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Most frequently arsenic is nominally monovalent (As1–) in pyrite (FeS2) and substituted for S. Nominally trivalent arsenic (As3+) has been reported previously in hydrothermal Peruvian pyrite and was considered to be substituted for Fe based on the negative correlation between the concentrations of the two elements. Here, we provide the first observation of the incorporation of As3+in goldfieldite [Cu12(As,Sb,Bi)2Te2S13] and As5+in colusite [Cu26V2(As,Sb)4Sn2S32] inclusions in As1–-pyrite from high-sulfidation deposits in Peru. This information was obtained by combining spatially resolved electron probe (EPMA), synchrotron-based X‑ray fluorescence (SXRF), and absorption spectroscopy (micro-XANES and micro-EXAFS) with new high energy-resolution XANES spectroscopy (HR-XANES). The two Cu sulfide inclusions range from several to one hundred micrometers in size, and the As3+/As5+concentration varies from a few parts per million (ppm) to a maximum of 17.33 wt% compared to a maximum of 50 ppm As1–in pyrite. They also contain variable amounts of Sn (18.47 wt% max), Te (15.91 wt% max), Sb (8.54 wt% max), Bi (5.53 wt% max), and V (3.25 wt% max). The occurrence of As3+/As5+-containing sulfosalts in As1–-containing pyrite grains indicates that oxidizing hydrothermal conditions prevailed during the late stage of the mineralization process in the ore deposits from Peru. From an environmental perspective, high concentrations of potentially toxic As, contained in what appear to be non-As-bearing pyrite, may pose a heretofore unrecognized threat to ecosystems in acid mine drainage settings. More generally, the combination of techniques used in this study offers a new perspective on the mineralogy and crystal chemistry of hazardous elements in pyrite, such as highly toxic and little studied thallium.

Minor-element concentrations in marine ferromanganese nodules are primarily controlled by the mineralogy, which itself depends on redox conditions at the sediment-water interface. Results are presented for the first in-depth X-ray microstructural and microspectroscopic investigation of a mixed hydrogenetic-diagenetic nodule, which is representative of ferromanganese deposits on abyssal plains. The measurements were conducted by micro-X-ray diffraction and X-ray absorption spectroscopy (both XANES and EXAFS) on hydrogeneous and diagenetic regions of the nodule. The hydrogeneticdiagenetic interface was imaged by X-ray microfluorescence, after which regions of interest were chosen to represent mineralogical and chemical transformations that occurred at the early stage of suboxic diagenesis. In the hydrogenetic nodule (oxic environment), Mn is speciated as Fe-vernadite, a nanocomposite material composed of intergrown feroxyhite (δ-FeOOH) and monodispersed phyllomanganate layers having no interlayer Mn (vernadite). In the diagenetic nodule (suboxic environment), Mn is speciated dominantly as Mg-rich 10 Å vernadite, which consists of a random intergrowth of vernadite and its transformation product todorokite. The authigenic 10 Å vernadite precipitated from the components of vernadite in Fe-vernadite that were dissolved in suboxic microenvironments of the sediment. Direct evidence supporting a redox-driven dissolution reaction is provided by the valence composition of Mn, as measured by micro-XANES, which is 0.69Mn4++ 0.24Mn3++ 0.07Mn2+(average = 3.62 ± 0.04 v.u.) for Fe-vernadite and 0.61Mn4++ 0.23Mn3++ 0.16Mn2+(average 3.28 ± 0.04 v.u.) for 10 Å vernadite. Ni and Cu, derived mainly from dissolved vernadite and oxidized organic matter, replace structural Mn3+/4+in both the MnO2layer and todorokite domains of 10 Å vernadite. Pure todorokite in highly diagenetic regions of the nodule has an average formula of Mg2+0.167(Mn4+0.783Mn3+0.099Co3+0.002Ni2+0.076Cu2+0.040)O2·nH2O, with an atomic ratio of (Cu+Ni+Co)/Mn = 0.13, which is slightly lower than 0.167 (⅙), the maximum metal uptake capacity reported for marine nodules. By analogy with synthetic todorokites we infer that Mg2+, which has a hydrated diameter close to that of the [3 × 3] tunnel size of todorokite, and Mn3+and Cu2+, which prefer Jahn-Teller distorted octahedra, play a crucial role in templating the topotactic transformation of 10 Å vernadite to todorokite and stabilizing todorokite in suboxic marine sediments.

