Gabriele Giuli

Iron local structure in tektites and impact glasses by extended X-ray absorption fine structure and high-resolution X-ray absorption near-edge structure spectroscopy

Abstract The local structure of iron in three tektites has been studied by means of Fe K-edge extended X-ray absorption fine structure (EXAFS) and high-resolution X-ray absorption near-edge structure (XANES) spectroscopy in order to provide quantitative data on distance and Fe coordination number. The samples studied are a moldavite and two australasian tektites. Fe model compounds with known Fe oxidation state and coordination number were used as standards in order to extract structural information from the XANES pre-edge peak. EXAFS-derived grand mean distances and Fe coordination numbers for the three tektite samples are constant within the estimated error ( =2.00 A ± 0.02 A, CN = 4.0 ± 0.4). In contrast to other data from the literature on Fe-bearing silicate glasses, the tektites spectra could not be fitted with a single Fe-O distance, but rather were fit with two independent distances (2 × 1.92 A and 2 × 2.08 A). High-resolution XANES spectra of the three tektites display a pre-edge peak whose intensity is intermediate between those of staurolite and grandidierite, thus suggesting a mean coordination number intermediate between 4 and 5. Combining the EXAFS and XANES data for Fe, we infer the mean coordination number to be close to 4.5. Comparison of the tektites XANES spectra with those of a suite of different impact glasses clearly shows that tektites display a relatively narrow range of Fe oxidation state and coordination numbers, whereas impact glasses data span a much wider range of Fe oxidation states (from divalent to trivalent) and coordination numbers (from tetra-coordinated to esa-coordinated). These data suggest that the tektite production process is very similar for all the known strewn fields, whereas impact glasses can experience a wide variety of different temperature–pressure–oxygen fugacity conditions, leading to different Fe local structure in the resulting glasses. These data could be of aid in discriminating between tektite-like impact glasses and impact glasses sensu strictu.

V oxidation state and coordination number in silicate glasses by XAS

Abstract The local structure of vanadium (V)in synthetic glasses of basaltic composition has been studied by means of high-resolution V K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to obtain quantitative data on distances, V coordination number (CN), and oxidation state. The compositions and experimental conditions were chosen so as to verify the effect of bulk-glass composition (using a diopside-anorthite composition, a sodium disilicate glass, and an iron-titanium-bearing basaltic glass) and V content (from 0.1 to 5 V2O5 wt%) on the structural role of V in these glasses. The combined analysis of high-resolution XANES spectra and EXAFS data indicate that on average, the Fe-free glasses synthesized in air show vanadium in the V5+ state, mainly in tetrahedral coordination (less than 20% [5]V5+) and with distances of 1.697 (± 0.020) Å, in agreement with the values found for tetrahedral V in minerals. In contrast, the Fe-bearing basaltic glasses display a mixture of V5+ in fourfold and fivefoldcoordination, 40% [4]V5+-60% [5]V5+ in proportion, and the EXAFS-derived distances and coordination numbers are in agreement with this interpretation. No significant changes in the V local structure were found in the glasses analyzed as a function of V-contents in the 0.1 to 5 V2O5 wt% range. The data obtained suggest that the structural role of vanadium in these melts is rather insensitive to bulk composition, in terms of V and alkali content, but can be strongly affected by the presence of other transition elements, e.g., Fe3+ competing with V to enter the tetrahedral framework.

REDUCTION AND SORPTION OF CHROMIUM BY Fe(II)-BEARING PHYLLOSILICATES: CHEMICAL TREATMENTS AND X-RAY ABSORPTION SPECTROSCOPY (XAS) STUDIES

The reduction of hexavalent chromium species in aqueous solutions by interaction with Fe(II)-bearing solid surfaces was studied using a 0.96 × l0−3 M Cr(VI) solution and iron-rich clays with different Fe(II)/Fe(III) ratios, layer charge, and exchange properties, i.e., chlorite, corrensite, and montmorillonite. Experimental studies demonstrated that Fe(II)-bearing phyllosilicates reduce aqueous Cr(VI) ions at acidic pH. Chlorite and corrensite, owing to the high Fe(II)/Fe(III) ratio, are electrochemically reactive, as rapid Cr(VI) reduction indicated. In contrast, montmorillonite showed minimum to nil reactivity towards Cr(VI). Furthermore, corrensite, which is high in both Fe(II)/Fe(III) ratio and exchange capacity, adsorbs the greatest amount of chromium.X-ray absorption spectroscopy at Al, Mg, Fe, and Cr K-edges was used to investigate the adsorbed chromium species. The montmorillonite sample, unaffected by treatment with Cr(VI) solution, displays no change at any investigated edge. Edge shape and energy also do not change for the Mg and Al spectra in corrensite, and changes are minor in chlorite. By contrast, the Fe K-edge changes both in chlorite and corrensite, and indicates an increase of Fe(III) in treated samples at the expense of pre-existing Fe(II). Cr K-edge spectra show that chlorite and corrensite sorb Cr(III), which implies its reduction from Cr(VI) in the interacting solution.

