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Cr K-edge X-ray absorption near-edge structure (XANES) spectra were recorded on Cr:MgO periclase and Cr:(Mg,Fe)O ferropericlase synthesized at different pressures (4 and 12 GPa) and temperatures (1200 to 1400 [degrees]C) at reducing oxygen fugacity conditions (~iron-wustite buffer IW to IW -2), and on Cr:MgSi[O.sub.3] perovskite with 0.5 wt% [Cr.sub.2][O.sub.3]. [sup.57]Fe Mossbauer spectra were collected on the Fe-containing samples. The aim of the study was to determine the Cr oxidation state in phases found in the Earth's lower mantle, and to examine the possible relationship with the Fe oxidation state in the same materials. To calculate the amount of [Cr.sup.2+], the intensity of the shoulder at the low-energy side of the edge crest was quantified using the area of the corresponding peak in the derivative XANES spectra (Berry and O'Neill 2004). In Cr:(Mg,Fe)O the relative [Cr.sup.2+] content reached at most 12.5% but results from Mossbauer spectroscopy combined with chemical composition data suggest that some [Cr.sup.2+] oxidized during cooling through the reaction [Cr.sup.2+] + [Fe.sup.3+] [right arrow] [Cr.sup.3+] + [Fe.sup.2+]. In iron-free Cr-MgO, the [Cr.sup.2+] content is much higher and reaches ~40%. In Cr:MgSi[O.sub.3] perovskite with 0.006 Cr pfu (similar to estimated lower mantle abundance), chromium is mainly divalent. Keywords: XANES, Cr oxidation state, ferropericlase, periclase, Cr:MgSi[O.sub.3] perovskite

We applied different independent techniques (electron microprobe analysis, structure refinement, and X-ray absorption spectroscopy) to unravel the possible mechanisms of Sc incorporation in the pyrope-grossular join. Samples were synthesized at elevated pressure and temperature by adding 5 wt% of [Sc.sub.2][O.sub.3] to selected nominal compositions (pyrope, [pyrope.sub.60][grossular.sub.40], [pyrope.sub.20][grossulars.sub.80], and grossular). In this way, the site of incorporation was not pre-determined, and only depends on the availability of a mechanism for local charge-balance. The EXAFS spectra of the two end-members could be analyzed by a multi-shell fit procedure, whereas the chemical heterogeneity of the Sc-doped solid-solution terms prevented this approach. However, the available information allows detection of different mechanisms of incorporation, which are active as a function of the bulk composition. In pyrope, Sc mainly enters the dodecahedral X site, and the local charge balance is achieved by incorporation of Mg at the adjacent tetrahedral Z site. Local charge-balance requirements suggest that a Z site occupied by Mg bridges two X sites occupied by Sc. When the entrance of Ca provides relaxation of the averaged structure, Sc may enter all the three available cation sites via the coupled heterovalent exchange [sup.X][Sc.sub.1][sup.Z][Sc.sub.1][sup.X][Mg.sub.-1][sup.Z][Si.sub.-1] and the homovalent exchange [sup.Y][Sc.sub.1][sup.Y][Al.sub-1]. In the samples of this work, there is an apparent limit in the Sc incorporation at the Y site, which is in contrast to the favored mechanism of incorporation in Sc-doped andradites. This limit can be explained in terms of relative dimensions of the structural sites when Al is the dominant Y cation. These results must be taken into account when evaluating trace-element behavior in garnets for geochemical purposes. In particular, they explain why [D.sub.Sc] can be treated together with [D.sub.REE] in models based on the elastic strain theory in garnets close to the pyrope composition, but deviate from the parabolic fit in grossular-rich garnets. Keywords: Crystal structure, garnet, trace elements and REE, scandium, XAS, XRD data, crystal synthesis

The redox conditions prevailing during the formation of vanadiferous titanomagnetites from three layered intrusions (Bushveld; Koillismaa; Skaergaard) have been estimated from the valence state of vanadium using synchrotron X-ray absorption near edge structure spectroscopy (XANES). Using a high energy-resolution X-ray emission spectrometer, we show that vanadium occurs mostly as [V.sup.3+], with minor [V.sup.4+]. The most concentrated samples (up to 2.4 wt% [V.sub.2][O.sub.3]) contain approximately 10% of vanadium as [V.sup.4+]. Both [V.sup.3+] and [V.sub.4+] occur in the octahedral site of the spinel structure. Considering the low magnetite/melt [V.sup.4+] partition coefficients, this suggests that vanadium ores crystallized under specific oxidizing conditions. Keywords: Trace elements, XAS (XANES), igneous petrology, new technique

Electron-microprobe analyses (EMPA) and Mossbauer spectra (MS) have been acquired for natural tourmalines sampled near Crummock Water and Haweswater, English Lake District. In the former, the dravite-schorl series is prevalent, whereas in the latter, the dravite species is dominant. All tourmaline samples presently studied are chemically disordered. The compositional and MS data give an insight into the thermal and fluid evolution experienced by the tourmalinite rocks. The tourmalines from Crummock Water crystallized in a closed, low [fo.sub.2] environment, under the influence of a magmatic fluid. The tourmalines from Haweswater are of hydrothermal origin and were formed trader low, variable T and low [fo.sub2] conditions, from a fluid of constant composition.