In an effort to shed light on the intricate structure of ferrihydrite, its pair distribution function (PDF) derived from high-energy X‑ray scattering (HEXS) data was refined with the single-phase akdalaite model, possessing 20% of the Fe atoms in tetrahedral coordination, and a modified akdalaite model in which Fe has only octahedral coordination. The second model is analogous to the predominant f-phase (ABAC stacking sequence) of classical multi-phase ferrihydrite. The contribution from the disordered d-phase component (randomly stacked ABA and ACA double-layer fragments) of the classical model was recovered in the modified akdalaite description by increasing the atomic motion of the ABAC motif above the double-layer distance 4.2 Å to simulate aperiodic stacking faults. Results show that the original and modified akdalaite representations provide near-identical fits to the ferrihydrite PDF. In the original single-phase and periodic model, the plurality of the Fe-O and Fe-Fe distances resulting from phase mixtures and defects are reconciled artificially by taking a large unit cell with three independent Fe sites, two Fe coordinations, and under-constrained atomic positions. Correlation matrices reveal that many fitted parameters are linearly correlated, thus explaining the crystallographic and chemical inconsistencies of the as-refined akdalaite model which have been identified in the literature. Structurally more constrained, the modified akdalaite model does not suffer from bias and provides a more robust description of the PDF data. However, because structural defects and inhomogeneities are not physically present but introduced artificially in PDF modeling, the crystallographic description of ferrihydrite by real-space modeling of HEXS data has an idealized character. To facilitate further understanding of the ferrihydrite structure, the PDF data are provided as supplementary material1for interlaboratory testing, and as a resource as more sophisticated tools may be brought to bear on this complex problem.

Ferrihydrite is the main form of ferric iron in surficial environments and a key reactive nanoparticle that regulates nutrient availability and the mobility of metal(loid) contaminants, yet its structure is not completely elucidated. Two models exist to date: the “f-phase” in which Fe is fully octahedral and the “akdalaite-model” possessing 20% of the Fe atoms in tetrahedral coordination. In this study, germanium was used as a structural probe to re-examine the validity of the latter model. Germaniumbearing ferrihydrites containing 0.2, 0.6, 1.4, 2.2, 2.9, 3.8, 12, and 15 wt% Ge were synthesized in the laboratory at 25 and 65 °C. X-ray diffraction analyses showed all the precipitates to be six-line ferrihydrite. Semi-quantitative energy-dispersive X-ray microanalyses (TEM) indicate that the precipitates made from solutions having Fe/Ge molar ratios of two and four have Fe/Ge atomic ratios of 3.8-3.9 and 4.4-5.1, respectively, which suggest a limit of Ge uptake in ferrihydrite of about 20 at% relative to total cations. Based on TEM examinations, these high Ge-bearing ferrihydrites are homogenous and consist of equant and plate-like crystallites about 5-6 nm in size. Furthermore, it appears that higher Ge concentrations in solution have no significant effect on the crystallite size, supporting the incorporation of Ge in the ferrihydrite structure. Extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that the Fe atoms in both the low and high Ge-bearing ferrihydrites are in octahedral coordination and that Ge occurs in the ferrihydrite structure by filling the empty tetrahedral sites and coordinating to 4 edge-sharing FeO6trimers through sharing a common oxygen (Ge-O-Fe linkage). Incorporation of the Ge tetrahedra in the ferrihydrite structure requires redistribution of Fe occupancy along the alternating O/OH layers while forming an ordered distribution of octahedral Fe and tetrahedral Ge. The local structure around Ge mimics a Keggin-like motif in two different, yet equivalent, orientations. It appears that the split diffraction peak at 1.46 and 1.51 Å is a characteristic feature of Ge-rich ferrihydrite and suggests that it is a fingerprint of increased order due to significant Ge incorporation in the tetrahedral sites. The findings can be rationalized in terms of the incorporation of Ge in the so-called “f-phase” of the classical ferrihydrite model, and demonstrate the flexibility of the model in terms of accommodating a Keggin-like cluster without the need of imposing unrealistic constraints as in the akdalaite model. Direct comparison of the imaginary parts of the Fourier transforms for ferrihydrite and maghemite further confirms the absence of tetrahedral Fe in ferrihydrite. The absence of tetrahedral Fe substantiates the use of goethite-like or akaganeite-like models to describe the polyhedral structure of ferrihydrite used in modeling sorption reactions at the ferrihydrite-water interface.

