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1.
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   

2.
The electron spin resonance (ESR) spectrum of Cr3+ in a synthetic single crystal of forsterite doped with Cr2O3 was studied at room temperature in the X-band frequency range. The dependence of the observed spectra on the crystal orientation with respect to the applied magnetic field was investigated. The ESR spectra are described by the spin Hamiltonian \(H = \beta HgS + D(S_Z^{\text{2}} - {\text{1/3}}S{\text{(}}S{\text{ + 1)) + }}E{\text{(}}S_x^{\text{2}} - S_y^{\text{2}} {\text{)}}\) with S=3/2. The spin resonance reveals that the chromium ions are located at both the M1 and M2 positions. Other possible substitutional or interstitial Cr3+ positions may be possible, but were not observed. The site occupancy numbers of Cr3+ at M1 and M2 are roughly 1.2×10?4 and 0.8×10?4, respectively, assuming that chromium is oxidized completely. The preference of the chromium ions for M1 was interpreted qualitatively in terms of crystal field criteria. The rhombic and axial spin Hamiltonian parameters, D and E, and the directions of the magnetic axes obtained for M1 and M2 are consistent with the respective oxygen coordination polyhedra.  相似文献   

3.
The nine adiabatic elastic stiffness constants of synthetic single-crystal fayalite, Fe2SiO4, were measured as functions of pressure (range, 0 to 1.0 GPa) and temperature (range, 0 to 40° C) using the pulse superposition ultrasonic method. Summary calculated results for a dense fayalite polycrystalline aggregate, based on the HS average of our single-crystal data, are as follows: Vp = 6.67 km/s; Vs = 3.39km/s; K= 127.9 GPa; μ = 50.3 GPa; (?K/?P)T = 5.2; (?μ/?P)T=1.5;(?K/?T)P= ?0.030 GPa/K;and,(?/?T)P =-0.013 GPa/K (the pressure and temperature data are referred to 25° C and 1 atm, respectively). Accuracy of the single-crystal results was maintained by numerous cross and redundancy checks. Compared to the single-crystal elastic properties of forsterite, Mg2SiO4, the fayalite stiffness constants, as well as their pressure derivatives, are lower for each of the on-diagonal (C ij for which i=j) values, and generally higher for the off-diagonal (C ij for which i≠j) data. As a result, the bulk moduli (K) and dK/dP for forsterite and fayalite are very similar, but the rigidity modulus (μ) and dμ/dP for polycrystalline fayalite are much lower than their forsterite counterparts. The bulk compression properties derived from this study are very consistent with the static-compression x-ray results of Yagi et al. (1975). The temperature dependence of the bulk modulus of fayalite is somewhat greater (in a negative sense) than that of forsterite. The rigidity dependencies are almost equivalent. Over the temperature range relevant to this study, the elastic property results are generally consistent with the data of Sumino (1978), which were obtained using the RPR technique. However, some of the compressional modes are clearly discrepant. The elastic constants of fayalite appear to be less consistent with a theoretical HCP model (Leibfried 1955) than forsterite, reflecting the more covalent character of the Fe-O bonding in the former.  相似文献   

4.
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   

5.
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   

6.
We present structural information obtained on spinel and alumina at high temperature (298-2400 K) using in-situ XANES at the Mg and Al K-edges. For spinel, [4](Alx,Mg1−x)[6](Al2−x,Mgx)O4, with increasing temperature, a substitution of Mg by Al and Al by Mg in their respective sites is observed. This substitution corresponds to an inversion of the Mg and Al sites. There is a significant change in the Al K-edge spectra between crystal and liquid, which can be attributed to a change of the [6]Al normally observed in corundum at room temperature, to a mixture of [6]Al-[4]Al in the liquid state. This conclusion is in good agreement with previous 27Al NMR experiments. Furthermore, both experiments at the Al and Mg K-edges are in good agreement with XANES calculation made using FDMNES code.  相似文献   

