Simultaneous incorporation of Mn and Al in the goethite structure   总被引：1，自引：0，他引：1  

Mariana Alvarez  Elsa E. Sileo 《Geochimica et cosmochimica acta》2007,71(4):1009-1020

Two series of (Al,Mn)-substituted goethites were synthesized from ferrihydrite made in alkaline media, with different Al/Mn mole ratios ([Al + Mn]/Fe molar ratio up to 0.12). Powder X-ray diffraction and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the simultaneous substitution in goethite. XRD patterns revealed that all the obtained solids remain in a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the increasing Mn substitution and consequent decrease of Al substitution causes an increase in the unit cell volume. This change is accompanied by the increment of the various Me-Me distances. XANES spectra at the Al and Mn K-edge confirm the octahedral coordination of Al and the trivalent oxidation state of the Mn ion in all the synthesized samples. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom remains practically constant upon (Mn,Al) substitution. Measurements in the Mn K-edge show that distances Mn-Me suffer different changes with the increase in Mn substitution: a marked decrease in E and a slight decrease in E′, while DC remains constant. E and E′ values correspond to the distance between one Mn and one neighboring Me (Fe, Mn, Al) atom, both situated in two polyhedra linked by an edge. These polyhedra belong to the same double row of the goethite structure. DC value corresponds to the distance between one Mn and one Me (Fe, Mn, Al) atom, situated in two octahedral linked by one corner and belonging to two adjacent double chains. All the intermetallic distances are minor than the corresponding singly substituted goethites, this fact is attributed to the structure contraction due to the presence of Al(III) which restrains the axial distortion of Mn. Dissolution-time curves, resulting from exposure to 6 M HCl at 318 K, show that the dissolution rate slows with increasing Al substitution and consequent decrease of Mn substitution, and the shape of the curve becomes increasingly sigmoidal for mixed goethite with large Al content and Al-goethite. Dissolution kinetics of most samples are well described by the Kabai equation. Al dissolves almost congruently with respect to Fe, implying that it is homogeneously distributed in the structure. However, the convex χ_Mn:χ_Fe curve indicates that Mn tends to be concentrated in the outer layers of the goethite particles.  相似文献   

A Fe K-edge XAS study of amethyst     

Francesco Di Benedetto  Francesco D’Acapito  Gabriele Fornaciai  Massimo Innocenti  Giordano Montegrossi  Luca A. Pardi  Silvia Tesi  Maurizio Romanelli 《Physics and Chemistry of Minerals》2010,37(5):283-289

An X-ray absorption spectroscopy (XAS) study of the Fe local environment in natural amethyst (a variety of α-quartz, SiO₂) has been carried out. Room temperature measurements were performed at the Fe K-edge (7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra suggest Fe to occur mainly in the trivalent state, although a fraction of Fe²⁺ is identified. EXAFS spectra, on the other hand, reveal an unusual short distance for the first coordination shell:  = 1.78(2) Å, the coordination number being 2.7(5). These results allow to establish that Fe replaces Si in its tetrahedral site, and that numerous local distortions are occurring as a consequence of the presence of Fe³⁺ variably compensated by protons and/or alkaline ions, or uncompensated. The formal valence of Fe, on the basis of both experimental and DFT structural features, can be either 4+ or 3+. Taking into account the XANES evidences, we suggest that Fe mainly occurs in the trivalent state, compensated by protons, and that a minor fraction of Fe⁴⁺ is stabilised by the favourable local structural arrangement.  相似文献   

Strontium in coral aragonite: 1. Characterization of Sr coordination by extended absorption X-ray fine structure     

Adrian A. Finch  Nicola Allison  Steven R. Sutton  Matthew Newville 《Geochimica et cosmochimica acta》2003,67(6):1197-1202

Aragonite was analyzed from Porites lobata, Pavona gigantea, Pavona clavus, and Montastrea annularis corals by Sr K-edge extended absorption X-ray fine structure (EXAFS) and compared with aragonite, strontianite, and mechanically mixed standards. Bulk analyses were performed and data compared with equivalent micro-EXAFS analyses on small (∼400 μm³) analytical volumes with a microfocused X-ray beam. As a result of the architecture of the coral skeleton, the crystals within the microanalytical volume are not randomly oriented, and the microanalytical X-ray absorption spectra show orientational dependence. However, refinement of bulk and microanalytical data provided indistinguishable interatomic distances and thermal vibration parameters in the third shell (indicative of Sr speciation). The Sr K-edge EXAFS of all the coral samples refine, within error, to an ideally substituted Sr in aragonite, in contrast to previous studies, in which significant strontianite was reported. Some samples from that study were also analyzed here. Strontianite may be less widely distributed in corals than previously thought.  相似文献   

