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Marine mammals detoxify organic methylmercury (MeHg) as inorganic mercury selenide (HgSe), yet the nature of the reaction intermediate species and the tissue-specific redistribution of Hg species in the body are unknown. We report that the identity and proportion of the dominant Hg species in long-finned pilot whale (Globicephala melas) tissues can be obtained from the bulk variation of isotopic values of δ202Hg against the extent of demethylation (percentage of total Hg as MeHg, %MeHg) using an alternating regularized inversion method. Our analysis of isotope data from two previous studies supports that MeHg is demethylated as a tetraselenolate species (Hg(Sec)4), which further transforms into HgSe. Hg(Sec)4 occurs in the liver, kidneys, muscle, heart, and brain, whereas HgSe biomineralization occurs only in the liver and kidneys. This study provides a mathematical approach that facilitates probing the molecular-level chemistry of mercury in biological tissues using bulk isotopic data.

The robust application of stable mercury (Hg) isotopes for mercury source apportionment and risk assessment necessitates the understanding of mass-dependent fractionation (MDF) as a result of internal transformations within organisms. Here, we used high energy-resolution X-ray absorption near edge structure spectroscopy and isotope ratios of total mercury (δ202THg) and methylmercury (δ202MeHg) to elucidate the chemical speciation of Hg and the resultant MDF as a result of internal MeHg demethylation in waterbirds. In three waterbirds (Clark's grebe, Forster's tern, and south polar skua), between 17 and 86% of MeHg was demethylated to inorganic mercury (iHg) species primarily in the liver and kidneys as Hg-tetraselenolate [Hg(Sec)4] and minor Hg-dithiolate [Hg(SR)2] complexes. Tissular differences between δ202THg and δ202MeHg correlated linearly with %iHg [Hg(Sec)4 + Hg(SR)2] and were interpreted to reflect a kinetic isotope effect during in vivo MeHg demethylation. The product-reactant isotopic enrichment factor (ϵp/r) for the demethylation of MeHg → Hg(Sec)4 was -2.2 ± 0.1‰. δ202MeHg values were unvarying within each bird, regardless of Hg(Sec)4 abundance, indicating fast internal cycling or replenishment of MeHg relative to demethylation. Our findings document a universal selenium-dependent demethylation reaction in birds, provide new insights on the internal transformations and cycling of MeHg and Hg(Sec)4, and allow for mathematical correction of δ202THg values as a result of the MeHg → Hg(Sec)4 reaction.

The speciation of Mn, Fe, As and Zn in a fast-growing (0.02mm/yr), shallow-marine ferromanganese nodule has been examined by micro X-ray fluorescence, micro X-ray diffraction, and micro X-ray absorption spectroscopy. This nodule exhibits alternating Fe-rich and Mn-rich layers reflecting redox variations in water chemistry. Fe occurs as two-line ferrihydrite. The As is strictly associated with Fe and is mostly pentavalent, with an environment similar to that of As sorbed on or coprecipitated with synthetic ferrihydrite. The Mn is in the form of turbostratic birnessite with ~;10 percent trivalent manganese in the layers and probably ~;8 percent corner-sharing metal octahedra in the interlayers. The Zn is enriched on the rim of the nodule, associated with Mn. The Zn is completely (>90 percent) tetrahedrally coordinated and sorbed in the interlayers of birnessite on vacant layer Mn sites. The Zn and Mn species are similar to ones found in soils, suggesting common structural principles, despite the differing formation conditions in these systems.