7.
A series of Cr-substituted goethites with (Cr:Fe molar ratio up to 0.12) were prepared. Thermal analysis of the solids indicates the formation of cation-deficient compounds that are more stable towards the transformation to hematite as the Cr content increases. Powder X-ray diffraction (PXRD) and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the whole series of the substituted solids. XRD patterns demonstrate that the order around Fe remains typical of a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the incorporation of Cr causes a slight decrease in the cell volume with the c-cell parameter following the Vegard’s law. This decrease is accompanied by changes in opposite directions of the various Me-Me distances. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom changes slightly upon Cr substitution: Measurements in the Cr K-edge show that the Cr environment remains unchanged in the whole series. All the observed trends in both average Rietveld and local EXAFS distances can be traced back to the differences in the coordination polyhedra around Cr and Fe. The polyhedron around Cr is more symmetric and can be described as Cr(OH0.5)6 as opposed to the polyhedron around Fe that contains two distinct sets of ligands, FeO3(OH)3. The effects caused by substitution are governed by this difference, rather than by the smaller size of Cr(III) as compared to Fe(III). Simultaneous use of XAS and Rietveld refinement of XRD data permits tracing the trends in the average long range ordering (Me-Me distances) to local changes in distances and angles when Cr3+ substitutes Fe3+ in goethite. Complex changes in the various interatomic distances and angles may result in deceivingly simple long-range trends. These trends are therefore of limited value as probes for the atomic scale changes. On the other hand, XAS provide direct information on the fundamental, atomic-scale changes.  相似文献   

8.
End-member synthetic fayalite and forsterite and a natural solid-solution crystal of composition (Mg1.80,Fe0.20)SiO4 were investigated using Raman spectroscopy. Polarized single-crystal spectra were measured as a function of temperature. In addition, polycrystalline forsterite and fayalite, isotopically enriched in 26Mg and 57Fe, respectively, were synthesized and their powder spectra measured. The high-wavenumber modes in olivine consist of internal SiO4 vibrations that show little variation upon isotopic substitution. This confirms conclusions from previous spectroscopic studies that showed that the internal SiO4 vibrations have minimal coupling with the lower-wavenumber lattice modes. The lowest wavenumber modes in both forsterite and fayalite shift in energy following isotopic substitution, but with energies less than that which would be associated with pure Mg and Fe translations. The low-wavenumber Raman modes in olivine are best described as lattice modes consisting to a large degree of mixed vibrations of M(2) cation translations and external vibrations of the SiO4 tetrahedra. The single-crystal spectra of forsterite and Fo90Fa10 were recorded at a number of temperatures from room temperature to about 1200 °C. From these data the microscopic Grüneisen parameters for three different Ag modes for both compositions were calculated, and also the structural state of the solid solution crystal was investigated. Small discontinuities observed in the wavenumber behavior of a low-energy mixed Mg/T(SiO4) mode between 700 and 1000 °C may be related to minor variations in the Fe–Mg intracrystalline partitioning state in the Fo90Fa10 crystal, but further spectroscopic work is needed to clarify and quantify this issue. The mode wavenumber and intensity behavior of internal SiO4 vibrations as a function of temperature are discussed in terms of crystal field and dynamic splitting and also 1 and 3 coupling. Crystal-field splitting increases only very slightly with temperature, whereas dynamical-field splitting is temperature dependent. The degree of 1–3 coupling decreases with increasing temperature.  相似文献   

9.
The dependence of Mg/Fe ordering on oxygen partial pressure in natural olivine crystals of volcanic origin has been studied by X-ray diffraction. Two natural crystals with 10% and 12% fayalite have been investigated and the atomic positions, anisotropic temperature factors, extinction coefficients and site occupancies have been refined, reaching R-values of 2.2%. After subjecting the crystals to oxygen partial pressures of 10?16 bar and 10?21 bar the crystals were studied again. In total six crystals were studied and the distribution coefficients K D determined. The natural untreated crystals had K D=1.09 and 1.06, e.g., a slight preference of Fe in (M1). p(O2) of 10?16 bar increased the ordering of Fe in (M1) to K D=1.2, while p(O2)=10?21 bar reversed K D to 0.8 with ordering of Fe in (M2). These experiments suggest that Mg/Fe ordering in olivines is primarily determined by the prevailing oxygen partial pressure.  相似文献   

10.
Synchrotron radiation has been used to collect high-resolution Fe K absorption near-edge spectra of a suite of Fe minerals and compounds having a range of Fe environments. These spectra, along with those of previous workers, indicate that the number, position, and intensity of near-edge features are characteristic of Fe valence and general site geometry. For example, the crest of the K-edge for Fe2+ in a six-coordinated site in the oxides studied is about 3 eV lower in energy than that for Fe3+ in a similar site. The K-edge crest for Fe3+ in a four-coordinated site is 1 to 2 eV lower than for Fe3+ in a regular site. The shape of the edge crest is sensitive to the details of first-neighbor bonding distances, tending to be broader in species with irregular Fe sites and varying in energy according to the average bond length. Comparison with Ca2+ and Zn2+ spectra from the literature is made and the applicability and utility of edge measurements discussed.  相似文献   

