Garry J Foran

XANES calibrations for the oxidation state of iron in a silicate glass

Abstract Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 °C. The Fe3+/ΣFe ratios were determined previously by 57Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe3+/ΣFe ratios from XANES spectra were investigated. The energy of the 1s → 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s → 4s and crest (1s → 4p) transitions. The Fe3+/ΣFe ratios determined from linear combinations of end-member spectra (Fe3+/ΣFe ~0 and ~1) were found to deviate significantly from the Mössbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe2+ or Fe3+ coordination with the Fe3+/ΣFe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe3+/ΣFe values may be obtained from XANES spectra, with an accuracy comparable to Mössbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.

Fingerprinting the water site in mantle olivine

Nominally anhydrous minerals such as olivine contain trace amounts of water and may accommodate the entire water budget of the upper mantle. Here we report for the first time synthetic olivines, crystallized experimentally under upper mantle conditions, that reproduce the most common and intense infrared hydroxyl stretching bands (at 3572 and 3525 cm−1) observed in spinel peridotite mantle olivines. These bands arise from water accommodated at point defects associated with the trace element Ti, and we suggest that this is the most important defect site in the shallow upper mantle. Additional hydrated defects may occur at higher pressures. We also identify bands related to water associated with Fe3+; these are unlikely to reflect equilibrium with the mantle, and indicate water incorporation during exhumation or retrogression. Water must be present at the defect site appropriate for the mantle, at the conditions of interest, for partitioning, seismic wave speed, and deformation experiments on hydrous olivine to be relevant.

The effect of composition on Cr2+/Cr3+ in silicate melts

Abstract Chromium K-edge X-ray absorption near-edge structure (XANES) spectra were recorded at room temperature for 27 CaO-MgO-Al2O3-SiO2 (CMAS) glass compositions quenched from melts equilibrated at various oxygen fugacities (fO₂) at 1400 °C. Values of Cr2+/ΣCr were determined from the intensity of a shoulder on the main absorption edge, attributed to the 1s → 4s transition, which is characteristic of Cr2+ in these glasses. For each composition, Cr2+/ΣCr could be quantified as a function of fO₂, using a theoretical expression, from as few as three samples (Cr2+/ΣCr ≈ 0, 0.5, and 1). This allowed logK’, or the reduction potential of the Cr3+/2+ half-reaction, and hence the relative change in the ratio γmeltCr³⁺O1.5/γmeltCr²⁺O, to be determined for each composition. At constant fO₂, log[Cr2+/Cr3+] was found to decrease linearly with increasing optical basicity. The variation in logK’ with composition is controlled by γmeltCr³⁺O1.5, corresponding to the capacity of the melt to stabilize both the charge and the preferred solvation site of Cr3+. The method was then applied to spectra recorded in situ at 1400 °C for a synthetic mid-ocean ridge basalt (MORB) composition, allowing Cr2+/ΣCr to be quantified in a Fe-bearing melt for the first time. Cr2+/ΣCr was found to vary from ~0.45 at the nickel-nickel oxide (NNO) fO₂ buffer to ~0.90 at iron-wüstite (IW). This indicates that Cr2+ is likely to be the dominant oxidation state in terrestrial basaltic melts.

A furnace design for XANES spectroscopy of silicate melts under controlled oxygen fugacities and temperatures to 1773 K.

A controlled-atmosphere furnace has been constructed for X-ray absorption spectroscopy experiments under imposed oxygen fugacities at temperatures up to 1773 K. The use of the furnace is demonstrated in a study of the oxidation state of Cr in a basaltic silicate melt (mid-ocean ridge basalt) by K-edge XANES spectroscopy. This is the first time the Cr(2+)/Cr(3+) ratio has been identified directly in an Fe-bearing melt. At typical terrestrial oxygen fugacities around half the Cr is present as Cr(2+), even though this oxidation state has never been identified in a terrestrial material and only Cr(3+) is observed after quenching to a glass. Cr(2+) oxidizes to Cr(3+) on cooling in the presence of Fe(3+) according to the electron exchange reaction Cr(2+) + Fe(3+) --> Cr(3+) + Fe(2+). This illustrates the importance of the in situ determination of metal oxidation states in melts.

Characterization of the local structure of amorphous GaAs produced by ion implantation

The first report of the structural parameters of amorphous GaAs produced by ion implantation, as determined with extended x-ray absorption fine structure measurements, is presented herein. Relative to a crystalline sample, the nearest-neighbor bond length and Debye–Waller factor both increased for amorphized material. In contrast, the coordination numbers about both Ga and As atoms in the amorphous phase decreased to ∼3.85 atoms from the crystalline value of four. All structural parameters were independent of both implant temperature and ion dose, the latter extending two orders of magnitude beyond that required for amorphization, and as a consequence, were considered representative of intrinsic, amorphous GaAs as opposed to an implantation-induced extrinsic structure.