The crystal structures of biogenic Mn oxides produced by three fungal strains isolated from stream pebbles were determined using chemical analyses, XANES and EXAFS spectroscopy, and powder X-ray diffraction. The fungi-mediated oxidation of aqueous Mn2+produces layered Mn oxides analogous to vernadite, a natural nanostructured and turbostratic variety of birnessite. The crystallites have domain dimensions of ~10 nm in the layer plane (equivalent to ~35 MnO6octahedra), and ~1.5-2.2 nm perpendicularly (equivalent to ~2-3 layers), on average. The layers have hexagonal symmetry and from 22 to 30% vacant octahedral sites. This proportion likely includes edge sites, given the extremely small lateral size of the layers. The layer charge deficit, resulting from the missing layer Mn4+cations, is balanced mainly by interlayer Mn3+cations in triple-corner sharing position above and/or below vacant layer octahedra. The high surface area, defective crystal structure, and mixed Mn valence confer to these bio-minerals an extremely high chemical reactivity. They serve in the environment as sorption substrate for trace elements and possess catalytic redox properties.

X-ray diffraction (XRD) and Mn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy were combined to elaborate a structural model for phyllomanganates (layer-type Mn oxides) lacking 3D ordering (turbostratic stacking). These techniques were applied to a sample produced by a common soil and freshwater bacterium (Pseudomonas putida), and to two synthetic analogs, δ-MnO2and acid birnessite, obtained by the reduction of potassium permanganate with MnCl2and HCl, respectively. To interpret the diffraction and spectroscopic data, we applied an XRD simulation technique utilized previously for well-crystallized birnessite varieties, complementing this approach with single-scattering-path simulations of the Mn K-edge EXAFS spectra. Our structural analyses revealed that all three Mn oxides have an hexagonal layer symmetry with layers comprising edgesharing Mn4+O6octahedra and cation vacancies, but no layer Mn3+O6octahedra. The proportion of cation vacancies in the layers ranged from 6 to 17%, these vacancies being charge-compensated in the interlayer by protons, alkali metals, and Mn atoms, in amounts that vary with the phyllomanganate species and synthesis medium. Both vacancies and interlayer Mn were most abundant in the biogenic oxide. The diffracting crystallites contained three to six randomly stacked layers and have coherent scattering domains of 19.42 Å in the c* direction, and of 60.85 Å in the a-b plane. Thus, the Mn oxides investigated here are nanoparticles that bear significant permanent structural charge resulting from cation layer vacancies and variable surface charge from unsaturated O atoms at layer edges.

