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1.
In a recent paper entitled “Water in minerals detectable by electron energy-loss spectroscopy EELS” by R. Wirth, it has been claimed that OH–- and H2O-bearing minerals exhibit a characteristic peak in the ELNES spectra at about 528 eV prior to the onset of the O K-edge at 532 eV, which could be used for (semi-)quantitative determination of water- or OH-contents on a nanometer scale. It is shown here by parallel electron energy-loss spectroscopy (PEELS) recorded in a transmission electron microscope (TEM) that O K-pre-edge peaks with very high intensities may also exist in water-free compounds and minerals, in particular when they contain transition metals. These spectral features arise from covalent mixing of the metal and oxygen states, which introduces oxygen p character in unoccupied states of mainly metal character. The point is illustrated by the comparison of hematite (α-Fe2O3) and lepidocrocite (γ-FeOOH) O K-edge PEELS spectra which exhibit similar intensities of the pre-edge peak, despite of their grossly different OH– contents. As a consequence, the general validity of the method proposed by Wirth is questioned. Received: 4 December 1997 / Revised, accepted: 22 April 1998  相似文献   

2.
O K- and Ti L23-core-loss spectra of fresnoite Ba2TiSi2O8 (BTS) and Sr2TiSi2O8 (STS), which is isotypic to BTS, have been measured by electron energy-loss spectroscopy (EELS). The energy-loss near-edge structures (ELNES) of the O K edge have been identified on the basis of theoretical simulations and interpretations of the X-ray absorption near-edge structures (XANES), which have been modelled in the framework of self-consistent full multiple-scattering (FMS) theory using FEFF8. Herewith, the K-absorption spectra of oxygen (E) and the local partial electron density of states (DOS) of all atoms have been calculated. For BTS, the observed spectral features in the O K-edge spectra are interpreted in terms of mixing between the central O p and neighbouring Ba 5d and 4f, Si 3p and 3d, and Ti 3d orbitals. The observed differences in the O K-edge spectra for STS and BTS can mainly be attributed to three properties: (1) The lack of high local partial Sr unoccupied DOS with 4f symmetry near the Fermi level compared to the high Ba 4f unoccupied DOS results in differences of overlapping O 2p – cation orbitals. (2) The differences in the ionic radii of Sr and Ba result in a larger unit cell for BTS and, thus, in larger oxygen-cation bonding distances. (3) In comparison to STS, the strength of the incommensurate 2-D structural modulation is significantly weaker in BTS, i.e. distortions of coordination polyhedra occur to a much lesser extent. All these effects alter the oxygen-cation hybridization and, hence, result in a variation of the O 1s p transition and consequently of the O K-edge spectral shape. The observed peak broadening in Ti L23 ELNES of STS compared to BTS is correlated with strong displacive modulations hosted in STS.  相似文献   

3.
The analysis of the extended energy-loss fine structure (EXELFS) of the Si K-edge for sixfold-coordinated Si in synthetic stishovite and fourfold-coordinated Si in natural α-quartz is reported by using electron energy-loss spectroscopy (EELS) in combination with transmission electron microscopy (TEM). The stishovite Si K-edge EXELFS spectra were measured as a time-dependent series to document irradiation-induced amorphization. The amorphization was also investigated through the change in Si K- and O K-edge energy-loss near edge structure (ELNES). For α-quartz, in contrast to stishovite, electron irradiation-induced vitrification, verified by selected area electron diffraction (SAED), produced no detectable changes of the EXELFS. The Si K-edge EXELFS were analysed with the classical extended X-ray absorption fine structure (EXAFS) treatment and compared to ab initio curve-waved multiple-scattering (MS) calculations of EXAFS spectra for stishovite and α-quartz. Highly accurate information on the local atomic environment of the silicon atoms during the irradiation-induced amorphization of stishovite is obtained from the EXELFS structure parameters (Si−O bond distances, coordination numbers and Debye-Waller factors). The mean Si−O bond distance R and mean Si coordination number N changes from R=0.1775 nm and N=6 for stishovite through a disordered intermediate state (R≈0.172 nm and N≈5) to R≈0.167 nm and N≈4.5 for a nearly amorphous state similar to α-quartz (R=0.1609 nm and N=4). During the amorphization process, the Debye-Waller factor (DWF) passes through a maximum value of as it changes from for sixfold to for fourfold coordination of Si. This increase in Debye-Waller factor indicates an increase in mean-square relative displacement (MSRD) between the central silicon atom and its oxygen neighbours that is consistent with the presence of an intermediate structural state with fivefold coordination of Si. The distribution of coordination states can be estimated by modelling the amorphization as a decay process. Using the EXELFS data for amorphization, a new method is developed to derive the relative amounts of Si coordinations in high-pressure minerals with mixed coordination. For the radiation-induced amorphization process of stishovite the formation of a transitory structure with Si largely in fivefold coordination is deduced. Received: 18 December 1996 / Revised, accepted: 20 June 1997  相似文献   