Structure and specification of iron complexes in aqueous solutions determined by X-ray absorption spectroscopy     

M.J. Apted  G.A. Waychunas  G.E. Brown 《Geochimica et cosmochimica acta》1985,49(10):2081-2089

X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe²⁺ and Fe³⁺ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe²⁺-O bond distance, closely matching octahedral Fe²⁺-O bonds obtained from solid oxide model compounds. Solution Fe²⁺-O bond distances decrease with chloride/iron ratio, pH, and total FeCl₂ concentration. A slight intensification of the $\text{1s → 3d}$ transition with increasing FeCl₂ concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe³⁺ solutions show a pronounced increase in the $\text{1s → 3d}$ transition intensities between 1.0 M FeCl₃/7.8 M Cl^? to 1.0 M FeCl₃/ 15 M Cl^?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe³⁺-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H₂O)₆³⁺ or trans-Fe(H₂O)₄Cl₂ or both; Fe³⁺-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe³⁺-Cl bond distances of 2.25 Å].  相似文献   

Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay     

L. Charlet  A.C. Scheinost  C. Tournassat  A. Géhin  S. Coudert  J. Brendle 《Geochimica et cosmochimica acta》2007,71(23):5731-5749

The mobility and availability of the toxic metalloid selenium in the environment are largely controlled by sorption and redox reactions, which may proceed at temporal scales similar to that of subsurface water movement under saturated or unsaturated conditions. Since such waters are often anaerobic and rich in Fe²⁺, we investigated the long-term (?1 month) kinetics of selenite sorption to montmorillonite in the presence of Fe²⁺ under anoxic conditions. A synthetic montmorillonite was used to eliminate the influence of structural Fe. In the absence of aqueous Fe²⁺, selenite was sorbed as outer-sphere sorption complex, covering only part of the positive edge sites, as verified by a structure-based MUSIC model and Se K-edge XAS (X-ray absorption spectroscopy). When selenite was added to montmorillonite previously equilibrated with Fe²⁺ solution however, slow reduction of Se and formation of a solid phase was observed with Se K-edge XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine-structure) spectroscopy. Iterative transformation factor analysis of XANES and EXAFS spectra suggested that only one Se reaction product formed, which was identified as nano-particulate Se(0). Even after one month, only 75% of the initially sorbed Se(IV) was reduced to this solid species. Mössbauer spectrometry revealed that before and after addition and reduction of Se, 5% of total sorbed Fe occurred as Fe(III) species on edge sites of montmorillonite (≈2 mmol kg⁻¹). The only change observed after addition of Se was the formation of a new Fe(II) species (15%) attributed to the formation of an outer-sphere Fe(II)-Se sorption complex. The combined Mössbauer and XAS results hence clearly suggest that the Se and Fe redox reactions are not directly coupled. Based on the results of a companion paper, we hypothesize that the electrons produced in the absence of Se by oxidation of sorbed Fe(II) are stored, for example by formation of surface H₂ species, and are then available for the later Se(IV) reduction. The slow reaction rate indicates a diffusion controlled process. Homogeneous precipitation of an iron selenite was thermodynamically predicted and experimentally observed only in the absence of clay. Interestingly, half of Fe was oxidized in this precipitate (Mössbauer). Since DFT calculations predicted the oxidation of Fe at the water-FeSe solid interface only and not in the bulk phase, we derived an average particle size of this precipitate which does not exceed 2 nm. A comparison with the Mössbauer and XAS spectra of the clay samples demonstrates that such homogenous precipitation can be excluded as a mechanism for the observed slow Se reduction, emphasizing the role of abiotic, heterogeneous precipitation and reduction for the removal of Se from subsurface waters.  相似文献   

Aqueous Cr(VI) reduction by pyrite: Speciation and characterisation of the solid phases by X-ray photoelectron, Raman and X-ray absorption spectroscopies     