The nature and proportion of Zn species present in an agricultural soil overlaid by a dredged contaminated sediment have been untangled by the novel combination of three non-invasive synchrotron-based x-ray techniques: x-ray microfluorescence ({mu}SXRF), microdiffraction ({mu}XRD), and absorption spectroscopy (EXAFS). One primary (franklinite) and two secondary (phyllomanganate and phyllosilicate) Zn-containing minerals were identified in the initial soil, and another primary (ZnS) and a new secondary (Fe-(oxyhydr)oxide) Zn species in the covered soil. The quantitative analysis of EXAFS spectra recorded on bulk samples indicated that ZnS and Zn-Fe (oxyhydr)oxides amounted to 71+-10% and 27+-10%, respectively, and the other Zn species to less than 10%. The two new Zn species found in the covered soil result from the gravitational migration of ZnS particles initially present in the sediment, and from their further oxidative dissolution and fixation of leached Zn on Fe (oxyhydr) oxides.

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 [Mn.Sup.2+] 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 Mn[O.sub.6] octahedra), 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 [Mn.sup.4+] cations, is balanced mainly by interlayer [Mn.sup.3+] 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. Keywords: Manganese oxide, birnessite, biominerals, phyllomanganate, crystal structure DOI: 10.2138/am.2010.3516

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, [delta]-Mn[O.sub.2] and acid birnessite, obtained by the reduction of potassium permanganate with Mn[Cl.sub.2] and 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 edge-sharing [Mn.sup.4+][O.sub.6] octahedra and cation vacancies, but no layer [Mn.sup.3+][O.sub.6] octahedra. 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 [Angstrom] in the [c.sup.*] direction, and of 60-85 [Angstrom] 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. Keywords: Geomicrobiology, Pseudomonas putida, Mn oxide, XAS, biogenic Mn oxide, XRD data, crystal structure, new minerals

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 Si[O.sub.2]-MnO-[H.sub.2]O chemical composition, and a bulk Mh/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 [+ or -] 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 [Angstrom] with weak modulations at 5.3, 3.5, and 2.72 [Angstrom], and a faint band at 1.605 [Angstrom]. A surrogate was synthesized in an [O.sub.2]-free atmosphere by mixing and aging at 150 [degrees]C an Mn[Cl.sub.2], and 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 [Angstrom] 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 [Angstrom] for the scale and 50-60 [Angstrom] for the surrogate material. The divalent oxidation state of manganese was confirmed by XANES spectroscopy, and the presumed existence of trioctahedral [[Mn.sup.2+].sub.3],(O,OH) clusters having a clay-like local structure was corroborated by the existence of a weak vibration band at 600-650 [cm.sup.-1] in 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 Si[O.sub.4] rings. 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.

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: [a.sub.p] = 2.9513(4) [Angstrom], [b.sub.p] = 2.9547(4) [Angstrom], [c.sub.p] = 7.334(1) [Angstrom], [[alpha].sub.p] = 78.72(2)[degrees], [[Beta].sub.p]= 101.79(1)[degrees], [[gamma].sub.p] = 122.33(1)[degrees], and space group P[bar]1. This sub-cell is equivalent to the base-centered sub-cell with parameters: a = 5.174 [Angstrom], b = 2.848 [Angstrom], c = 7.334 [Angstrom], [alpha] = 90.53[degrees], [beta] = 103.20[degrees], and [gamma] = 90.07[degrees]. A structure model has been refined using the Rietveld technique. NaBi consists of vacancy-free Mn-bearing octahedral layers whose negative charge arises mostly from the substitution of [Mn.sup.3+] for [Mn.sup.4+]. The departure from the hexagonal symmetry of layers results from Jahn-Teller distortion of [Mn.sup.3+] octahedra, which are elongated along the a axis, segregated in [Mn.sup.3+]-rich rows parallel to the b axis, and separated from each other along the a axis by two [Mn.sup.4+]-rows. Structural sites of interlayer Na cations and [H.sub.2]O 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 [H.sub.2]O 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 [gamma] = 6b (Drits et al. 1997).