4.
The decrease of the mean Si-O-Si angle in vitreous silica upon densification from 2.20 to 2.36 gcm-3 has been followed by oxygen and silicon K-edge XANES spectroscopy. Multiple scattering calculations using clusters of two shells around the oxygen and silicon atoms, respectively, are in good agreement with experimental absorption spectra and confirm mean Si-O-Si angles between 130 and 144° for these samples, and a decrease of the mean angle with densification. The experimental spectra also exhibit features due to scattering at outer (>2) shells around the photoabsorbers.  相似文献   

5.
Fe L-, S L-, and O K-edge X-ray absorption spectra of natural monoclinic and hexagonal pyrrhotites, Fe1-xS, and arsenopyrite, FeAsS, have been measured and compared with the spectra of minerals oxidized in air and treated in aqueous acidic solutions, as well as with the previous XPS studies. The Fe L-edge X-ray absorption near-edge structure (XANES) of vacuum-cleaved pyrrhotites showed the presence of, aside from high-spin Fe2+, small quantity of Fe3+, which was higher for a monoclinic mineral. The spectra of the essentially metal-depleted surfaces produced by the non-oxidative and oxidative acidic leaching of pyrrhotites exhibit substantially enhanced contributions of Fe3+ and a form of high-spin Fe2+ with the energy of the 3d orbitals increased by 0.3–0.8 eV; low-spin Fe2+ was not confidently distinguished, owing probably to its rapid oxidation. The changes in the S L-edge spectra reflect the emergence of Fe3+ and reduced density of S s–Fe 4s antibonding states. The Fe L-edge XANES of arsenopyrite shows almost unsplit eg band of singlet Fe2+ along with minor contributions attributable to high-spin Fe2+ and Fe3+. Iron retains the low-spin state in the sulphur-excessive layer formed by the oxidative leaching in 0.4 M ferric chloride and ferric sulphate acidic solutions. The S L-edge XANES of arsenopyrite leached in the ferric chloride, but not ferric sulphate, solution has considerably decreased pre-edge maxima, indicating the lesser admixture of S s states to Fe 3d orbitals in the reacted surface layer. The ferric nitrate treatment produces Fe3+ species and sulphur in oxidation state between +2 and +4.  相似文献   

6.
2p (L 2,3) X-ray absorption spectra are presented for a range of minerals to demonstrate the usefulness of L-edge spectroscopy as a symmetry- and valenceselective probe. 2p XAS provides a sensitive fingerprint of the electronic states of 3 d transition metals and can be applied to phases containing mixtures of such elements. Calculated spectra for 3d n → 2p 5 3d n+1 transitions provide a basis for the interpretation of the measured spectra. Thus, in principle, multiple valence states of a particular 3 d metal can be precisely characterized from a single L-edge spectrum. Examples of vanadium L-edge spectra are presented for a range of minerals; these complex spectra hold information concerning the presence of vanadium in multiple valence states. The Cu L-edge spectrum of sulvanite (Cu3 VS4) indicates the presence of both Cu+ and Cu2+; the V L-edge spectrum of the same sample shows that both V2+ and V5+ are present. Spectral simulations representing mixtures of Fe d 5 and Fe d 6 states are used to quantify Fe3+/∑Fe in a spinel, a glass, and an amphibole, all of which contain Fe as a major component. To illustrate the sensitivity of 2p XAS in a dilute system, the Fe L-edge spectrum of amethyst (α-SiO2: Fe) has been recorded; this spectrum shows that ～68% of the Fe in amethyst is Fe2+, and ～32% is Fe3+. Although previous studies on amethyst using other spectroscopic methods cite evidence for Fe4+, there is no indication in the L-edge spectrum for Fe4+ in amethyst. Comparison of theoretical and experimental spectra not only allows the valence states of 3 d ions to be recognised, but also provides site-symmetry information and crystal field parameters for each ion site.  相似文献   