Martine Mullet  Frédéric Demoisson  Laurent J. Michot 《Geochimica et cosmochimica acta》2007,71(13):3257-3271

Optical microscopy, confocal Raman micro-spectrometry, X-ray photoelectron micro-spectroscopy (XPS) and synchrotron based micro-X-ray fluorescence (XRF), micro-X-ray absorption near edge spectroscopy (XANES) and micro-extended X-ray absorption fine structure (EXAFS) were used to investigate the reduction of aqueous Cr(VI) by pyrite. Special emphasis was placed on the characterisation of the solid phase formed during the reaction process. Cr(III) and Fe(III) species were identified by XPS analyses in addition to non-oxidised pyrite. Optical microscopy images and the corresponding Raman spectra reveal a strong heterogeneity of the samples with three different types of zones. (i) Reflective areas with E_g and A_g Raman wavenumbers relative to non-oxidised pyrite are the most frequently observed. (ii) Orange areas that display a drift of the E_g and A_g pyrite vibration modes of −3 and −6 cm⁻¹, respectively. Such areas are only observed in the presence of Cr(VI) but are not specifically due to this oxidant. (iii) Bluish areas with vibration modes relative to a corundum-like structure that can be assigned to a solid solution Fe_2−xCr_xO₃, x varying between 0.2 and 1.5. The heterogeneity in the spatial distribution of chromium observed by optical microscopy and associated Raman microspectroscopy is confirmed by μ-XRF. In agreement with both solution and XPS analyses, these spectroscopies clearly confirm that chromium is in the trivalent state. XANES spectra in the iron K-edge pre-edge region obtained in rich chromium areas reveal the presence of ferric ion thus revealing a systematic association between Cr(III) and Fe(III). In agreement with Raman analyses, Cr K-edge EXAFS can be interpreted as corresponding to Cr atoms involved in a substituted-type hematite structure Fe_2−xCr_xO₃.  相似文献   

Arsenic and iron speciation in uranium mine tailings using X-ray absorption spectroscopy     

《Applied Geochemistry》2013

In northern Saskatchewan, Canada, high-grade U ores and the resulting tailings can contain high levels of As. An environmental concern in the U mining industry is the long-term stability of As within tailings management facilities (TMFs) and its potential transfer to the surrounding groundwater. To mitigate this problem, U mill effluents are neutralized with lime to reduce the aqueous concentration of As. This results in the formation of predominantly Fe³⁺–As⁵⁺ secondary mineral phases, which act as solubility controls on the As in the tailings discharged to the TMF. Because the speciation of As in natural systems is critical for determining its long-term environmental fate, characterization of As-bearing mineral phases and complexes within the deposited tailings is required to evaluate its potential transformation, solubility, and long-term stability within the tailings mass. In this study, synchrotron-based bulk X-ray absorption spectroscopy (XAS) was used to study the speciation of As and Fe in mine tailings samples obtained from the Deilmann TMF at Key Lake, Saskatchewan. Comparisons of K-edge X-ray absorption spectra of tailings samples and reference compounds indicate the dominant oxidation states of As and Fe in the mine tailings samples are +5 and +3, respectively, largely reflecting their generation in a highly oxic mill process, deposition in an oxidized environment, and complexation within stable oxic phases. Linear combination fit analyses of the K-edges for the Fe X-ray absorption near edge spectra (XANES) to reference compounds suggest Fe is predominantly present as ferrihydrite with some amount of the primary minerals pyrite (8–15% in some samples) and chalcopyrite (5–15% in some samples). Extended X-ray absorption fine structure (EXAFS) analysis of As K-edge spectra indicates that As⁵⁺ (arsenate) present in tailings samples is adsorbed to the ferrihydrite though an inner-sphere bidentate linkage.  相似文献   

Theoretical analysis of X-ray absorption near-edge structure in forsterite, Mg2SiO4-Pbnm, and fayalite, Fe2SiO4-Pbnm, at room temperature and extreme conditions     

Ziyu Wu  Annibale Mottana  Augusto Marcelli  Calogero Rino Natoli  Eleonora Paris 《Physics and Chemistry of Minerals》1996,23(3):193-204