X-ray diffraction, infrared spectroscopy, and X-ray absorption (XANES and EXAFS) spectroscopy were combined to characterize poorly crystalline Mn-rich silicate scale deposited from brine at a geothermal field in Indonesia. The precipitate has a vitreous pink-amber appearance, a nearly pure SiO2-MnO-H2O chemical composition, and a bulk Mn/Si atomic ratio of 0.63. X-ray microfluorescence indicated that the sample consists of Mn-free and Mn-containing silica domains, whose Mn/Si ratio (1.2 ± 0.4) ranges between those of 2:1 and 1:1 phyllosilicates. The XRD pattern is characterized by a broad scattering band spanning from ~6 to ~2.5 Å with weak modulations at 5.3, 3.5, and 2.72 Å, and a faint band at 1.605 Å. A surrogate was synthesized in an O2-free atmosphere by mixing and aging at 150 °C an MnCl2and a sodium meta-silicate solution for 3 hours. Its XRD pattern was similar to the scale sample, but the two reflections at 2.72 and 1.605 Å were enhanced and the former was asymmetrical as in randomly stacked layered compounds. Diffraction data are consistent with a mixture of amorphous silica and a poorly ordered manganoan sheet silicate with a domain size of 40.50 Å for the scale and 50.60 Å for the surrogate material. The divalent oxidation state of manganese was confirmed by XANES spectroscopy, and the presumed existence of trioctahedral [Mn2+]3(O,OH) clusters having a clay-like local structure was corroborated by the existence of a weak vibration band at 600-650 cm-1in infrared spectra. The connectivity of Mn octahedra was examined using EXAFS spectroscopy by comparing the local structure of Mn in the scale sample and in a large series of Mn compounds having different kinds of Mn octahedra and Si tetrahedra linkages. The manganoan scale has a polyhedral local structure resembling that of sheet silicates, in which metal octahedra are joined along edges and share corners with ditrigonal SiO4rings. Similar short-range ordering has been described in Fe- and Al- silicate scales, but the Mn-silicate scale has a lower domain size owing to the greater misfit between the lateral dimensions of the Mn and Si sheets. The strain induced by the shrinkage of the Mn sheet to fit the Si sheet(s) is alleviated by the nanometer size of the two-dimensional hydrous silicate domains.

Selected-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 pH 4 in the presence of various heavy metal cations (Me), including Pb, Cd, Zn, and Cu.

The structure of a synthetic analogue of Na-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) Å, bP= 2.9547(4) Å, cP= 7.334(1) Å, αP= 78.72(2)°, βP= 101.79(1)°, γP= 122.33(1)°, and space group P1̄. This sub-cell is equivalent to the base-centered sub-cell with parameters: a = 5.174 Å, b = 2.848 Å, c = 7.334 Å, α = 90.53°, β = 103.20°, and γ = 90.07°. A structure model has been refined using the Rietveld technique. NaBi consists of vacancy-free Mnbearing octahedral layers whose negative charge arises mostly from the substitution of Mn3+for Mn4+. The departure from the hexagonal symmetry of layers results from 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 types 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).

The 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.

The 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.

Selected-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 pH 4 in the presence of various heavy metal cations (Me), including Pb, Cd, Zn, and Cu.

The structure of a synthetic analogue of Na-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) Å, bP= 2.9547(4) Å, cP= 7.334(1) Å, αP= 78.72(2)°, βP= 101.79(1)°, γP= 122.33(1)°, and space group P1̄. This sub-cell is equivalent to the base-centered sub-cell with parameters: a = 5.174 Å, b = 2.848 Å, c = 7.334 Å, α = 90.53°, β = 103.20°, and γ = 90.07°. A structure model has been refined using the Rietveld technique. NaBi consists of vacancy-free Mnbearing octahedral layers whose negative charge arises mostly from the substitution of Mn3+for Mn4+. The departure from the hexagonal symmetry of layers results from 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 types 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).

X-ray microprobes are among the most important new analytical techniques to emerge from third generation synchrotron facilities. Here we show how X-ray fluorescence, diffraction, and absorption can be used in parallel to determine the structural form of trace elements in heterogeneous matrices at the micrometer-scale of resolution. Scanning X-ray microfluorescence (μSXRF) and microdiffraction (μSXRD) first are used to identify the host solid phase by mapping the distributions of elements and solid species, respectively. Micro-extended X-ray absorption fine structure (μEXAFS) spectroscopy is then used to determine the mechanism of trace element binding by the host phase at the molecular scale. To illustrate the complementary application of these three techniques, we studied how nickel is sequestered in soil ferromanganese nodules, an overwhelmingly complex natural matrix consisting of submicrometer to nanometer sized particles with varying structures and chemical compositions. We show that nickel substitutes for Mn3+in the manganese layer of the MnO2- Al(OH)3mixed-layer oxide lithiophorite. The affinity of Ni for lithiophorite was characteristic of micronodules sampled from soils across the U.S.A. and Europe. Since many natural and synthetic materials are heterogeneous at nanometer to micrometer scales, the synergistic use of μSXRF, μSXRD, and μEXAFS is expected to have broad applications to earth and materials science.