7.
The Fe L 2,3-edge spectra for a range of natural minerals and synthetic solid solutions have been measured using the technique of parallel electron energy-loss spectroscopy (PEELS) recorded in a transmission electron microscope (TEM). The Fe L 2,3 -edges of the minerals are characterised by two white-line features and exhibit electron energy-loss near-edge structure (ELNES) characteristic of Fe valence state. For divalent iron, the Fe L 3 -edge spectra are dominated by a sharp peak (white-line) at ca. 707.8 eV, followed by a broader and less intense peak at ca. 710.5 eV. The ELNES on the Fe L 3 -edge of trivalent iron consists of a white-line with its maximum at ca. 709.5 eV and a preceeding peak at ca. 708.0 eV. Mineral solid solutions that contain both Fe2+ and Fe3+ exhibit an Fe L 3 -edge shape that is composed of Fe L 3 -edges from the respective Fe2+- and Fe3+-bearing end members. The integral Fe L 2,3 -edge white-line intensity ratios I(L 3 )/I(L 2 ) show clear differences for Fe2+ and Fe3+. We demonstrate the feasibility of quantification of the ferrous/ferric ratio in minerals by determining the integral Fe L 2,3 -edge white-line intensity ratios I(L 3 )/I(L 2 ) as a function of the ferric iron concentration resulting in an universal curve within the experimental errors. The application of the universal curve combined with the high spatial resolution using the PEELS/TEM allows the quantification of the ferric iron concentration on a scale down to 10 nm, which is illustrated from a sample of ilmenite containing hematite exsolution lamellae that shows different Fe L 2,3 -edge shapes consistent with variations in the Fe2+-Fe3+ ratio over distances of ca. 100 nm. Received: 30 July 1997 / Revised, accepted: 26 October 1997  相似文献   

8.
Metal K- and L3-, sulfur K- and arsenic K- and L3-edge X-ray absorption near-edge spectra of a series of metal disulfides, FeS2 (both pyrite and marcasite), CoS2, NiS2, and CuS2, and their isomorphs, FeAsS and CoAsS, are presented. The features in this region of these spectra are interpreted using band structure and molecular orbital calculations in terms of the transitions from the 1s or 2p3/2 state to unoccupied states. The 3d transition metal L3-edge spectra of these materials show dependence on the degree of multiplet splitting in the final state, and thus offer less information on the electronic ground state. There are substantial differences in the spectra of the isostructural materials, whereas the spectra of the isotopes pyrite and marcasite show several similarities, illustrating the dependence of near-edge region on electronic structure.  相似文献   

9.
We report strong magnetic linear dichroism (MLD) at the Fe L23 and O K edges of the antiferromagnetic compound hematite -Fe2 O3 in high-resolution orientation- and temperature-dependent electron energy-loss spectroscopy (EELS). Large intensity differences of corresponding spectral features are observed when the Fe L23 and O K edges are measured with momentum transfer either parallel or perpendicular to the magnetization. The resultant difference spectra for the Fe L23 edges is consistent with the MLD observed in X-ray absorption spectroscopy. For the first time we have observed MLD at the O K edge, where the magnetic origin of this dichroism is demonstrated by temperature-dependent investigations across the Morin transition temperature TM= 263 K, at which the Fe electron spins, i.e. the magnetic moments, rotate by 90°. The O K edge MLD is interpreted in terms of superexchange between the spins of the Fe 3d and O 2p electrons through overlapping Fe 3d and O 2p orbitals. The experiments were performed in a transmission electron microscope (TEM), yielding information about the anisotropic electronic structure at nanoscale spatial resolution when operated with a focused electron probe. The effects of MLD at the Fe L23 edges on the determination of Fe3+/Fe in hematite at submicrometre scale using different independent quantification methods are discussed.  相似文献   