We calculated the forsterite Mg K-edge and the fayalite Fe K-edge X-ray absorption spectra both for the M 1 and M 2 sites and for the overall edge by using the one-electron multiple-scattering theory. The validity of the theoretical model is well illustrated by comparison of calculations with experimental data at the Mg K-edge of MgO (periclase) and at the Mg and Fe K-edges spectra of forsterite and fayalite. Starting from these results at room conditions, we calculated the Mg and Fe K-edges X-ray absorption spectra of forsterite and fayalite at low and high temperatures and at high pressures as well. Variations of fine structures occur mostly in the intermediate multiple scattering (IMS) regions and as a result of the applied pressure. In order to demonstrate the capability of XAS to lead to deeper knowledge of structure relevant to Earth's upper mantle we also attempted calcuating the high-P edge for Fe ²⁺ in low-spin using a different occupation of valence electrons. If a change in spin state really occurs in fayalite, our simple model shows that XAS would evidence it easily even with low resolution.  相似文献   

Sulfur K- and L-edge X-ray absorption spectroscopy of sphalerite,chalcopyrite and stannite     

Dien Li  G. M. Bancroft  M. Kasrai  M. E. Fleet  B. X. Yang  X. H. Feng  K. Tan  Mingsheng Peng 《Physics and Chemistry of Minerals》1994,20(7):489-499

Sulfur K-edge x-ray absorption spectra (XANES and EXAFS) and L-edge XANES of sphalerite (ZnS), chalcopyrite (CuFeS₂) and stannite (Cu₂FeSnS₄) have been recorded using synchrotron radiation. The K- and L-edge XANES features are interpreted using a qualitative MO/energy band structure model. The densities of unoccupied states at the conduction bands of sphalerite, chalcopyrite and stannite are determined using S K- and L-edge XANES features (up to 15 eV above the edge), combined with published metal K-edge XANES. The SK- and L-edge XANES also indicate that, for sphalerite, the Fe²⁺ 3d band at the fundamental gap has little or no bonding hybridization with S 3p and S 3s orbitals; for chalcopyrite, the Cu⁺ 3d and Fe³⁺ 3d bands have strong mixing with S 3p and S 3s states, while for stannite the Cu⁺ 3d band strongly hybridizes with S 3p and S 3s orbitals, but the Fe²⁺ 3d band does not. The post-edge XANES features (15–50 eV above the edge) of sphalerite, chalcopyrite and stannite are similar. These features are related to the tetrahedral coordination of sulfur in all these structures, and interpreted by a multiple scattering model. The resonance energies from both the K-edge and L-edge XANES for these minerals are well correlated with reciprocal interatomic distances and lattice spaces. Sulfur K-edge EXAFS analyses using Fourier transform and curve fitting procedures are presented. Comparison of the structural parameters from EXAFS with x-ray structure data shows that the first shell bond distances (BD) from EXAFS are usually accurate to ±0.02 Å, and that coordination numbers (CN) are generally accurate to ±20 percent. For sphalerite, EXAFS analysis yields the structure parameters for the first three neighbour shells around a sulfur atom; the BD and CN even for the third shell are in close agreement with the x-ray structure, and the Debye-Waller term decreases from the first shell to the third shell. It is shown that sphalerite (ZnS) is a good model compound for EXAFS analysis of sulfur in chalcogenide glasses and metalloproteins.  相似文献   

Abiotic reduction of uranium by Fe(II) in soil     

Drew E. Latta  Maxim I. Boyanov  Kenneth M. Kemner  Edward J. O’Loughlin  Michelle M. Scherer 《Applied Geochemistry》2012

Structural Fe(II) has been shown to reduce several oxidized environmental contaminants, including NO₃, chlorinated solvents, Cr(VI), and U(VI). Studies investigating reduction of U(VI) by soils and sediments, however, suggest that abiotic reduction of U(VI) by Fe(II) is not significant, and that direct enzymatic reduction of U(VI) by metal-reducing bacteria is required for U(VI) immobilization as U(IV). Here evidence is presented for abiotic reduction and immobilization of U(VI) by structural Fe(II) in a redoximorphic soil collected from a hillside spring in Iowa. Oxidation of Fe(II) in the soil after reaction with U(VI) was demonstrated by Mössbauer spectroscopy and reduction of U(VI) by the pasteurized soil using U L_III-edge X-ray absorption spectroscopy (XAS). XAS indicates that both reduced U(IV) and oxidized U(VI) or U(V) are present after U(VI) interaction with the Fe(II) containing soil. The EXAFS data show the presence of a non-uraninite U(IV) phase and evidence of the oxidized U(V) or U(VI) fraction being present as a non-uranyl species. Little U(VI) reduction is observed by soil that has been exposed to air and oxidation of Fe(II) to goethite has occurred. Soil characterization based on chemical extractions, Mössbauer spectroscopy, and Fe K-edge XAS indicate that the majority of Fe(II) in the soil is structural in nature, existing in clay minerals and possibly a green rust-like phase. These data provide compelling evidence for abiotic reduction of U(VI) by structural Fe(II) from soil near Fe-rich oxic–anoxic boundaries in natural environments. The work highlights the potential for abiotic reduction of U(VI) by Fe(II) in reduced, Fe-rich environments.  相似文献   