X-ray microprobes are among the most important new analytical techniques to emerge from third generation synchrotron facilities. Here we show how X-ray fluorescence, diffraction, and absorption can be used in parallel to determine the structural form of trace elements in heterogeneous matrices at the micrometer-scale of resolution. Scanning X-ray microfluorescence (μSXRF) and microdiffraction (μSXRD) first are used to identify the host solid phase by mapping the distributions of elements and solid species, respectively. Micro-extended X-ray absorption fine structure (μEXAFS) spectroscopy is then used to determine the mechanism of trace element binding by the host phase at the molecular scale. To illustrate the complementary application of these three techniques, we studied how nickel is sequestered in soil ferromanganese nodules, an overwhelmingly complex natural matrix consisting of submicrometer to nanometer sized particles with varying structures and chemical compositions. We show that nickel substitutes for Mn3+in the manganese layer of the MnO2- Al(OH)3mixed-layer oxide lithiophorite. The affinity of Ni for lithiophorite was characteristic of micronodules sampled from soils across the U.S.A. and Europe. Since many natural and synthetic materials are heterogeneous at nanometer to micrometer scales, the synergistic use of μSXRF, μSXRD, and μEXAFS is expected to have broad applications to earth and materials science.

The structural transformation of high pH Na-rich buserite (NaBu) to H-exchanged hexagonal birnessite (HBi) at low pH was studied by simulation of experimental X-ray diffraction patterns. Four HBi samples were prepared by equilibration of NaBu at constant pH in the range pH 5-2. The samples differ from each other by the presence of one (at pH 2 and 3) or two (at pH 4 and 5) phases, and by the structural heterogeneity of these phases which decreases with decreasing pH. The sample obtained at pH 5 is a 4:1 physical mixture of a 1H phase (a = 4.940 Å, b = a/√3 = 2.852 Å, c = 7.235 Å, β = 90°, γ = 90°) and of a 1M phase (a = 4.940 Å, b = a/√3 = 2.852 Å, c = 7.235 Å, β = 119.2°, γ = 90°) in which successive layers are shifted with respect to each other by +a/3 along the a axis as in chalcophanite. Both the 1H and 1M phases contain very few well-defined stacking faults at pH 5. At pH 4, the sample is a 8:5 physical mixture of a 1H phase containing 15% of monoclinic layer pairs and of a 1M phase containing 40% of orthogonal layer pairs. Any further decrease of the pH leads to the formation of a single defective 1H phase. This 1H phase contains 20% and 5% of monoclinic layer pairs at pH 3 and 2, respectively. Independent of pH, all phases contain 0.833 Mnlayercations, 0.167 vacant layer sites, and 0.167 interlayer Mn cations located either above or below layer vacancies per octahedron. A structural formula is established at each pH.

The structural transformation of high pH Na-rich buserite (NaBu) to H-exchanged hexagonal birnessite (HBi) at low pH was studied by simulation of experimental X-ray diffraction patterns. Four HBi samples were prepared by equilibration of NaBu at constant pH in the range pH 5-2. The samples differ from each other by the presence of one (at pH 2 and 3) or two (at pH 4 and 5) phases, and by the structural heterogeneity of these phases which decreases with decreasing pH. The sample obtained at pH 5 is a 4:1 physical mixture of a 1H phase (a = 4.940 Å, b = a/√3 = 2.852 Å, c = 7.235 Å, β = 90°, γ = 90°) and of a 1M phase (a = 4.940 Å, b = a/√3 = 2.852 Å, c = 7.235 Å, β = 119.2°, γ = 90°) in which successive layers are shifted with respect to each other by +a/3 along the a axis as in chalcophanite. Both the 1H and 1M phases contain very few well-defined stacking faults at pH 5. At pH 4, the sample is a 8:5 physical mixture of a 1H phase containing 15% of monoclinic layer pairs and of a 1M phase containing 40% of orthogonal layer pairs. Any further decrease of the pH leads to the formation of a single defective 1H phase. This 1H phase contains 20% and 5% of monoclinic layer pairs at pH 3 and 2, respectively. Independent of pH, all phases contain 0.833 Mnlayercations, 0.167 vacant layer sites, and 0.167 interlayer Mn cations located either above or below layer vacancies per octahedron. A structural formula is established at each pH.

