A Fe K-edge XAS study of amethyst

Chemical Geology, 2001

High-resolution X-ray Absorption Near-Edge Structure XANES spectra of iron allow to take into account the effects of the coordination numbers on the quantification of redox values. Volcanic glasses show split pre-edge features, arising from a bimodal distribution between the relative contributions of ferric and ferrous iron. The chemical shift between these two oxidation states, 2 eV, has been resolved using a 400 Si monochromator. High-resolution pre-edge spectroscopy shows the distribution of ferric and ferrous iron between various coordination states. Ferrous iron is mostly fivefold-coordinated and minority fourfold-coordinated while ferric iron occurs in fourfold and sixfold-coordinated sites. The importance of 6 Fe 3q in basaltic glasses may explain the formation of magnetite during glass oxidation. The increase of 4 Fe 3q in the more silicic, pantelleritic glass is consistent with the peralkaline character of this glass. The increase of the proportion of tetrahedral Fe 3q , accompanied by more covalent Fe 3q-O bonds, is consistent with the chemical dependence of redox equilibria in magmatic systems, in which the most differentiated terms correspond to more oxidizing compositions.

Spectroscopic methods in mineralogy

The number of mineral species known to date rapidly approaches 4000, and yet they represent but a small fraction of all the known inorganic and organic compounds. Nevertheless, minerals represent an ideal field of activity for X-ray absorption spectroscopy (XAS), because the investigation of their crystal-chemical peculiarities takes an enormous advantage of the property of this method of being atom-selective, even in the presence of a wide range of competing atoms located in similar structural environments. As a matter of fact, XAS on minerals proved to be a useful probing method as early as for W. Kossel's pioneer studies of in the 1930's, just after the fine structures occurring at and near the absorption edge had been first detected. However, XAS did not really become consolidated in mineral studies until the 1980's, when synchrotron sources became available to users. A concise, but complete review of the historical and recent applications of XAS to minerals and to their analogues synthesized for geological/geophysical purposes i.e., to better understand the mechanisms by which the Earth evolves, is here given. Special reference will be made to transition metals (Ca, Ti, Cr, Mn, Fe, Ni) which absorb in the hard X-ray spectral region (> 4 KeV) and to the geologicallysignificant elements (O, Na, Mg, Al, Si, S and K) which absorb in the soft X-ray region (500-4000 eV).

Geochimica et Cosmochimica Acta, 2011

Ferrihydrite (Fh) is a short-range ordered nanocrystalline iron(III) (oxyhydr)oxide that has been recognized to play an important role in contaminant sequestration and in iron cycling in geological and biological systems. Despite intensive research for the two last decades, the structure of Fh is still a subject of debate. In the present study, we report extended X-ray absorption fine structure (EXAFS)

Physics and Chemistry of Minerals, 2010

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 Cu I in the mineral. It was concluded that the best formal oxidation state representation for cubanite is Cu I (Fe 2 ) V S 3 -II .

Geochimica Et Cosmochimica Acta, 1994

Initial results are reported on a new microprobe technique for determining ratios in Fe-bearing minerals. The technique is based on the energy shift of a pre-edge peak in X-ray absorption near-edge structure (XANES) spectra obtained using the synchrotron X-ray fluorescence microprobe. A linear relationship between pre-edge peak energy and iron oxidation state was observed for oxide and silicate standards. Reasonable iron oxidation state results were obtained for pallasitic olivine, acmite, altered magnetites and synthetic wüstite. Direct measurements of in coexisting phases is feasible using this technique.

Physics and Chemistry of Minerals, 1994

Sulfur K-edge x-ray absorption spectra (XANES and EXAFS) and Ledge XANES of sphalerite (ZnS), chalcopyrite (CuFeS2) and stannite (Cu2FeSnS4) have been recorded using synchrotron radiation. The K-and Ledge 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 Ledge XANES features (up to 15 eV above the edge), combined with published metal K-edge XANES. The S K-and Ledge XANES also indicate that, for sphalerite, the Fe 2 § 3 d band at the fundamental gap has little or no bonding hybridization with S 3p and S 3s orbitals; for chalcopyrite, the Cu § 3d and Fe 3+ 3d bands have strong mixing with S 3p and S 3s states, while for stannite the Cu § 3 d band strongly hybridizes with S 3 p and S 3 s orbitals, but the Fe 2 § 3 d 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 Ledge 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/~, 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.