Surface complexation and precipitate geometry for aqueous Zn(II) sorption on ferrihydrite I: X-ray absorption extended fine structure spectroscopy analysis     

G.A Waychunas  C.C FullerJ.A Davis 《Geochimica et cosmochimica acta》2002,66(7):1119-1137

“Two-line” ferrihydrite samples precipitated and then exposed to a range of aqueous Zn solutions (10⁻⁵ to 10⁻³ M), and also coprecipitated in similar Zn solutions (pH 6.5), have been examined by Zn and Fe K-edge X-ray absorption spectroscopy. Typical Zn complexes on the surface have Zn-O distances of 1.97(.02) Å and coordination numbers of about 4.0(0.5), consistent with tetrahedral oxygen coordination. This contrasts with Zn-O distances of 2.11(.02) Å and coordination numbers of 6 to 7 in the aqueous Zn solutions used in sample preparation. X-ray absorption extended fine structure spectroscopy (EXAFS) fits to the second shell of cation neighbors indicate as many as 4 Zn-Fe neighbors at 3.44(.04) Å in coprecipitated samples, and about two Zn-Fe neighbors at the same distance in adsorption samples. In both sets of samples, the fitted coordination number of second shell cations decreases as sorption density increases, indicating changes in the number and type of available complexing sites or the onset of competitive precipitation processes. Comparison of our results with the possible geometries for surface complexes and precipitates suggests that the Zn sorption complexes are inner sphere and at lowest adsorption densities are bidentate, sharing apical oxygens with adjacent edge-sharing Fe(O,OH)₆ octahedra. Coprecipitation samples have complexes with similar geometry, but these are polydentate, sharing apices with more than two adjacent edge-sharing Fe(O,OH)₆ polyhedra. The results are inconsistent with Zn entering the ferrihydrite structure (i.e., solid solution formation) or formation of other Zn-Fe precipitates. The fitted Zn-Fe coordination numbers drop with increasing Zn density with a minimum of about 0.8(.2) at Zn/(Zn + Fe) of 0.08 or more. This change appears to be attributable to the onset of precipitation of zinc hydroxide polymers with mainly tetrahedral Zn coordination. At the highest loadings studied, the nature of the complexes changes further, and a second type of precipitate forms. This has a structure based on a brucite layer topology, with mainly octahedral Zn coordination. Amorphous zinc hydroxide samples prepared for comparison had a closely similar local structure. Analysis of the Fe K-edge EXAFS is consistent with surface complexation reactions and surface precipitation at high Zn loadings with little or no Fe-Zn solid solution formation. The formation of Zn-containing precipitates at solution conditions two or more orders of magnitude below their solubility limit is compared with other sorption and spectroscopic studies that describe similar behavior.  相似文献   

Characterization of synthetic hedenbergite (CaFeSi2O6)–petedunnite (CaZnSi2O6) solid solution series by X-ray single crystal diffraction     

Matthias Heuer  Alexandra L. Huber  Geoffrey D. Bromiley  Karl Thomas Fehr  Klaus Bente 《Physics and Chemistry of Minerals》2005,32(8-9):552-563