Natural apatites used in fertilizer industries often contain trace amounts of Cd, which may reach concentrations of several tens to a few hundred parts per million. Cd is not eliminated during the production of phosphate fertilizers, and its concentration in the final product can exceed environmental norms. Knowledge of the chemical state of Cd in apatite ores is a prerequisite for the design of technical processes of extraction. In the present study, Cd K-edge EXAFS spectroscopy was used to investigate the structural environment of Cd present in sedimentary apatite ores from West Africa. These apatites are fluorinated and contain goethite, quartz, and crandallite as ancillary phases detected by X-ray diffraction or EXAFS spectroscopy. Cd K-edge EXAFS spectra for two natural samples were analyzed and compared with those for Cd-containing reference minerals, including hydroxylapatite, goethite, otavite, and crandallite. A good spectral resemblance was observed between natural products and synthetic apatite containing small amounts of Cd. This spectral likeness indicates that the majority of Cd atoms are diluted in the apatitic framework and do not form Cd10(PO4)6(OH,F)2clusters. This finding was confirmed by quantitative analysis of the EXAFS spectra, which indicated that Cd atoms are surrounded by nearest 0 atoms at 2.33 Å, next-nearest P atoms at ~3.53 Å, and a third-nearest shell of Ca atoms at ~4.02 Å. A comparison of these data with those obtained for synthetic apatites allowed us to assess that Cd occupies both Ca crystallographic sites with a slight preference for the Ca2 site.

Synthetic Ca-exchanged birnessite (CaBi) was studied by X-ray and selected-area electron diffraction (XRD, SAED). The substructure of CaBi may be described with a fourlayer monoclinic subcell with a = 5.150, b = 2.844, c = 4cʹ = 28.16 Å , and b = 90.38. Two different superstructures of CaBi were distinguished. CaBi type I has cell parameters A = 3a = 15.45, B = 3b = 8.472 Å . The stacking sequence in this unit cell may be described as defect-free OSOS, where successive layers are shifted relative to their predecessors by 0 (O) or b/2 (S) along the b axis. The complete description of stacking involves the structure of the layer itself, the structure of the interlayer, and the shift from this layer to the next one. CaBi type II can be described as a regular interstratification of AP= 3⁄2(a - b), BP= 4b, γ =5 118.98 and AP= 3⁄2(a + b), BP= 24b, γ = 118.98 supercells that are connected by a mirror plane in projection on the a-b plane. Its stacking sequence is a random interstratification of OSOS (90%) and OOOS (10%) structural fragments. Most CaBi crystals appeared to consist of intergrown type I and type II sub-crystals.

Solution chemical techniques were used to study the conversion of synthetic Na-rich buserite (NaBu) to hexagonal (H+-exchanged) birnessite (HBi) at low pH. The low-pH reaction is broadly characterized by the exchange of structural Na+with solution H+and the partial loss of Mn2+to the aqueous phase. The desorption of Na+in two temporally distinct steps indicates the existence of two types of binding sites for this cation. Mn2+appears to originate from a partial disproportionation of Mn31 in the NaBu layers, according to the sequence

Synthetic Na-rich birnessite (NaBi) and its low pH form, hexagonal birnessite (HBi), were studied by X-ray and selected-area electron diffraction (XRD. SAED). SAED patterns were also obtained for synthetic Sr-exchanged bimessite (SrBi) microcrystals in which Sr was substituted for Na. XRD confirmed the one-layer monoclinic stmcture of NaBi and the one-layer hexagonal structure of HBi with subcell parameters a = 5.172 Å, b = 2.849 Å, c = 7.34 Å, β = 103.3° and a = 2.848 Å, c = 7.19 Å, γ = 120°, respectively. In addition to super-reflection networks, SAED patterns for NaBi and SrBi contain satellite reflections.