Infrared (IR) spectra in the OH stretching region and Mossbauer spectra (MS) have been acquired for natural tourmalines from magmatic and hydrothermal deposits in the state of Minas Gerais, Brazil. In the magmatic environment, samples of tourmaline were collected from granitic pegmatites and granitoids. In the former, the elbaite-schorl series is prevalent, while in the latter the schorl species is dominant. Tourmalines from hydrothermal gold deposits are typically intermediate members of the dravite-schorl series, whereas the uvite and Fe-bearing uvite species are present in tourmaline samples from a magnesite deposit. Different behaviors are discussed in terms of local lattice environment, geological history, fluid/rock interaction or magma evolution, and degree of crystallinity.

Iron oxidation state and coordination geometry have been determined by Fe K-edge X-ray absorption near edge spectroscopy (XANES) for three sets of silicate glasses of peralkaline rhyolitic composition with different peralkalinity values. These compositions were chosen to investigate the effect of alkali content (and oxygen fugacity) on the Fe oxidation state. The samples were produced by means of hydro thermal vessels at 800 °C with oxygen fugacity conditions ranging from NNO-1.61 to NNO+2.96 log units. Comparison of the pre-edge peak data with those of Fe model compounds of known oxidation state and coordination number allowed determination of the Fe oxidation state and coordination number in all glasses analyzed. Within each group of samples, Fe tends to oxidize with increasing oxygen fugacity as expected. However, alkali content is shown to have a strong effect on the Fe3+/(Fe3++Fe2+) ratio at constant oxygen fugacity: this ratio varies from 0.25 to 0.55 (±0.05) for the least peralkaline series, and from 0.45 to 0.80 (±0.05) for the most peralkaline series. Moreover, pre-edge peak data clearly indicate that Fe3+ is in fourfold coordination in the most peralkaline glasses. Extrapolation of pre-edge peak data suggests the presence of both fourfold and fivefold coordination for trivalent Fe in the other two series. Divalent Fe is suggested to be mainly in fivefold coordination in all the three glass series. The presence of minor amounts of sixfold- and fourfold-coordinated Fe cannot be ruled out by XANES data alone. XANES data suggest that the amount of alkalis also affects the Fe3+ coordination environment resulting in a decrease in the average coordination numbers. Extended X-ray absorption fine structure (EXAFS) data of the most oxidized and peralkaline sample indicate that Fe3+ is in tetrahedral coordination with = 1.85 Å (±0.02). This value compares well with literature data for [4]Fe3+ in crystalline phases (e.g., in tetra-ferriphlogopite or rodolicoite) or in silicate glasses (e.g., phono lite glasses) supporting the XANES-determined coordination number obtained for the most peralkaline glasses. Calculated NBO/T ratios decrease slightly with Fe oxidation because of the higher fraction of network forming Fe, thus increasing the polymerization of the tetrahedral network. [ABSTRACT FROM AUTHOR]

The Fe oxidation state, coordination geometry, and distances have been determined by Fe K-edge XANES and EXAFS for a set of silicate glasses of phonolite composition produced at different oxygen fugacity conditions with the aim of determining the effect of iron oxidation state and local structural environment on the viscosity of the corresponding melts. Comparison of the pre-edge peak data with those of Fe model compounds with known oxidation state and coordination number allowed for determination of the Fe oxidation state and coordination number for all the glasses analyzed. The Fe3+/(Fe3++Fe2+) ratio varies from 0.44 to 0.93 (±0.05) in the glasses studied. The determined values are in excellent agreement (within 0.03 difference) with those independently measured by the titration method. Moreover, pre-edge peak data clearly indicate that Fe3+ is in fourfold coordination, whereas Fe2+ exists both in fourfold and fivefold coordination for this phonolitic composition, although the presence of minor amounts of sixfold-coordinated Fe cannot be ruled out by XANES data alone. EXAFS data of the most oxidized sample indicate that Fe3+ is in tetrahedral coordination with = 1.85 Å (±0.01). This value compares well with literature data for [4]Fe3+ (e.g., in tetra-ferriphlogopite or rodolicoite). Calculated NBO/T ratios decrease with Fe oxidation (from 0.23 to 0.19). For phonolitic glasses of this study, going from reducing to oxidizing conditions results in a higher fraction of network-forming Fe, thus increasing the polymerization of the tetrahedral network and producing shorter (and stronger) bond distances. Both the polymerization increase and the structural variations in the Fe local environment can qualitatively explain the strong increase in melt viscosity observed at higher oxygen fugacity. [ABSTRACT FROM AUTHOR]

A high-temperature furnace with an induction heater coil has been designed and constructed for in situ X-ray spectroscopic experiments under controlled atmospheric conditions and temperatures up to 3275 K. The multi-purpose chamber design allows working in backscattering and normal fluorescence mode for synchrotron X-ray absorption and emission spectroscopy. The use of the furnace is demonstrated in a study of the in situ formation of Cr oxide between 1823 K and 2023 K at logPO2 values between -10.0 and -11.3 using X-ray absorption near-edge spectroscopy. The set-up is of particular interest for studying liquid metals, alloys and other electrically conductive materials under extreme conditions. [ABSTRACT FROM AUTHOR]

Presents a study of Brazilian tourmalines from different geological environments using infrared and M ö ssbauer spectroscopy. Sampling and analytical techniques employed in the study; Description of tourmaline samples derived from different geological settings; Results of infrared and M ö ssbauer spectroscopy.