XAS determination of the Fe local environment and oxidation state in phonolite glasses

Abstract 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

Effect of aluminum on Ti-coordination in silicate glasses: A XANES study

Abstract The structure of glasses in the K2O-Al2O3-TiO2-SiO2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air from high temperature fusions, represent the addition of Al2O3 to a base of composition K2TiSi4O11 in amounts corresponding to 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/aluminum ratios crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edge spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. Changes in the relative intensities of peaks in the Al K-edge, however, suggest variation in the intertetrahedral (T-O-T) angle. We associate the decrease of this angle for the glasses of peraluminous composition with the presence of triclusters of tetrahedra. The pre-edge peak absorption features in the Ti K-edge XANES spectra indicate that the average Ti coordination decreases with the addition of Al2O3. We infer depletion of fivefold-coordinated titanium (possibly as alkali titanyl complexes), which are dominant in the Al-free glass, by the formation of fourfold coordinated Ti and alkali aluminate complexes (up to a concentration of 40% in the most peraluminous glass). Significant amounts of [V]Ti remain present, even at peraluminous compositions, in further support of tricluster formation as a mechanism for Al incorporation.

Sulfur-metal orbital hybridization in sulfur-bearing compounds studied by X-ray emission spectroscopy.

The electronic structure and ligand environment of sulfur was investigated in various sulfur-containing compounds with different structures and chemical states by using X-ray emission spectroscopy (XES). Calculations were performed using density functional theory (DFT) as implemented in the StoBe code. The sulfur chemical state and atomic environment is discussed in terms of the molecular orbitals and partial charges that are obtained from the calculations. The main spectral features can be modeled using our calculational approach. The sensitivity of the Kbeta emission to the cation and the local symmetry is discussed.

Octahedral versus tetrahedral coordination of Al in synthetic micas determined by XANES

Abstract We used the JUMBO monochromator at SSRL to measure the Al K-edge X-ray absorption spectra of synthetic micas having variable Al content and occupancy, from 0 to 2/4 in the octahedral M positions, and 0 to ⅔ in the tetrahedral T positions. The measured Al K edges differ markedly, but the differences may have a common explanation: (1) Micas containing ⅓ Al in M or ¼ Al in T have K edges that differ in the energy and intensity of the first two features, which are related to interaction of Al with its first-shell nearest neighbors (O and OH or F). They are nearly identical to the K edges of reference minerals such as albite (tetrahedral Al only) or grossular (octahedral Al only). (2) Micas containing Al in both M and T have K edges that can be interpreted as a weighed combination of the simple edges.

Effect of alkalis on the Fe oxidation state and local environment in peralkaline rhyolitic glasses

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 hydrothermal 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 Fe 3+ /(Fe 3+ +Fe 2+ ) 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 Fe 3+ 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 Fe 3+

Al-Fe disorder in synthetic epidotes: A single-crystal X-ray diffraction study

Abstract A single-crystal X-ray diffraction study of synthetic pseuowollastonite (CaSiO3) shows that this crystal has monoclinic C2/c symmetry, with a = 6.8394(5), b = 11.8704(9), c = 19.6313(7) Å, P = 90.667(6)°, and V= 1593.7(2) Å3. Basic features of the C2/c structure are similar to those previously determined with a C1̄ symmetry but fewer sites exist for the monoclinic space group: five symmetrically nonequaivalent Ca sites, three Si sites, and nine O sites.

Experimental and theoretical XANES and EXAFS study of tetra-ferriphlogopite

Experimental and theoretical EXAFS and XANES data on Fe coordination and bond distances have been obtained for a natural tetra-ferriphlogopite. XANES data show a predominance of trivalent iron in tetrahedral coordination. The use of theoretical multiple scattering calculations helps distinguish the contributions to the total spectrum coming from the different iron atoms. EXAFS spectra were reduced and analyzed using the GNXAS programs and Fe has been confirmed to be in tetrahedral coordination, with refined structural data Fe-O = 1.86 A, Fe-Si = 3.24 A, Fe-O-Si = 138°. Moreover, a further contribution from Fe is detected corresponding to a Fe-O distance equal to 2.22 A. This latter contribution (20 mol % of the total Fe ca. ) is interpreted to arise from divalent Fe in octahedral coordination, in excellent agreement with the structural and chemical data.