Clinopyroxenes of the solid solution series hedenbergite (CaFeSi₂O₆)–petedunnite (CaZnSi₂O₆) were synthesized at temperatures of 825–1200°C and pressures of 0.5–2.5 GPa. Compositions were determined by electron microprobe analysis. Selected crystals were investigated by means of single crystal diffraction and structure refinement and the structural distortion was studied depending on the substitution of iron by zinc on the octahedral M1 site. It is shown that the coordination of the M1 site has the most significant effect on M–O bond lengths, with changes on the other sites accommodating this distortion. The mean quadratic elongation and the octahedral angle variance as quantitative measures of the distortion of the coordination polyhedron were correlated with former results of ⁵⁷Fe Mössbauer spectroscopy at 298 K. The results presented now complete an earlier work on synthetic, crystalline powders of the same material and deliver exact structural data that were not possible to obtain by Rietveld refinements on powder data.  相似文献   

The site of Fe in Fe-bearing MgSiO3 enstatite and perovskite. a theoretical-, x-ray multiple-scattering study at Fe K-edge     

François Farges 《Physics and Chemistry of Minerals》1995,22(5):318-322

Two polycrystalline-, Fe-bearing MgSiO₃ enstatite and perovskite have been probed by x-ray absorption near edge structure (XANES) spectroscopy at the Fe K-edge under ambient conditions. The perovskite sample was synthesized at 260 kbar and 1973 K in a multianvil apparatus. The experimental XANES spectrum has been compared to ab-initio-, x-ray multiple-scattering calculations (Feff 6 code). Calculations confirm that the Fe K-edge arises mainly from multiple scattering involving the first shell of oxygen neighbors around Fe. In Fe-enstatite, these calculations are consistent with Fe²⁺ as substituted in the M2 site of this orthopyroxene, in good agreement with crystal structure refinements and previous XANES studies. In perovskite, Feff 6 suggests that Fe is likely to be substituted to Mg within the (8+4)-coordinated sites of that perovskite. No evidences for 6-coordinated Fe were found. These results are consistent with a previous anharmonic analysis of the extended x-ray absorption fine structure (EXAFS) study that evidenced the presence of 8-coordinated Fe in the same perovskite sample.  相似文献   

Quantitative antimony speciation in shooting-range soils by EXAFS spectroscopy   总被引：1，自引：0，他引：1  

Andreas C. Scheinost  Andre Rossberg  Irene Xifra  Ann-Kathrin Leuz  C. Annette Johnson 《Geochimica et cosmochimica acta》2006,70(13):3299-3312

The Sb speciation in soil samples from Swiss shooting ranges was determined using Sb K-edge X-ray absorption spectroscopy (XAS) and advanced statistical data analysis methods (iterative transformation factor analysis, ITFA). The XAS analysis was supported by a spectral data set of 13 Sb minerals and 4 sorption complexes. In spite of a high variability in geology, soil pH (3.1-7.5), Sb concentrations (1000-17,000 mg/kg) and shooting-range history, only two Sb species were identified. In the first species, Sb is surrounded solely by other Sb atoms at radial distances of 2.90, 3.35, 4.30 and 4.51 Å, indicative of metallic Sb(0). While part of this Sb(0) may be hosted by unweathered bullet fragments consisting of PbSb alloy, Pb L_III-edge XAS of the soil with the highest fraction (0.75) of Sb(0) showed no metallic Pb, but only Pb²⁺ bound to soil organic matter. This suggests a preferential oxidation of Pb in the alloy, driven by the higher standard reduction potential of Sb. In the second species, Sb is coordinated to 6 O-atoms at a distance of 1.98 Å, indicative of Sb(V). This oxidation state is further supported by an edge energy of 30,496-30,497 eV for the soil samples with <10% Sb(0). Iron atoms at radial distances of 3.10 and 3.56 Å from Sb atoms are in line with edge-sharing and bidentate corner-sharing linkages between Sb(O,OH)₆ and Fe(O,OH)₆ octahedra. While similar structural units exist in tripuhyite, the absence of Sb neighbors contradicts formation of this Fe antimonate. Hence the second species most likely consists of inner-sphere sorption complexes on Fe oxides, with edge and corner-sharing configuration occurring simultaneously. This pentavalent Sb species was present in all samples, suggesting that it is the prevailing species after weathering of metallic Sb(0) in oxic soils. No indication of Sb(III) was found.  相似文献   

From the green color of eskolaite to the red color of ruby: an X-ray absorption spectroscopy study     

Émilie Gaudry  Philippe Sainctavit  Farid Juillot  Federica Bondioli  Philippe Ohresser  Isabelle Letard 《Physics and Chemistry of Minerals》2006,32(10):710-720

