi micro-crystals and contain super-cell reflections distributed parallel to [100] * with a periodicity which is not commensurate with that of the MeBi sub-structure (a */2.15 and a*/5.25, respectively). The super-cell reflections result from the ordered distribution within MeBi layers of vacant layer sites capped by Me as pairs along the a axis. Within each pair, vacant sites are separated by 2a for type III, and by 5a for type IV. In one-layer monoclinic structures, the apparent incommensurability arises from the +a/3 shift between adjacent layers having a similar one-dimensional periodic distribution of interlayer Me located above and below vacant octahedra sharing three corners with Mnlayer octahedra (TC sites). Tetrahedral coordination of these Me cations in TC sites, as in ZnBi, leads to the formation of strong H-bonds between adjacent layers. A similar incommensurate effect occurs in one layer hexagonal MeBi if octahedrally coordinated Me cations periodically distributed along the a axis are located above and/or below empty tridentate cavities sharing three edges with Mnlayer octahedra ( VITE sites, PbBi). SAED patterns of type V contain only sub-cell reflections and were observed mostly for PbBi and CuBi micro-crystals. Three different conditions can lead to the absence of supercell reflections: (1) a low amount of sorbed Me (PbBi); (2) the presence of Me having a similar scattering power as that of Mn on a single side of vacant layer sites (CuBi); or (3) a random distribution of interlayer species

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

International audienceHydration of the <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, bi-hydrated – 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 1W-to-0W transitions towards lower RH values. For all samples, layer thickness of 0W, 1W, 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. Layer thickness however increased with increasing RH conditions

Structure of synthetic K-rich birnessite obtained by high-temperature decomposition of KMnO4. I. Two-layer polytype from 800°C experiment.

International audienceThe structure of a synthetic potassium birnessite (KBi) obtained as a finely dispersed powder by thermal decomposition of KMnO4 at 800°C was for the first time studied by single crystal X-ray diffraction (XRD). It is shown that KBi has a two-layer cell with a = 2.840(1) Ä, and c = 14.03(1) Ä, and space group P63/mmc. In contrast to the structure model proposed by Kim et al., 1 the refined model demonstrates the sole presence of Mn4+ in the octahedral layers, the presence of 0.12 vacant layer sites per octahedron being responsible for the layer charge deficit. This layer charge deficit is compensated for 1) by the presence of interlayer Mn 3+ above or below vacant layer octahedra sharing three O layer with neighboring Mnlayer octahedra to form a triple-corner surface complex ( VITC sites), and 2) by the presence of interlayer K in prismatic cavities located above or below empty tridentate cavities, sharing three edges with neighboring Mnlayer octahedra ( VITE sites). As compared to the structure model proposed by Kim et al., 1 this VITE site is shifted from the center of the prismatic cavity towards its edges. A complementary powder XRD study confirmed the structure model of the main defect-free KBi phase and allowed to determine the nature of stacking disorder in a defective accessory KBi phase admixed to the defect-free KBi

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

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

Structure of synthetic Na-birnessite: Evidence for a triclinic one-layer unit cell

International audienceThe structure of a synthetic analogue of sodium birnessite (NaBi) was studied by powder X-ray diffraction (XRD). It is shown that NaBi has a one-layer triclinic structure with sub-cell parameters aP = 2.9513(4) Ä, bP = 2.9547(4) Ä, cP = 7.334(1) Ä, αP = 78.72(2)°, βP = 101.79(1)°, γP = 122.33(1)°, and space group P1bar. This sub-cell is equivalent to the base-centered sub-cell with parameters a = 5.174 Ä, b = 2.848 Ä, c = 7.334 Ä, α = 90.53°, β = 103.20°, γ = 90.07°. A structure model has been refined using the Rietveld technique. NaBi consists of vacancy-free manganese octahedral layers whose negative charge arises mostly from the substitution of Mn3+ for Mn4+. The departure from the hexagonal symmetry of layers results from the Jahn-Teller distortion of Mn3+ octahedra, which are elongated along the a axis, segregated in Mn3+-rich rows parallel to the b axis, and separated from each other along the a axis by two Mn4+-rows. Structural sites of interlayer Na cations and H2O have been determined as well as their occupancies. The sub-cells of the two NaBi modifications described by Drits et al. (1997) as type I and II likely contain four sites for interlayer species, two of which are occupied by Na and the other two by H2O molecules. In the two NaBi varieties, these pairs of sites are split along the c axis and related by a center of symmetry. This splitting is consistent with the modulated structure of both NaBi types, which arises from the periodic displacement of interlayer species along the b axis with a periodicity λ = 6b (Drits et al. 1997)

Birnessite polytype systematics and identification by powder X-ray diffraction

International audienceThe polytypes of birnessite with a periodic stacking along the c* axis of one-, two-, and three-layers are derived in terms of an anion close-packing formalism. Birnessite layers may be stacked so as to build two types of interlayers: P-type in which basal O atoms from adjacent layers coincide in projection along the c* axis, thus forming interlayer prisms; and, O-type in which these O atoms form interlayer octahedra. The polytypes can be categorized into three groups that depend on the type of interlayers: polytypes consisting of homogeneous interlayers of O- or P-type, and polytypes in which both interlayer types alternate. Ideal birnessite layers can be described by a hexagonal unit-cell (ah = bh ≈ 2.85 Ä and γ = 120°) or by an orthogonal C-centered cell (a = √3 b, bh ≈ 2.85 Ä and γ = 90°); and, hexagonal birnessite polytypes (1H, 2H1, 2H2, 3R1, 3R2, 3H1, and 3H2) have orthogonal analogues (1O, 2O1, 2O2, 1M1, 1M2, 3O1, and 3O2). X-ray diffraction (XRD) patterns from different polytypes having the same layer symmetry and the same number of layers per unit cell exhibit hkl reflections at identical 2θ positions. XRD patterns corresponding to such polytypes differ only by their hkl intensity distributions, thus leading to possible ambiguities in polytype identification. In addition, the characteristics of the birnessite XRD patterns depends not only on the layer stacking but also on the presence of vacant layer sites, and on the type, location and local environment of interlayer cations. Several structure models are described for birnessite consisting of orthogonal vacancyfree or of hexagonal vacancy-bearing layers. These models differ by their stacking modes and by their interlayer structures, which contain mono-, di-, or tri-valent cations. Calculated XRD patterns for these models show that the hkl intensity distributions are determined by the polytype, with limited influence of the interlayer structure. Actual structures of phyllomanganates can thus be approximated by idealized models for polytype identification purpose. General rules for this identification are formulated. Finally, the occurrence of the different polytypes among natural and synthetic birnessite described in the literature is considered with special attention given to poorly understood structural and crystal-chemical features