Journal of Non-Crystalline Solids, 2011

a b s t r a c t X-ray absorption near edge structure spectroscopy has been used at the O K and Fe L 2,3 edges to investigate the electronic and atomic structure of (Fe 2 O 3 ) x (Na 2 O) 0.30 (SiO 2 ) 0.70 − x (x b 0.2) obtained by melt-quench technique. The Fe L 2,3 edge spectra show that the Fe atoms are in octahedral coordination. The O K edge spectra reveal no change in coordination of Fe and increased degree of hybridization between O 2p and Fe 3d orbitals with iron doping. It is estimated that about 10% Fe 2+ and 90% Fe 3+ are in these glasses by peak fitting analysis of Fe L 3 edge. The ligand-field splitting of Fe 3d orbital is about 1.6 eV.

Journal of the American Chemical Society, 2004

Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe(4)S(4)](1+,2+,3+) clusters. The results are quantitatively and qualitatively compared with DFT calculations. The change in covalency upon redox in both the [Fe(4)S(4)](1+/2+) (ferredoxin) and the [Fe(4)S(4)](2+/3+) (HiPIP) couple are much larger than that expected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higher than that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculations in terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronic relaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIP redox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple is metal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributions of these RAMO differences to ET processes ...

The Canadian Mineralogist, 2002

Ferriallanite-(Ce), ideally CaCeFe 3+ AlFe 2+ (SiO 4)(Si 2 O 7)O(OH), is the analogue of allanite-(Ce) with Fe 3+ dominant in the octahedral site M(1). It is of metasomatic origin and occurs in an alkaline granitic pegmatite of Mount Ulyn Khuren in the Altai Range (about 48°32'N, 92°55'E), Mongolian People's Republic. Ferriallanite-(Ce) is black, opaque or translucent, with a brown streak; the luster is resinous. The Mohs hardness is 6; it is brittle, without observed cleavage or parting. The fracture is conchoidal to uneven. Its density is 4.22 g/cm 3. It is biaxial (-), ␣ 1.825(2), ␤ 1.855(5), ␥ 1.880(5), 2V calc 83°. The strongest five peaks in the X-ray powder-diffraction pattern [d(Å)(I)(hkl)] are: 2.93(65)(113), 2.72(80)(120,013), 2.63(60)(311), 2.18(100)(122,123,401), 2.14(80)(221,403,223,014). The crystal structure has been refined to R = 0.026 using 2296 observed reflections collected with a four-circle X-ray diffractometer [a 8.962(2), b 5.836(2), c 10.182(2) Å, ␤ 115.02(1)°, space group P2 1 /m]. The position of the hydrogen atom has been found. On the basis of the chemical data, structure refinement, Mössbauer spectroscopy and chargedistribution calculations, the cations have been assigned to the independent crystallographic sites and the following crystal chemical formula has been obtained: (Ca 0.97 Ce 0.03)(Ce 0.89 Ca 0.11)(Fe 3+ 0.80 Ti 4+ 0.14 Al 0.06)(Al 0.56 Fe 3+ 0.44) (Fe 2+ 0.93 Mn 0.07)(Si 0.94 Al 0.06 O 4)(Si 2 O 7)O(OH). In the same sample, a single-point electron-microprobe analysis reveals an ideal composition CaCeFe 3+ 2 Fe 2+ (SiO 4)(Si 2 O 7)O(OH), but not enough material is available for a full characterization of this potentially new epidotegroup mineral.

2006

The electronic structures of the Heusler type compounds Fe 3-x V x Si in the concentration range between x = 0 and x = 1 have been probed by photoemission spectroscopy (PES). The observed shift of Si 2p corelevel and the main valence band structres indicate a chemical potential shift to higher energy with increasing x. It is also clarified that the density of state at Fermi edge is owing to the collaboration of V 3d and Fe 3d derived states. Besides the decrease of the spectral intensity near Fermi edge with increasing x suggests the formation of pseudo gap at large x.