Andrew J. Berry

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.

Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle

Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle or hydrous melting at temperatures only modestly greater than those found today. Here we determine the original Fe3+/ΣFe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe, preserved as melt inclusions in olivine, to be 0.10 ± 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe3+/ΣFe value, together with a water content of only 0.2–0.3 wt% (ref. 7), excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe3+/ΣFe ratio would be ∼1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (∼1,700 °C), rather than melting under hydrous conditions (3–5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.

A Mössbauer study of the oxidation state of Fe in silicate melts

Abstract Fe3+/ΣFe ratios were determined from Mössbauer spectra recorded for a series of 17 anorthitediopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities from fO₂ ~ 105 bars (Fe3+/ΣFe = 1) to 10-13 bars (Fe3+/ΣFe = 0) at 1682 K. Fe3+/Fe2+ was found to be proportional to fO₂ to the power of 0.245 ± 0.004, in excellent agreement with the theoretical value of 0.25 expected from the stoichiometry of the reaction Fe2+O + 0.25 O2 = Fe3+O1.5. The uncertainty in the Fe3+/ΣFe ratios determined by Mössbauer spectroscopy was estimated as ± 0.01 (1σ) from the fit of the data to the theoretical expression, which is significantly less than that quoted for previous measurements on silicate glasses; this results from fitting the spectra of a large number of systematically varying samples, which allows many of the ambiguities associated with the fitting procedure to be minimized. Fe3+/ΣFe ratios were then determined for samples of the anorthitediopside eutectic composition equilibrated at selected values of fO₂, to which up to 30 wt% Fe2O3 had been added. Fe3+/ΣFe was found to vary with ΣFe (or FeOT), but both the 1 wt% and high FeOT data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe3+/Fe2+ ∝fO₂1/4) by the inclusion of a Margules term describing Fe2+-Fe3+ interactions. The large negative value of this term indicates a tendency toward the formation of Fe2+-Fe3+ complexes in the melt. The resulting expression, using the ideal exponent of 0.25, gave a fit to 289 Fe3+/ΣFe values, compiled from various literature sources, of similar quality as previous empirical models which found an exponent of ~0.20. Although the empirical models reproduce Fe3+/ΣFe values of glasses with high FeOT reasonably well, they describe the data for 1 wt% FeOT poorly. The non-ideal values of the exponent describing the dependence of Fe3+/ΣFe on fO₂ at high FeOT are an artifact of models that did not include a term explicitly to describe the Fe2+-Fe3+ interactions. An alternative model in which Fe in the silicate melt is described in terms of three species, Fe2+O, Fe3+O1.5, and the non-integral valence species Fe2.6+O1.3, was also tested with promising results. However, at present there is no model that fits the data within the assessed accuracy of the experimental measurements.

A XANES determination of the oxidation state of chromium in silicate glasses

Abstract Cr K-edge X-ray absorption near-edge structure (XANES) spectra were recorded for silicate glasses of various compositions in the system CaO + MgO + Al2O3 + SiO2 ± TiO2, quenched from melts equilibrated as a function of oxygen fugacity (fO₂) at 1400 °C. The spectra vary systematically with fO2 (logfO₂ ∼ 0 to -16) indicating changes in the Cr oxidation state. The intensity of a shoulder on the absorption edge (attributed to the 1s → 4s transition) was quantified using the corresponding peak in the XANES derivative spectrum and used to determine Cr2+/ΣCr. The resulting Cr2+/ΣCr values are in agreement with the theoretical dependence on logfO₂ suggesting that the 1s → 4s feature is diagnostic of Cr2+ in a silicate glass and σ(Cr2+/ΣCr) = ± 0.015. The Cr2+/ΣCr ratio for a given logfO₂ changes with the glass composition which may reflect the ability of the melt to accommodate the Jahn-Teller distorted coordination geometry which stabilizes Cr2+. Cr2+/ΣCr varies between ∼ 0.3 and 0.8 over the logfO₂ range bounded by the Ni/NiO and Fe/FeO fO₂ buffers, suggesting that Cr2+ may be important in natural melts even though this oxidation state has never been identified in a terrestrial material. The development of a methodology for determining Cr2+/ΣCr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature

The kinetics and mechanism of MgO dissolution

Abstract Reactions and atomic rearrangements at fluid–crystal interfaces play an important role in catalysis and in controlling the kinetics and mechanisms of dissolution. We have studied the attachment and reactions of water molecules at the MgO–water interface by combining measurements of 1 H and 2 D surface penetration and etch pit morphology with ab initio calculations. These studies show that the most common MgO cleavage surface, (001), is thermodynamically unstable when hydrated. Proton rearrangement on such surfaces precedes proton–cation exchange and provides a general mechanism for the detachment of ions during dissolution. The kinetics of dissolution are strongly influenced by the concentration of surface defects and a simple model based on the ab initio results predicts a dissolution rate of 10−10 mol cm−2 s−1 for a typical surface defect concentration of 0.1.

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.

Motional broadening: an important distinction between multiple-quantum and satellite-transition MAS NMR of quadrupolar nuclei

Abstract Multiple-quantum (MQ) and satellite-transition (ST) magic-angle spinning (MAS) are two very similar techniques used to obtain high-resolution or ‘isotropic’ NMR spectra of half-integer quadrupolar nuclei. In a variety of materials it is observed that some STMAS peaks are very broad compared with the corresponding MQMAS peaks, sometimes so broad that they are unobservable. We present 17 O ( I =5/2) NMR spectra of two materials, chondrodite (2Mg 2 SiO 4 · Mg(OH) 2 ) and clinohumite (4Mg 2 SiO 4 ·Mg(OH) 2 ), exhibiting this phenomenon and show that the cause is motional broadening arising from the combined effects of molecular reorientation, the quadrupolar interaction and MAS.

A XANES study of Cu speciation in high-temperature brines using synthetic fluid inclusions

Abstract Cu K-edge X-ray absorption near edge structure (XANES) spectra were recorded from individual synthetic brine fluid inclusions as a function of temperature up to 500 °C. The inclusions serve as sample cells for high-temperature spectroscopic studies of aqueous Cu-Cl speciation. Cu+ and Cu2+ can both be identified from characteristic pre-edge features. Mixed oxidation states can be deconvoluted using linear combinations of Cu+ and Cu2+ spectra. This work illustrates how complex Cu XANES spectra can be interpreted successfully. Cu2+ is the stable oxidation state in solution at room temperature and Cu+ at high temperatures. The change in oxidation state with temperature was completely reversible. Cu+ was found to occur exclusively as the linear species [CuCl2]- in solutions containing KCl with Cu:Cl ratios up to 1:6. In the absence of K+, there is evidence for higher order coordination of Cu+, in particular the tetrahedral complex [CuCl4]3-. The importance of such complexes in natural ore-forming fluids is yet to be determined, but may explain the vapor-phase partitioning of Cu as a Cl complex from a Cl-rich brine.

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.