11.
The isobaric (P=10 kb) temperature dependence of the electrical conductivities of forsterite, fayalite and forsterite-fayalite mixed crystals was measured with special regard to the thermodynamics of point defects in these minerals. Measurements, taken at increasing and decreasing temperature, were performed on synthetic powders of the following compositions: Fo 100/Fa 0, Fo 90/Fa 10, Fo 80/Fa 20, Fo 60/Fa 40, and Fo 0/Fa100. Control of oxygen partial pressure was achieved with solid state buffers (Fa/Q/M, Fa/Q/I, and Fe/FeO). Activities of the binary components were controlled by equilibrating the sample with its neighbouring phases. All values for σ, obtained with controlled pO2 and fixed activities of the binary components, agree well upon either heating or cooling. From the gradient of lg σ vs. 1/T plots, the following activation energies were estimated: 2,461 eV (970°–1075°C) and 0.984 eV (522°–970°C) for Fo 100/Fa 0 equilibrated with MgO; 0.777 eV and 0.683 eV for Fo 90/Fa 10 and Fo 80/Fa 20 equilibrated with enstatite and pO2 controlled by Fe/FeO buffer; 0.622 eV, 0.528 eV, and 0.479 eV for Fo 90/Fa 10, Fo 80/Fa 20, and Fo 60/Fa 40 equilibrated with enstatite and pO2 controlled by Fa/Q/M buffer; and 0.524 eV and 0.383 eV for Fo 0/Fa 100 equilibrated with Q/I and Q/M respectively.  相似文献   

12.
Polarized optical absorption measurements were carried out on three single crystals of Mg2SiO4 (forsterite), differently doped with Cr. Two crystals containing average 0.013 and 0.027 weight% Cr, respectively, were pulled from the melt in air, whereas one crystal containing average 0.08 weight% Cr was pulled from the melt in an argon atmosphere. The absorption spectra of the three crystals agree with each other although the intensity of single absorption bands varies significantly. In all -polarized patterns a sharp absorption line around 18000cm-1 (550 nm) appears. Conjectures are presented to assign this line to the lasing center in Cr doped forsterite which very likely exists as Cr4+ at the fourfold coordinated Si site.  相似文献   

13.
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   

14.
The 57Fe Mössbauer spectra of deerites of different chemical composition, taken at several temperatures, show that Fe2+ and Fe3+ occupy all the six-coordinated lattice sites with a preference of Fe3+ probably for the M(1) to M(3) positions, and a preference of Fe2+ probably for the M(4) to M(6) and the M(7) to M(9) sites. The room and high temperature spectra reveal absorption patterns due to thermally activated Fe2+ → Fe3+ electron delocalization. The extent of electron delocalization is dependent on the chemical composition, e.g., the amount of ions (Mg, Mn, Al) substituting for Fe.  相似文献   

15.
X-ray photoelectron and absorption spectra have been obtained for natural specimens of cubanite and compared with the corresponding spectra for chalcopyrite. Synchrotron X-ray photoelectron spectra of surfaces prepared by fracture under ultra-high vacuum revealed some clear differences for the two minerals, most notably those reflecting their different structures. In particular, the concentration of the low binding energy S species formed at cubanite fracture surfaces was approximately double that produced at chalcopyrite surfaces. However, the core electron binding energies for the two S environments in cubanite were not significantly different, and were similar to the corresponding values for the single environment in chalcopyrite. High binding energy features in the S 2p and Cu 2p spectra were not related to surface species produced either by the fracture or by oxidation, and most probably arose from energy loss due to inter-band excitation. Differences relating to the Fe electronic environments were detectable, but were smaller than expected from some of the observed physical properties and Mössbauer spectroscopic parameters for the two minerals. X-ray absorption and photoelectron spectra together with the calculated densities of states for cubanite confirmed an oxidation state of CuI in the mineral. It was concluded that the best formal oxidation state representation for cubanite is CuI(Fe2)VS 3 ?II .  相似文献   

16.
New polarized infrared reflectance spectra of pure synthetic forsterite and natural Fo86-olivine have been recorded from 5000 to 100cm-1. Out of the 35 expected infrared active modes, 33 have been observed (8 B1u, 12 B2u, 13 B3u). The observed frequency shift from pure forsterite to Fo86-olivine is consistent with the higher mass of the substituted iron. The substitution of only 14% of iron also reduces the overal far-infrared reflectivity of olivine as compared to pure forsterite. Several discrepancies associated with previous studies of forsterite are explained by our investigation. We suggest that some of the previous investigations were complicated by polarization mixing.  相似文献   