10.
X-ray absorption spectroscopy at the Mg K-edge is used to obtain information on magnesium environment in minerals, silicate and alumino-silicate glasses. First-principles XANES calculations are performed for minerals using a plane-wave density functional formalism with core-hole effects treated in a supercell approach. The good agreement obtained between experimental and theoretical spectra provides useful information to interpret the spectral features. With the help of calculation, the position of the first peak of XANES spectra is related to both coordination and polyhedron distortion changes. In alumino-silicate glasses, magnesium is found to be mainly 5-fold coordinated to oxygen whatever the aluminum saturation index value. In silicate glasses, magnesium coordination increases from 4 in Cs-, Rb- and K-bearing glasses to 5 in Na- and Li-bearing glasses but remains equal as the polymerization degree of the glass varies. The variation of the C feature (position and intensity) is strongly related to the alkali type providing information on the medium range order.
Stéphanie RossanoEmail:
  相似文献   

11.
Ti 2p X-ray absorption spectra for a series of minerals have been measured. Crystal field multiplet calculations can explain the spectral shape. The asymmetry of the e g , peak is shown to be related to distortions of the Ti IV octahedron. It is found, theoretically as well as experimentally, that the absorption spectra are more sensitive to tetragonal distortions than to trigonal distortions. A number of silicate minerals and metamict minerals containing titanium are measured and Ti III has not been observed in any of these minerals. A comparison is made to the 1s X-ray absorption, and the potential of both for the study of minerals is discussed.  相似文献   

12.
Si K-, L- and O K-edge ELNES spectra of natural α-quartz and synthetic coesite on one side and synthetic stishovite on the other show characteristic differences that can be related, by comparison with multiple-scattering (MS) calculations, to the fourfold vs. sixfold coordination of Si in these polymorphs of SiO2. It is shown by MS calculations on large clusters that the outer shells contribute relatively little to the overall topologies of the spectra. Therefore, distinction between fourfold- and sixfold-coordinated Si is possible even on a nm scale and probably also in amorphous substances.  相似文献   

13.
Sedimentary biogenic silica from Redeyef in Gafsa basin (southern Tunisia) was analysed for its 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra and complemented by X-ray diffraction and SEM observations. The 29Si MAS NMR spectrum is characterized by the abundance of hydroxylated silicon, displayed in resonance intensities and reflects a clear tendency towards dissolution of diatomaceous amorphous silica and the occurrence of the hydrated silica, which is the main component that ensures the diagenetic transition via the mechanism of dissolution–precipitation to other more crystalline silica phases, after the lost of its hydroxyls groups (water) by heating (burial). 27Al MAS NMR reveals two coordinations of Al; the octahedrally coordinated Al suggests the presence of clay relics trapped during crystal growth or a microcrystalline zeolite (clinoptilolite detected by SEM observations), while the tetrahedrally coordinated Al suggests the presence of minor quantities of minerals with tetrahedral Al, such as an Al-rich fluid and/or minerals such as feldspars.  相似文献   

14.
 A series of titanium silicate glasses along the composition joins SiO2–TiO2, Na2SiO3–TiO2, K2SiO3–TiO2, and CaSiO3–TiO2 have been examined using titanium (Ti) L-edge X-ray absorption near-edge spectroscopy (XANES). XANES spectra were collected at the Canadian Synchrotron Radiation Facility (CSRF), University of Wisconsin, Madison, using the Spherical Grating Monochromator (SGM) beamline. Glass spectra were compared with spectra obtained from crystalline analogues with differing Ti coordination environments: βrBa2TiO4 ([4]Ti); fresnoite ([5]Ti); and rutile and anatase ([6]Ti). The Ti L-edge data indicate that for homogeneous TiO2–SiO2 glasses Ti coordination is [5]Ti at TiO2 contents below 3.6 wt% but predominantly [4]Ti with some [5]Ti present above this content. There is no evidence for [6]Ti. For the Na2 O-containing glasses the L-edge data indicate that Ti is [4]Ti with some [5]Ti at low TiO2 contents, becomes a mix of [4]Ti and [5]Ti with increasing TiO2 content, but is exclusively [5]Ti by 14.3 wt% TiO2. The K2O composition glasses exhibit similar behavior but contain a greater proportion of [4]Ti and less [5]Ti than the equivalent Na2O-bearing glasses. These findings are consistent with previous Raman and XANES pre-edge studies. Alkaline-earth-containing glasses behave somewhat differently, with [5]Ti occurring in low TiO2 content glasses, becoming a mix of [4]Ti and [5]Ti, and then gradually changing to predominantly [4]Ti at higher TiO2 compositions. Finally, we have obtained data for a fresnoite composition (Ba2TiSi2O8) glass. Previous pre-edge and Raman data had suggested that this composition glass contained [5]Ti; however, our Ti L-edge data indicate that Ti is almost exclusively [4]Ti, although some [5]Ti may also be present. Received: 20 December 2000 / Accepted: 10 July 2001  相似文献   