The best known cause for colors in insulating minerals is due to transition metal ions as impurities. As an example, Cr³⁺ is responsible for the red color of ruby (α-Al₂O₃:Cr³⁺) and the green color of eskolaite (α-Cr₂O₃). Using X-ray absorption measurements, we connect the colors of the Cr _x Al_2−x O₃ series with the structural and electronic local environment around Cr. UV–VIS electronic parameters, such as the crystal field and the Racah parameter B, are related to those deduced from the analysis of the isotropic and XMCD spectra at the Cr L_2,3-edges in Cr_0.07Al_1.93O₃ and eskolaite. The Cr–O bond lengths are extracted by EXAFS at the Cr K-edge in the whole Cr _x Al_2−x O₃ (0.07≤x< 2) solid solution series. The variation of the mean Cr–O distance between Cr_0.07Al_1.93O₃ and α-Cr₂O₃ is evaluated to be 0.01₅ Å (≈1%). The variation of the crystal field in the Cr _x Al_2−x O₃ series is discussed in relation with the variation of the averaged Cr–O distances.  相似文献   

A temperature dependent X-ray Absorption Fine Structure study of dynamic X-site disorder in almandine: a comparison to diffraction data     

Simona Quartieri  G. Antonioli  G. Artioli  C. A. Geiger  P. P. Lottici 《Physics and Chemistry of Minerals》1997,24(3):200-205

X-ray Absorption Fine-Structure (XAFS) measurements in the temperature range 20-473K at the Fe K-edge of synthetic almandine (Fe₃Al₂Si₃O₁₂) allow us to determine the temperature dependence of the XAFS Debye-Waller (DW) factors for each of the two crystallographically independent Fe?O bond distances (Fe(1)?O and Fe(2)?O). The vibrational anisotropy of Fe²⁺ in the dodecahedral X-site has been documented. The XAFS DW factor of the longer Fe(2)?O bond is larger and has a greater T-dependence than the shorter Fe(1)?O bond, which remains constant over the measured T range. The XAFS DW factors are compared with the uncorrelated part of the DW factors derived from a single-crystal X-ray diffraction (XRD) study on almandine and are interpreted on the basis of the Einstein model. A comparison with diffraction data shows that the shorter Fe(1)–O bond is significantly affected by correlation in the atomic displacements.  相似文献   

Correlation between local structure and molar ratio of Au (III) complexes in aqueous solution: An XAS investigation     

Xing Chen  Wangsheng Chu  Dongliang Chen  Zhonghua Wu  A. Marcelli  Ziyu Wu   《Chemical Geology》2009,268(1-2):74-80

The local and geometrical structure around gold (III) e.g., Au³⁺ ions in aqueous solution with different OH⁻/Cl⁻ molar ratios, has been investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of [AuCl_n(OH)_4−n]⁻ solutions have been calculated and the multiple-scattering spectral features have been attributed to Cl d-states, axial water molecules and the replacement of Cl⁻ ligands by OH⁻ ligands. A square–planar geometry for [AuCl_n(OH)_4−n]⁻ with two axial water molecules has been identified. Moreover, a spectral correlation between XANES features and the type of planar atoms has been identified. By extended X-ray absorption fine structure spectra (EXAFS), the planar Au bond distances in the solutions have also been determined, e.g., 2.28 Å for Au–Cl and 1.98 Å for Au–O, respectively. The same EXAFS analysis provides evidence that the peak at about 4.0 Å in solutions with the lowest OH⁻/Cl⁻ molar ratio arises from collinear Cl–Au–Cl multiple-scattering contributions. For the first time, a complete detailed reconstruction of the hydration structure of an Au ion at different pH values has been achieved.  相似文献   

Effect of citrate on the local Fe coordination in ferrihydrite, arsenate binding, and ternary arsenate complex formation     

Christian Mikutta  Jakob Frommer  Ralf Kaegi 《Geochimica et cosmochimica acta》2010,74(19):5574-6504

