Max Wilke

Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study

Abstract High-resolution Fe K-edge XANES spectra of a series of crystalline Fe2+- and Fe3+-bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe2+, including 5-coordinated trigonal bipyramidal Fe2+, and from 4 to 6 O atoms for Fe3+. Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe2+ and Fe3+ is 1.4 ± 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average preedge position for mixed Fe2+-Fe3+ compounds occurring between positions for Fe2+ and Fe3+. The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe2+ and Fe3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs. preedge intensity can be observed, depending on the coordination environment of Fe2+ and Fe3+, with an accuracy in redox determination of ±10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe3+/Fe2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe2+/Fe3+.

Microscopic structure of water at elevated pressures and temperatures

We report on the microscopic structure of water at sub- and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley’s K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6 at 600 °C and p = 134 MPa.

Speciation of Fe in silicate glasses and melts by in-situ XANES spectroscopy

Abstract In-situ X-ray absorption spectroscopy at the Fe K-edge was used to characterize the local structural environment of Fe2+ and Fe3+ in silicate melts at high temperature (up to 1050 °C) in comparison to their quenched glassy analog at room temperature. Measurements were performed on binary alkali-silicate compositions and on haplogranitic compositions, which were doped with about 5 wt% Fe2O3. Changes in the structural environment of Fe were evaluated by analyzing both the pre-edge feature and the first maximum of the EXAFS of the spectra. In most cases, the spectra collected at high temperature differed from those of the quenched samples. At reducing conditions, the melts showed slightly higher amounts of low-coordinated Fe2+ than their glassy counterparts. This finding is consistent with results of earlier studies (e.g., Jackson et al. 1993), but the observed change in speciation is smaller than reported by these authors. At oxidizing conditions, glasses and melts displayed a more heterogeneous behavior. The spectra of alkali-silicate compositions indicate higher amounts of low-coordinated Fe3+ in the melt, whereas no significant difference between melt and glass was observed for Fe3+ in haplogranitic compositions, even if the latter are peralkaline. The amount of non-bridging O atoms in the glass/melt system appears to play an important role particularly for Fe3+. However, more complex relationships between Fe and other structural components, especially Al, are possible.

The origin of S4+ detected in silicate glasses by XANES

Abstract The origin of sulfite (S4+) species in silicate glasses was evaluated using XANES at the S K-edge. Systematic investigations show that the presence of S4+ species in silicate glasses is an analytical artifact related to changes in the sulfur species caused by irradiation with an electron beam during EMPA or by irradiation with an intense focused X-ray beam during synchrotron analysis. The data shown here indicate that S2- and S6+ are the only significant sulfur species occurring in silicate glasses synthesized under geologically relevant conditions.

Determination of the iron oxidation state in Earth materials using XANES pre-edge information

Fe K-edge XANES spectra have been measured in more than 35 Fe(II) and Fe(III)-bearing minerals. The separation between the average pre-edge centroid positions for Fe(II) and Fe(III) is 1.4 +/- 0.1 eV. Examination of calculated pre-edge features of mechanical mixtures of phases containing different proportions of Fe(II) and Fe(III) reveals that different trends of pre-edge position vs. pre-edge intensity can be observed, depending on the coordination environment. Both pre-edge parameters have been used to estimate the ferric/ferrous ratio in 12 natural minerals.

The oxidation state of iron in silicic melt at 500 MPa water pressure

Abstract The dependence of the ferric–ferrous ratio in silicate melts on oxygen fugacity was studied in the system SiO 2 (Qz)–NaAlSi 3 O 8 (Ab)–CaAl 2 Si 2 O 8 (An)–H 2 O using Mossbauer spectroscopy. Experiments were performed under water-saturated conditions at 500 MPa, and at temperatures of 850 and 950 °C, covering a range typical for magmatic processes. The oxygen fugacity was varied in the f O 2 range from Cu–Cu 2 O buffer to slightly more reducing conditions than the wustite–magnetite buffer. The iron redox ratio was determined by analyzing the Mossbauer parameter distribution that was modeled based on experimental spectra collected at room temperature on the quenched samples. The obtained iron redox ratios show a linear dependence on oxygen fugacity on a logarithmic scale for both temperatures. The iron redox ratio (Fe 3+ /Fe 2+ ) decreases with temperature for a given oxygen fugacity. The spectroscopic data at 850 °C are in good agreement with Fe 3+ /Fe 2+ ratios derived from element partitioning but show considerable deviations from iron redox ratios predicted by the empirical equation given by Kress and Carmichael [Contrib. Mineral. Petrol. 108 (1991) 82]. This indicates that an extrapolation of this equation to such low temperatures may have large errors. A sample quenched slowly through the temperature range near and below T g shows considerable differences in the obtained Mossbauer spectra compared to more rapidly cooled samples, indicating ordering of the iron environment at least in the mesoscopic range. The oxidation state, however, does not differ when compared to the more rapidly quenched melts.