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. 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 hexagonal supercells with [A.sub.H] = [B.sub.H] = [square root of (7b)]/[square root of (3), and [A.sub.H] = [B.sub.H] = [square root of (7b)], for ZnBi type I and II, respectively. The supercells in the two varieties 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 [A.sub.H] = [square root of (7b)]. In ZnBi micro-crystals of type I, interstratified twinned right- and left-handed fragments are similar to chalcophanite (Zn[Mn.sub.3][O.sub.7] * 3[H.sub.2]O; 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 that 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 [Mn.sub.layer] 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 [Mn.sub.layer] octahedra ([sup.VI]TE 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 super-cell reflections: (1) a low amount of sorbed Me (PbBi); (2) the presence of Me having a scattering power similar to that of Mn on a single side of vacant layer sites (CuBi); or (3) a random distribution of interlayer species.

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. Two main types of experimental XRD patterns were obtained as a function of the Me cations sorbed from solution, which exert a strong control on layer stacking sequence, as well as on the location and coordination of Me: (1) CdBi and PbBi samples correspond to a one-layer hexagonal (1H) structure, Ab[C.sub.b'A'.sup.C'b']AbC ..., and (2) ZnBi exhibits a one-layer monoclinic (1M) structure in which adjacent layers are shifted by +a/3, Ab[C.sub.b'A'c']Bc[A.sub.c'B'a']Ca[B.sub.a'C'b']AbC. Simulated XRD patterns show that octahedral layers contain 0.833 Mn cations ([Mn.sup.4+] and [Mn.sup.3+]) and 0.167 vacant octahedra; [Mn.sup.3+.sub.interlayer] and adsorbed [Me.sub.interlayer] compensate for the layer charge deficit. [Mn.sup.3+.sub.interlayer] is octahedrally coordinated in all samples and is located above or below vacant layer octahedra sharing three [O.sub.layer] with neighboring [Mn.sub.layer] octahedra to form a triple-corner surface complex ([sup.VI]TC sites). In ZnBi and CdBi samples, [Me.sub.interlayer] is also located in TC sites; all Cd is octahedrally coordinated whereas about 30% of Zn is tetrahedrally coordinated ([sup.IV]TC 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 [Mn.sub.layer] octahedra ([sup.VI]TE sites). Structural formulae calculated for each sample show that during the NaBu-to-MeBi structural transformation, interlayer [Na.sup.+] and [Mn.sup.2+] are replaced by Me and [H.sup.+] adsorbed from solution, whereas [Mn.sup.3+.sub.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.

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 ([micro]SXRF) and microdiffraction ([micro]SXRD) 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 ([micro]EXAFS) 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 [Mn.sup.3+] in the manganese layer of the Mn[O.sub.2][Al(OH).sub.3] mixed-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 [micro]SXRF, [micro]SXRD, and [micro]EXAFS is expected to have broad applications to earth and materials science.

Extended X-ray absorption fine structure spectroscopy and kinetics experiments were used to determine the mechanics of zinc uptake in hectorite dilute suspensions. Results show that zinc demonstrates a stronger affinity for the hectorite surface than cobalt.

X-ray technologies, including extended X-ray absorption fine structure spectroscopy were used to study the speciation of zinc in soils contaminated by smelters in France and Belgium. Results are discussed.

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 four-layer monoclinic subcell with a = 5.150, b = 2.844, c = 4c' = 28.16 [Angstrom], and [Beta] = 90.3 [degrees]. Two different superstructures of CaBi were distinguished. CaBi type I has cell parameters A = 3a = 15.45, B = 3b = 8.472 [Angstrom]. 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 [A.sub.p] = 3/2(a - b), [B.sub.p] = 4b, [Gamma] = 118.9 [degrees] and [A.sub.p] = 3/2(a + b), [B.sub.p] = -4b, [Gamma] = 118.9 [degrees] 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. As in Na-rich birnessite, the A = 3a superstructure arises from the ordered distribution of [Mn.sup.3+]-rich rows parallel to [010] and separated from each other along [100] by two [Mn.sup.4+] rows. In [Mn.sup.3+]-rich rows heterovalent Mn cations are regularly distributed according to [Mn.sup.3+] [Mn.sup.3+] [Mn.sup.44+] (CaBi type I, B = 3b) and [Mn.sup.3+] [Mn.sup.3+] [Mn.sup.4+] (CaBi type II, B = 4b) sequences. Super-periodicities along the b axis are induced by these regular distributions of heterovalent Mn atoms in [Mn.sup.3+]-rich rows and of associated interlayer Ca. No significant amount of layer vacancies was detected. Idealized structural formulae for CaBi type I and II are [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], respectively.