In oxic environments contaminated with arsenate (As(V)), small polyhydroxycarboxylates such as citrate may impact the structure of precipitating ferrihydrite (Fh) and thus the surface speciation of As(V). In this study, ‘2-line’ Fh was precipitated from ferric nitrate solutions that were neutralized to pH 6.5 in the presence of increasing citrate concentrations and in the absence or presence of As(V). The initial citrate/Fe and As/Fe ratios were 0-50 mol% and 5 mol%, respectively. The reaction products, enriched with up to 0.32 mol citrate per mole Fe, were characterized by X-ray diffraction, transmission electron microscopy, and Fe and As K-edge X-ray absorption spectroscopy. Citrate decreased the particle size of Fh by impairing the polymerization of Fe(O,OH)₆ octahedra via edge and corner linkages. In the presence of citrate and As(V), coordination numbers of Fe decreased by up to 28% relative to pure Fh. Citrate significantly reduced the static disorder of Fe-O bonds, implying a decreased octahedral distortion in Fh. Mean bond distances in Fh were not affected by citrate and remained constant within error at 1.98 Å for Fe-O, 3.03 Å for Fe-Fe1, and 3.45 Å for Fe-Fe2. Likewise, citrate had no effect on the As-Fe (3.31 Å) bond distance in As(V) coprecipitated with Fh. The As K-edge EXAFS data comply with the formation of (i) only monodentate binuclear (²C) As(V) surface complexes and (ii) combinations of ²C, monodentate mononuclear (¹V), and outersphere As(V) surface complexes. Our results suggest that increasing citrate concentrations led to a decreasing ¹V/²C ratio and/or that citrate increasingly impaired the formation of outersphere As(V) complexes. Moreover, citrate stabilized colloidal suspensions of Fh (pH 4.3-6.6, I ∼0.45 M) and reduced Fh formation at the expense of soluble Fe(III)-citrate complexes. At initial citrate/Fe ratios ?25 mol%, between 8% and 41% of total Fe was bound in Fe(III)-citrate complexes after Fh formation. Polynuclear Fe(III)-citrate species were found to bind As(V) via surface complexes indistinguishable by EXAFS from those of As(V) adsorbed to or coprecipitated with Fh. Our study implies that low molecular weight polyhydroxycarboxylates may enhance the mobility of As(V) in aqueous systems of high ionic strength (e.g., neutralizing acid mine drainage) by colloidal stabilization of suspended Fh particles and the formation of ternary As(V) complexes.  相似文献   

Arsenic uptake by natural calcite: An XAS study     

F. Bardelli  P. Costagliola  F. Di Benedetto  P. Lattanzi  M. Romanelli 《Geochimica et cosmochimica acta》2011,75(11):3011-3023

As-bearing travertine rocks from Tuscany (Italy), where previous studies suggested the existence of a CO₃²⁻ ⇔ AsO₃³⁻ substitution in the calcite lattice, were investigated with X-ray Absorption Spectroscopy (XAS) at the As K-edge (11,867 eV). In two of the studied samples, XANES indicates that As is in the 5+ oxidation state only, and EXAFS analysis reveals a local environment typical of arsenate species. For these samples, the lack of detectable second shell signals suggests a poorly ordered environment, possibly corresponding to an adsorption onto oxide and/or silicate phases. On the other hand, in the third sample XANES reveals a mixed As oxidation state (III and V). This sample also presents evident next nearest neighbor coordination shells, attributed to As-Ca and As-As contributions. The occurrence of next neighbor shells is evidence that part of As is incorporated in an ordered lattice. Furthermore, the local structure revealed by EXAFS is compatible with As incorporation in the calcite phase, as further supported by DFT simulations. The observation of next neighbors shells only in the As(III)-rich sample suggests the substitution of the arsenite group in place of the carbonate one (CO₃²⁻ ⇔ AsO₃³⁻). The conclusion of this work is that uptake of As by calcite is in general less favored than adsorption onto iron oxhydroxides, but could become environmentally important wherever the latter phenomenon is hindered.  相似文献   

Microtextures and crystal chemistry in P21/c pigeonites     

M. Tribaudino  D. Pasqual  G. Molin  L. Secco 《Mineralogy and Petrology》2003,77(3-4):161-176

Summary ?A single-crystal X-ray investigation was performed on crystals of P2₁/c natural pigeonite with varying Ca and Fe^* ( = Fe²⁺ + Mn²⁺) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae. The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated. The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples. In particular, it is shown that Ca and Fe^* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and decrease the difference between the M2–O3 distances. Received October 18, 2001; revised version accepted February 15, 2002  相似文献