Micro-XANES determination of ferric iron and its application in thermobarometry

Micro-X-ray absorption near-edge structure (XANES) analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh-pressure (UHP) eclogites. It is observed that omphacite and phengite contain significant amounts of Fe3+/Fetot (0.2–0.6), whereas only very low contents are present in garnet (Fe3+/Fetot=0.0–0.03), the latter being consistent with results from stoichiometric charge-balance calculations. Furthermore, considerable variations in the Fe3+/Fetot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry (eclogite and calc-silicate matrix) within one thin section. The oxidation state of isofacial minerals is thus likely to depend on the local fluid composition, which, in the studied case, is controlled by calcareous and meta-basic mineral compositions. These first in-situ measurements of ferric iron in an eclogite sample from the Dabie Shan, E China, are used to recalculate geothermobarometric data. Calculations demonstrate that the temperature during UHP metamorphism was as high as 780 °C, about 80–100 °C higher than previously estimated. Temperatures based on charge balance calculations often give erroneous results. Pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond (43.7 kbar at 750 °C). These P–T data result in a geothermal gradient of ca. 6 °C/km during UHP metamorphism in the Dabie Shan. However, accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for Fe3+ and the actual effect on Mg–Fe2+ partitioning is unknown. The present study further shows that micro-XANES is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample: a clear advantage compared to bulk methods.

Fragmentation of foamed silicic melts: an experimental study

We present the first experimental investigation of the fragmentation behavior of two-phase (melt+gas) rhyolitic systems under rapid decompression. Two-phase samples have been generated by foaming water-oversaturated rhyolitic melts up to 900°C and up to 18 MPa prior to rapid decompression in a fragmentation bomb. The fragmented particles or experimental pyroclasts were recovered for analysis. Several features of naturally foamed pumices have been reproduced, including the generation of both isotropic and tube pumices. We focus here on the fragmentation behavior. Fragmentation occurred through a layer-by-layer process, in the brittle regime of melt response. We investigated the influence of the magnitude of the decompression (4 to 18 MPa), the porosity (0 to 85 vol%) and the pore morphology (tube versus isotropic) on the fragment size distribution. Less vesicular samples (porosity 50 vol%) yield coarser fragments when decompressed below 15 MPa and finer fragments when decompressed above 15 MPa. Increasing decompression of the vesicular samples results in a decrease in fragment size of 0.2 Φ unit/MPa. The presence of tubes instead of isotropic pores in vesicular samples generates finer fragments under decompression. Implications for dome eruptions are discussed.

Calibration of zircon as a Raman spectroscopic pressure sensor to high temperatures and application to water-silicate melt systems

Abstract The shifts in wavenumber of the ν3(SiO4) (~1008 cm-1) Raman band of fully crystalline synthetic zircon with changing pressure (P) and temperature (T) were calibrated for application as a Raman spectroscopic pressure sensor in optical cells to about 1000 °C and 10 GPa. The relationship between wavenumber (ν) of this band and T from 22 to 950 °C is described by the equation ν (cm-1) = 7.54·10−9·T3 - 1.61·10−5·T2 - 2.89·10−2·T + 1008.9, where T is given in °C. The pressure dependence is nearly linear over the studied range in P. At ~25 °C, the ∂ν/∂P slope to 6.6 GPa is 5.69 cm-1/GPa, and that to 2 GPa is 5.77 cm-1/GPa. The ∂ν/∂P slope does not significantly change with temperature, as determined from experiments conducted along isotherms up to 700 °C. Therefore, this pressure sensor has the advantage that a constant ∂ν/∂P slope of 5.8 ± 0.1 cm-1/GPa can be applied in experiments to pressures of at least about 6.6 GPa without introducing a significant error. The pressure sensor was tested to determine isochores in experiments with H2O+Na2Si3O7 and H2O+NaAlSi3O8 fluids to 803 °C and 1.65 GPa. These pressures were compared to pressures calculated from the equation of state (EoS) of H2O based on the measured vapor dissolution or ice melting temperature for the same experiment. Pressures determined from the zircon sensor in runs in which NaAlSi3O8 melt dissolved in aqueous fluid were close to or lower than the pressure calculated from the EoS of H2O using the vapor dissolution or ice melting temperature. In experiments with H2O+Na2O+SiO2 fluids, however, the pressure obtained from the Raman spectrum of zircon was often significantly higher than that estimated from the EoS of H2O. This suggests that the pressures along some critical curves of water-silicate melt pseudobinary systems should be revised.

A confocal set-up for micro-XRF and XAFS experiments using diamond-anvil cells

A confocal set-up is presented that improves micro-XRF and XAFS experiments with high-pressure diamond-anvil cells (DACs). In this experiment a probing volume is defined by the focus of the incoming synchrotron radiation beam and that of a polycapillary X-ray half-lens with a very long working distance, which is placed in front of the fluorescence detector. This set-up enhances the quality of the fluorescence and XAFS spectra, and thus the sensitivity for detecting elements at low concentrations. It efficiently suppresses signal from outside the sample chamber, which stems from elastic and inelastic scattering of the incoming beam by the diamond anvils as well as from excitation of fluorescence from the body of the DAC.