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 [Co.sup.2+] to weakly soluble [Co.sup.3+] species, coupled with the reduction of [Mn.sup.4+] or [Mn.sup.3+] ions, initially present in the manganese oxide sorbent, to soluble [Mn.sup.2+]. 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 [Co.sup.2+] 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 [Co.sup.2+]/([C.sup.o2+] + [C.sup.o[3+]) 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 [Mn.sup.3+] according to [Mathematical Expression Omitted], where [-square] denotes a vacant site. Divalent Co sorbs above or below a vacant site ([[-square].sub.1]) and is then oxidized by the nearest [Mathematical Expression Omitted]. The resulting [Co.sup.3+] species fills the [[-square].sub.1] position while the reduced Mn migrates to the interlayer or into solution creating a new vacant site ([[-square].sub.2]). This reaction can be written: [Mathematical Expression Omitted]. This mechanism may replicate along a [Mn.sup.3+]-rich row, and, because the density of vacancies remains constant, it can result in relatively high Co concentrations, as well as domains rich in [Mathematical Expression Omitted]. During the low-pH buserite transformation, about one-half of the layer [Mn.sup.3+] that does not disproportionate migrates from the layer to the interlayer space creating new vacancies, with the displaced [Mn.sup.3+] residing above or below these vacancies. The second oxidation mechanism involves the replacement of [Mathematical Expression Omitted]; the latter may eventually migrate into layer vacancies depending on the chemical composition of octahedra surrounding the vacancy. The criterion for the migration of [Co.sup.3+] into layer vacancies is the need to avoid [Mathematical Expression Omitted] sequences. The suite of chemical reactions for this second mechanism can be schematically written: [Mathematical Expression Omitted] 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 [Mathematical Expression Omitted] species despite some non-reduced [Mn.sup.3+] in the sorbent. This result can be explained by the n-rich domains devoid sorption of [Mathematical Expression Omitted] onto vacant sites located in [Mathematical Expression Omitted] and [Mathematical Expression Omitted] of [Mn.sup.3+]. 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 [Mn.sup.3+] rather than [Mn.sup.4+]. Thermodynamic considerations indicate that under the solution pH conditions employed in this study [Mn.sup.3+] is the more likely electron sink for the oxidation of [Co.sup.2+]. This study also shows that the high affinity of Co for manganese oxides is not only due to its oxidation to weakly soluble [Co.sup.3+] species, but also because of the reducted layer strains from the substitution of [Co.sup.3+] for [Mn.sup.3+]. Results obtained for these model compounds were compared with those for natural Co-containing asbolane and lithiophorite (Manceau et al. 1987). This comparison indicates that the different structural mechanisms explored in the laboratory can satisfactorily account for the observations made on natural samples. Specifically, the present study proves that Co substitutes for Mn in natural phyllomanganates and allows us to eliminate the possibility of precipitation of discrete CoOOH particles.