17.
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18.
The electric field gradient (EFG) in Mg2SiO4 is calculated on the basis of the extended point ion model, including the local term of the overlap contribution. The agreement with experimental data deduced from the quadrupole coupling constants and principal axes at the Mg sites is quite good. The results of the present calculation exhibit a small overlap contribution to the EFG at M1 and a clearly bigger one at M2, whereas the lattice contribution to the EFG at M1 and M2 is reversed. The distinct overlap effects are assumed to be due to the particular Mg2SiO4 crystal structure and the different point symmetry at M1 and M2. The oxygen polarizability and charge used to calculate the EFG tensor were found to be smaller than the theoretical polarizability and formal charge, respectively. The sign of the Mg quadrupole coupling constants at M1 and M2, which has not been determined experimentally, results from the EFG calculation as positive.  相似文献   

19.
A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg2SiO4-Fe2SiO4 and Mg2Si2O6-Fe2Si2O6. The formulation makes explicit provision for nonconvergent ordering of Fe2+ and Mg2+ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene ± quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and Mössbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and (5) direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe(M2)Fe(M1)Fe(M2)Mg(M1)+Mg(M2)Fe(M1)] cannot differ significantly (± 1 cal/K) from zero over the temperature range of calibration (400°–1300° C). Consideration of the mixing properties of olivine-orthopyroxene solid solutions places tight constraints on the standard state thermodynamic quantities describing Fe-Mg exchange reactions involving olivine, orthopyroxene, pyralspite garnets, aluminate spinels, ferrite spinels and biotite. These constraints are entirely consistent with the standard state properties for the phases-quartz,-quartz, orthoenstatite, clinoenstatite, protoenstatite, fayalite, ferrosilite and forsterite which were deduced by Berman (1988) from an independent analysis of phase equilibria and calorimetric data. In conjunction with these standard state properties, the solution model presented in this paper provides a means of evaluating an internally consistent set of Gibbs energies of mineral solid solutions in the system Mg2SiO4-Fe2SiO4-SiO2 over the temperature range 0–1300° C and pressure interval 0.001–50 kbars. As a consequence of our analysis, we find that the excess Gibbs energies associated with mixing of Fe and Mg in (Fe, Mg)2SiO4 olivines, (Fe, Mg)3Al2Si3O12 garnets, (Fe, Mg)Al2O4 and (Fe, Mg)Fe2O4 spinels, and K(Mg, Fe)3AlSi3O10(OH)2 biotites may be satisfactory described, on a macroscopic basis, with symmetric regular solution type parameters having values of 4.86±0.12 (olivine), 3.85±0.09 (garnet), 1.96±0.13 (spinel), and 3.21±0.29 kcals/gfw (biotite). Applications of the proposed solution model demonstrate the sensitivity of petrologic modeling to activity-composition relations of olivine-orthopyroxene solutions. We explore the consequences of estimating the activity of silica in melts forming in the mantle and we develop a graphical geothermometer/geobarometer for metamorphic assemblages of olivine+orthopyroxene+quartz. Quantitative evaluation of these results suggests that accurate and realistic estimates of silica activity in melts derived from mantle source regions,P-T paths of metamorphism and other intensive variables of petrologic interest await further refinements involving the addition of trace elements (Al3+ and Fe3+) to the thermodynamic formulation for orthopyroxenes.  相似文献   

20.
The surface sulfur speciation of chalcopyrite leached by moderately thermophilic Sulfobacillus thermosulfidooxidans was investigated by employing scanning electron microscopy (SEM), X-ray diffraction (XRD) and sulfur K-edge X-ray absorption near edge structure spectroscopy (XANES), accompanying with the leaching behavior elucidation. Leaching experiment showed that there was an optimum range of the redox potential for chalcopyrite dissolution. Leaching products were found accumulating during the leaching process, which might be jarosite according to the XRD analysis. The sulfur K-edge spectra indicated that chalcocite might be the intermediate sulfur compound under a critical redox potential, which might explain the existence of optimum range of the redox potential and provide an evidence for the two-step leaching model of chalcopyrite at low Eh. In addition, the results of sulfur K-edge spectra showed jarosite would accumulate on mineral surface, which might be the main component of the passivation layer.  相似文献   

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