15.
The blue-green color of amazonite has been assigned by various authors to ions Pb+ (6 s)2 (6 p) and/or Pb3+ (6 s) in site of K+ of microcline. Owing to the complex which forms between the ion Pb3+ and the lone pairs of the oxygen atoms surrounding it, the peripheral electron of Pb3+ passes on the levels (6 p) of the latter, which results in a great similarity of the spectra of Pb+ and Pb3+ in amazonite (the transition energies are multiplied by a factor greater than 1), whereas, in the isolated state, these spectra are completely different from one another. An analytical development of the crystal field around a site K+ is established. Under the effect of the crystal field, the transition 2 P 1/2→2 P 3/2 (6 p) is split into two double transitions. The lower transition only falls in the visible domain (1.6–1.8 eV for Pb+), the second in U−V. The green color would arise from the ion Pb+, whereas the blue one would be attributed to the ion Pb3+. Received: 23 January 1997 / Revised, accepted: 10 September 1997  相似文献   

16.
Metal L2,3, sulfur K and oxygen K near-edge X-ray absorption fine structure (NEXAFS) spectra for chalcopyrite, bornite, chalcocite, covellite, pyrrhotite and pyrite have been determined from single-piece natural mineral specimens in order to assess claims that chalcopyrite should be regarded as CuIIFeIIS2 rather than CuIFeIIIS2, and that copper oxide species are the principal initial oxidation products on chalcopyrite and bornite exposed to air. Spectra were obtained using both fluorescence and electron yields to obtain information representative of the bulk as well as the surface. Where appropriate, NEXAFS spectra have been interpreted by comparison with the densities of unfilled states and simulated spectra derived from ab initio calculations using primarily the FEFF8 code and to a lesser extent WIEN2k. Metal 2p and S 2p photoelectron spectra excited by monochromatised Al Kα X-rays were determined for each of the surfaces characterised by NEXAFS spectroscopy. The X-ray excited Cu LMM Auger spectrum was also determined for each copper-containing sulfide. FEFF8 calculations were able to simulate the experimental NEXAFS spectra quite well in most cases. For covellite and chalcocite, it was found that FEFF8 did not provide a good simulation of the Cu L3-edge spectra, but WIEN2k simulations were in close agreement with the experimental spectra. Largely on the basis of these simulations, it was concluded that there was no convincing evidence for chalcopyrite to be represented as CuIIFeIIS2, and no strong argument for some of the Cu in either bornite or covellite to be regarded as Cu(II). The ab initio calculations for chalcopyrite and bornite indicated that the density of Cu d-states immediately above the Fermi level was sufficient to account for the Cu L3-edge absorption spectrum, however these incompletely filled Cu d-states should not be interpreted as indicating some Cu(II) in the sulfide structure. It was also concluded that the X-ray absorption spectra were quite consistent with the initial oxidation products on chalcopyrite and bornite surfaces being iron oxide species, and inconsistent with the concomitant formation of copper-oxygen species.  相似文献   