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 birnessite (SrBi) microcrystals in which Sr was substituted for Na. XRD confirmed the one-layer monoclinic structure of NaBi and the one-layer hoexagonal structure of HBi with subcell parameters a = 5.172 [Angstrom], b = 2.849 [Angstrom], c = 7.34 [Angstrom], [Beta] = 103.3[degrees] and a = 2.848 [Angstrom], c = 7.19 [Angstrom], [Gamma] = 120 [degrees], respectively. In addition to super-reflection networks, SAED patterns for NaBi and SrBi contain satellite reflections. On the basis of these experimental observations, structural models for NaBi and HBi are proposed. NaBi consists of almost vacancy-free Mn octahedral layers. The departure from the hexagonal symmetry of layers is caused by the Jahn-Teller distortion associated with the substitution of [Mn.sup.3+] for [Mn.sup.4+]. The supercell A = 3a parameter arises from the ordered distribution of [Mn.sup.3+]-rich rows parallel to [010] and separated from each other along [100] by two [Mn.sup.4+] rows. The superstructure in the b direction of NaBi type II (B = 3b) comes from the ordered distribution of Na cations in the interlayer space. The maximum value of the layer negative charge is equal to 0.333 v.u. per Mn atom and is obtained when [Mn.sup.3+]-rich rows are free of [Mn.sup.4+]. The idealized structural formula proposed for NaBi type II is [Na.sub.0.333]([Mathematical Expression Omitted])[O.sub.2]. NaBi type I has a lower amount of [Mn.sup.3+] and its ideal composition would vary from [Na.sub.0.167]([Mathematical Expression Omitted])[O.sub.2] to [Na.sub.0..25]([Mathematical Expression Omitted])[O.sub.2]. Satellites in SAED patterns of NaBi crystals result from the ordered distribution of [Mn.sup.4+] and [Mn.sup.2+] pairs in [Mn.sup.3+]-rich rows with a periodicity of 6b. The structure of HBi consists of hexagonal octahedral layers containing predominantly [Mn.sup.4+] with variable amounts of [Mn.sup.3+] and layer vacancies. The distribution of layer vacancies is inherited from the former [Mn.sup.3+] distribution in NaBi. Interlayer Mn cations are located above or below vacant layer sites. The driving force of the NaBi to HBi transformation is probably the destabilization of [Mn.sup.3+]-rich rows at low pH.

Solution chemical techniques were used to study the conversion of synthetic Na-rich buserite (NaBu) to hexagonal ([H.sup.+]-exchanged) birnessite (HBi) at low pH. The low-pH reaction is broadly characterized by the exchange of structural [Na.sup.+] with solution [H.sup.+] and the partial loss of [Mn.sup.2+] to the aqueous phase. The desorption of [Na.sup.+] in two temporally distinct steps indicates the existence of two types of binding sites for this cation. [Mn.sup.2+] appears to originate from a partial disproportionation of [Mn.sup.3+] in the NaBu layers, according to the sequence [Mathematical Expression Omitted] EXAFS measurements on Na-rich birnessite (NaBi) show that this mineral is primarily a layered structure formed by edge-sharing Mn[O.sub.6] octahedra, with no evidence for triple-corner (TC) sharing Mn. HBi is significantly different with strong evidence for TC-sharing Mn and therefore layer vacancies. The relative numbers of edge (E)-sharing and TC-sharing neighbors determined from EXAFS measurements on HBi is consistent with SAED results (Drits et al. 1997), which suggest that the layer vacancies are restricted to every third row of Mn cations, with 50% of the Mn sites along these rows vacant. The density of vacancies in the entire layer is therefore one in six of layer Mn sites. Polarized EXAFS measurements on orientated films of NaBi and HBi confirm the absence of TC-sharing Mn in NaBi and indicate that Mn adsorbed at layer vacancy sites in HBi at pH 4 is dominantly [Mn.sup.3+]. The intensity of the TC-sharing contribution to the Mn EXAFS spectra of HBi samples increases with increasing pH from pH 2 to 5, and supports a mechanism of formation involving both the direct migration of layer [Mn.sup.3+] to interlayer TC-sharing positions and re-adsorption of [Mn.sup.2+] from solution onto layer vacancy sites. The migration of [Mn.sup.3+] cations into the interlayer releases the steric strain associated with the Jahn-Teller distortion of these octahedra. This model of the NaBu-to-HBi conversion explains the transformation from orthogonal to hexagonal layer symmetry, respectively, as reported by Drits et al. (1997). Analysis of the Zn EXAFS spectrum of [Zn.sup.2+]-exchanged birnessite shows that [Zn.sup.2+] also occupies TC-sharing positions at layer vacancy sites. The results of this study strongly suggest that lattice cation vacancies are of critical importance in adsorption and electron transfer processes occurring at the surface of this mineral.