The geochemistry of Co at the Earth’s surface is closely associated with that of manganese oxides. This geochemical association results from the oxidation of highly soluble Co2+to weakly soluble Co3+species, coupled with the reduction of Mn4+or Mn3+ions, initially present in the manganese oxide sorbent, to soluble Mn2+. The structural mechanism of this Co immobilization-manganese oxide dissolution reaction was investigated at the buserite surface. Co-sorbed samples were prepared at different surface coverages by equilibrating a Na-exchanged buserite suspension in the presence of aqueous Co2+at pH 4. The structure of Co-sorbed birnessite obtained by drying buserite samples was determined by X-ray diffraction (XRD) and powder and polarized EXAFS spectroscopy. For each sample we determined the proportion of interlayer cations and layer vacancy sites, the Co2+/(Co2++ Co3+) ratio, the nature of Co sorption crystallographic sites, and the proportion of interlayer vs. layer Co. From this in-depth structural characterization two distinct oxidation mechanisms were identified that occur concurrently with the transformation of low pH monoclinic buserite to hexagonal H-rich birnessite (Drits et al. 1997; Silvester et al. 1997). The first mechanism is associated with the fast disproportionation of layer Mn3+according to 2Mn3+layer →Mn4+layer + ⃞layer+ Mn2+solution, where ⃞ denotes a vacant site. Divalent Co sorbs above or below a vacant site (⃞1) and is then oxidized by the nearest Mn3+layer. The resulting Co3+species fills the ⃞1position while the layer reduced Mn migrates to the interlayer or into solution creating a new vacant site (⃞2). This reaction can be written: Co2+solution+ ⃞1+ Mn3+layer →Co2+interlayer + ⃞1+ Mn3+layer → Co3+interlayer+ ⃞1+ Mn2+layer→Co3+layer + ⃞2+ Mn2+sol/inter. This mechanism may replicate along a Mn3+-rich row, and, because the density of vacancies remains constant, it can result in relatively high Co concentrations, as well as domains rich in Co3+layer- Mn4+layer. During the low-pH buserite transformation, about one-half of the layer Mn3+that does not disproportionate migrates from the layer to the interlayer space creating new vacancies, with the displaced Mn3+residing above or below these vacancies. The second oxidation mechanism involves the replacement of Mn3+interlayerby Co3+interlayer; the latter may eventually migrate into layer vacancies depending on the chemical composition of octahedra surrounding the vacancy. The criterion for the migration of Co31 into layer vacancies is the need to avoid Mn3+layer - Co3+layer- Mn3+layer sequences. The suite of chemical reactions for this second layer layer layer mechanism can be schematically written: Co2+solution+ Mn3+interlayer+ ⃞ → Mn2+solution+ Co3+interlayer+ ⃞ → Mn2+solution+ Co3+layer, the last step being conditional. In contrast to the first mechanism, this second mechanism decreases the density of vacant sites. At high surface coverage, Co-sorbed birnessite contains a substantial amount of unoxidized Co2+interlayerspecies despite some non-reduced Mn3+in the sorbent. This result can be explained by the sorption of Co2+onto vacant sites located in Co3+layer- and Mn4+layer-rich domains devoid of Mn3+. The number and size of these domains increase with the extent of oxidation and the total Co concentration in the solution, and this accounts for the decreasing capacity of buserite to oxidize Co. The weight of structural evidence indicates that Co is oxidized by Mn3+rather than Mn4+. Thermodynamic considerations indicate that under the solution pH conditions employed in this study Mn3+is the more likely electron sink for the oxidation of Co2+. This study also shows that the high affinity of Co for manganese oxides is not only due to its oxidation to weakly soluble Co3+species, but also because of the reducted layer strains from the substitution of Co3+for Mn3+.

The XRD pattern of hydromaghemite, previously interpreted as a mixture of hydroxylated maghemite, ferrihydrite, and hematite (Barrón et al. 2003; Liu 2008), has been reinterpreted as being from a new form of ferrihydrite called “ferrimagnetic ferrihydrite” (ferrifh, Michel et al. 2010; Barrón et al. 2012). Although ferrihydrite sensu stricto and ferrifh have distinct XRD patterns, their profiles have been fit with the same akdalaite-type model using atomic pair distribution (PDF) analysis (Michel et al. 2007, 2010). This ambivalence shows that the crystal structure problem is ill-conditioned, as often the case for the study of nanostructured materials by PDF (Billinge and Levin 2007).