17.
The determination of the oxidation states of copper and iron in sulfides, and chalcopyrite (CuFeS2) in particular, using 2p X-ray photoemission spectroscopy (XPS) and L2,3-edge X-ray absorption spectroscopy (XAS) is revisited. Reassessment of the published spectra derived by these methods produces consistent results and reveals the ‘d count’ in the copper compounds to be intermediate between d9 and d10. Nevertheless, these covalent copper compounds can be divided into those nominally monovalent and those nominally divalent. The Fe L2,3-edge XAS of chalcopyrite, along with Mössbauer data, confirm the presence of high-spin Fe3+. Chalcopyrite, despite recent published reports to the contrary, clearly belongs to the monovalent copper class.  相似文献   

18.
Synchrotron radiation has been used to collect Cu K-edge and Ag K-edge EXAFS from several tetrahedrite, (Cu,Ag)10(Zn,Fe,Cu)2Sb4S13, minerals. The results have been used to investigate the coordination environment of the Ag and Cu, and to determine which sites in the structure are occupied by silver atoms when they replace copper. The Ag EXAFS spectrum of a sample with high silver content reveals an interaction between silver and antimony which may explain the anomalous decrease in unit cell size found in natural tetrahedrites when the silver content increases beyond four atoms per unit formula.  相似文献   

19.
In recent years, the relevance of physico-chemical heterogeneity patterns in soils at the micron and submicron scale for the regulation of biogeochemical processes has become increasingly evident. For an organic surface soil horizon from a forested Histosol in Germany, microspatial patterns of element distribution (sulfur, phosphorus, aluminium, silicon) and S speciation were investigated by synchrotron-based X-ray spectromicroscopy. Microspatial patterns of S, P, Al and Si contents in the organic topsoil were assessed for a sample region of 50 μm × 30 μm by spatially resolving μ-XRF. Sulfur speciation at four microsites was investigated by focused X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge. The results show a heterogeneous distribution of the investigated elements on the (sub)micron scale, allowing the identification of diatoms, aluminosilicate mineral particles and sulfide minerals in the organic soil matrix. Evaluation of the S K-edge μ-XANES spectra acquired at four different microsites by linear combination fitting revealed a substantial microspatial heterogeneity of S speciation, characterized by the presence of distinct enrichment zones of inorganic sulfide and zones with dominant organic disulfide S within a few micrometers distance, and coexistence of different S species (e.g. reduced inorganic and organic S compounds) at a spatial scale below the resolution of the instrument (60 nm × 60 nm; X-ray penetration depth: 30 μm).  相似文献   

20.
E.A. Hetland  F.T. Wu  J.L Song   《Tectonophysics》2004,386(3-4):157-175
During 1998–1999, we installed a temporary broadband seismic network in the Changbaishan volcanic region, NE China. We estimated crustal structure using teleseismic seismograms collected at the network. We detected a near surface region of strong anisotropy directly under the main volcanic edifice of the volcanic area. We modeled 109 receiver functions from 19 broadband stations using three techniques. First we used a “slant-stacking” method to model the principal crustal P reverberation phases to estimate crustal thickness and the average crustal P to S speed ratio (vp/vs), assuming an average P-wave velocity in the crust. We then estimated crustal S-wave velocity (vs) and vp/vs profiles by modeling stacked receiver functions using a direct search. Finally, we inverted several receiver functions recorded at stations closest to the main volcanic edifice using least squares to estimate vs velocity profiles, assuming a vp/vs value. The results from the three estimation techniques were consistent, and generally we found that the receiver functions constrained estimates of changes in wave speeds better than absolute values. We resolved that the crust is 30–39 km thick under the volcanic region and 28–32 km thick away from the volcanic region, with a midcrust velocity transition at about 10–15 km depth. We estimated that the average crust P-wave velocity is about 6.0–6.2 km/s surrounding the main volcanic region, while it is slightly lower in the vicinity of the main volcanic edifice. The estimates of vp/vs were more ambiguous, but we inferred that the bulk crustal Poisson's ratio (which is related to vp/vs) ranges between 0.20 and 0.30, with a suggestion that the Poisson's ratio is lower under the central volcanic region compared to the surrounding areas. We resolved low S-wave velocities (down to about 3 km/s) in the middle crust in the region of the main volcanic edifice. The low velocity anomaly extends from about 5–10 to 15–25 km below the surface, probably indicating a region of elevated temperatures. We were unable to determine if partial melt is present with the data we considered in this paper.  相似文献   

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