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 [Cd.sub.10][(P[O.sub.4]).sub.6][(OH, F).sub.2] clusters. This finding was confirmed by quantitative analysis of the EXAFS spectra, which indicated that Cd atoms are surrounded by nearest O atoms at 2.33 [angstrom], next-nearest P atoms at [approximately]3.53 [angstrom], and a third-nearest shell of Ca atoms at [approximately]4.02 [angstrom]. 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.

Data from spectroscopic methods, particularly X-Ray Absorption Spectroscopy and Electron Paramagnetic Resonance, have made possible the improved understanding of the crystal chemistry of some minerals and help advance knowledge of mineral formation conditions in various low-temperature environments. Spectroscopic data obtained from the crystal chemistry of kaolinite and Fe-Mn oxyhydroxides have helped explain the intracrystalline distribution of elements, some mechanisms of sorption processes, defective structure of minerals and structural control of mineral formation.

A study was conducted on the oxidation state andcoordination number of Mn, Fe, Co an Ni found in manganese hydrous oxides usinghigh resolution x-ray absorption near-edge structure (XANES) spectroscopy. Results showed that Mn atoms are commonly tetravalent. Vernadite was also foundto contain the same number of corner-sharing octahedra as todorokite. XANES data also revealed that delta-MnO2 does not have a layered structure and shouldnot be considered as long-range disordered birnessite.

A study was conducted on the oxidation state and coordination number of Mn, Fe, Co and Ni found in manganese hydrous oxides using extended x-ray absorptionfine structure spectroscopy (EXAFS) along with electron and x-ray diffraction and chemical analysis. The study provided fresh examples of hybrid structures of low-temperature materials and verified their heterogeneity on a very fine scale.

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)2Te2S[sub 13]] 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. [ABSTRACT FROM AUTHOR]

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 [Angstrom], b = a/[square root of 3] = 2.852 [Angstrom], c = 7.235 [Angstrom], [Beta] = 90 [degrees], [Gamma] = 90 [degrees]) and of a 1M phase (a = 4.940 [Angstrom], b = a/[square root of 3] = 2.852 [Angstrom], c = 7.235 [Angstrom], [Beta] = 119.2 [degrees], [Gamma] = 90 [degrees]) 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 [Mn.sub.layer] cations, 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 origin of the observed phase and structural heterogeneities has been analyzed. 1H and 1M phases are assumed to inherit their specific structural and crystal chemical features from the two distinct NaBu modifications. NaBu type I, with a high proportion of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] cations, is thought to be responsible for the monoclinic layer stacking because this configuration allows Mn cations from adjacent layers to be as far as possible from each other, thus minimizing the electrostatic repulsion between these high charge cations. In contrast, NaBu type II has a high interlayer charge induced by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] substitutions. Consequently, the 1H phase has a high amount of interlayer protons and achieves compensation of the unfavorable overlap of layer and interlayer Mn cations, in projection on the ab-plane, by the presence of strong hydrogen bondings between layers. The higher proportion of defined stacking faults in both 1H and 1M phases at pH 4 compared to pH 5 can be attributed to the increase in reaction rate with decreasing pH. At lower pH (3 and 2) the formation of strong hydrogen bonds between adjacent layers controls the layer stacking mode and leads to the formation of a unique 1H phase. The proportion of well-defined stacking faults in this phase decreases from pH 3 to 2.