Deriving formation constants for aqueous metal complexes from XANES spectra: Zn2+ and Fe2+ chloride complexes in hypersaline solutions

Abstract The development of numerical modeling of reactive transport relies on the availability of thermodynamic properties for the solid, surface, aqueous, and vapor species stable at the conditions of interest. The lack of experimental studies and comprehensive activity-composition models severely limits the predictive capabilities of these models in systems involving highly saline fluids or low-density volatile-rich fluids. X-ray absorption near-edge structure spectroscopy (XANES) is a powerful technique to study the speciation of transition metals in aqueous fluids: it is element specific; sensitive to the oxidation state of the metal, the ligand field and the coordination geometry of the complex; and it is suitable for measuring trace amounts of metals (<1 wt%) in solutions over a wide pressure and temperature range. Formation constants for the aqueous complexes of transition metals can be determined from a series of XANES spectra obtained on solutions containing a constant amount of the metal of interest and variable concentrations of a ligand. The method relies on a non-linear, least-squares-fitting approach, with full distribution of species calculations based on a complete thermodynamic model for the experimental system under consideration. The technique is particularly suitable for following octahedral to tetrahedral transitions among weak chloride complexes of transition metals. The log K for the reaction Zn2+(octahedral) + 4Cl- = ZnCl42-(tetrahedral) at 25 °C is retrieved to be 0.1(6), within error of the accepted literature value. The same method applied to Fe2+-chloride complexes shows that the log K for the reaction Fe2+(octahedral) + 4Cl- = FeCl42-(tetrahedral) increases from -6.2(6) at 25 °C to -2.9(3) at 150 °C. This study confirms that tetrahedral chloride complexes play an important role in Fe transport in hypersaline brines especially at elevated temperatures, and shows that XANES is well suited to study systems that may be difficult to study with other techniques.

Structural perturbations within Ge nanocrystals in silica

Extended x-ray absorption fine structure (EXAFS) spectroscopy was used to identify structural perturbations in Ge nanocrystals produced in silica by ion implantation and annealing. Although the nanocrystals retained tetrahedral coordination, both the short- and medium-range orders were perturbed relative to bulk crystalline material. Equivalently, the nanocrystal interatomic distance distribution deviated from that of bulk crystalline Ge, exhibiting enhanced structural disorder of both Gaussian and non-Gaussian forms in the first, second, and third nearest-neighbor shells. The relative influences of nanocrystal size, bonding distortions, multiple phases, and a matrix-induced compression were considered.

Structural characterization of Cu nanocrystals formed in SiO2 by high-energy ion-beam synthesis

Cu nanocrystals (NCs) were produced by multiple high-energy ion implantations into 5‐μm-thick amorphous silica (SiO2) at liquid-nitrogen temperature. The Cu-rich SiO2 films were subsequently annealed to reduce irradiation-induced damage and promote NC formation. The NC size distribution and structure were studied utilizing a combination of Rutherford backscattering spectroscopy, x-ray diffraction, cross-sectional transmission electron microscopy, and extended x-ray-absorption fine-structure (EXAFS) spectroscopy. We present results derived from all four techniques, focussing on EXAFS measurements to study the local atomic structure surrounding Cu atoms, and comparing NC samples with bulk standards. Using a unique sample preparation method, we drastically improve the signal-to-noise ratio to extract high-quality EXAFS data to enable the determination of a non-Gaussian bond length distribution via the third-order cumulant. We quantify subtle concentration- and annealing-temperature-dependent changes in the C...

Bond length contraction in Au nanocrystals formed by ion implantation into thin SiO2

P. K. is grateful to the Humboldt Foundation in Germany for support. P.K., B.J., A.C., C.J.G., G.d.M.A., G.J.F., and M.C.R. were supported by the Australian Synchrotron Research Program.

An experimental determination of the effect of pressure on the Fe3+/∑Fe ratio of an anhydrous silicate melt to 3.0 GPa

Abstract The effect of pressure on the Fe3+/ΣFe ratio of an anhydrous andesitic melt was determined from 0.4 to 3.0 GPa at 1400 °C with oxygen fugacity controlled internally by the Ru + RuO2 buffer. Values of Fe3+/ΣFe were determined by Mössbauer spectroscopy on quenched glasses with a precision of ±0.01, one standard deviation. This precision was verified independently by XANES spectroscopy of the same samples. The XANES spectra show a systematic increase in energy and decrease in intensity of the 1s → 3d transition with increasing pressure. The results to 2.0 GPa are in good agreement with predictions from density and compressibility measurements fitted to a Murnaghan equation of state, but the datum at 3.0 GPa has higher Fe3+/ΣFe than predicted from the trend established by the lowerpressure data. This might be due to a coordination change in Fe3+ at high pressure; although there is no evidence for this in the Mössbauer spectra, such a change could account for the change in intensity of the 1s → 3d transition in the XANES spectra with pressure.