Maria Rita Cicconi

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 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+

Dioxygen oxidation Cu(II) → Cu(III) in the copper complex of cyclo(Lys-dHis-βAla-His): a case study by EXAFS and XANES approach.

A former spectroscopic study of Cu(II) coordination by the 13-membered ring cyclic tetrapeptide c(Lys-dHis-βAla-His) (DK13), revealed the presence, at alkaline pH, of a stable peptide/Cu(III) complex formed in solution by atmospheric dioxygen oxidation. To understand the nature of this coordination compound and to investigate the role of the His residues in the Cu(III) species formation, Cu K-edge XANES, and EXAFS spectra have been collected for DK13 and two other 13-membered cyclo-peptides: the diastereoisomer c(Lys-His-βAla-His) (LK13), and c(Gly-βAla-Gly-Lys) (GK13), devoid of His residues. Comparison of pre-edge peak features with those of Cu model compounds, allowed us to get information on copper oxidation state in two of the three peptides, DK13 and GK13: DK13 contains only Cu(III) ions in the experimental conditions, while GK13 binds only with Cu(II). For LK13/Cu complex, EXAFS spectrum suggested and UV-vis analysis confirmed the presence of a mixture of Cu(II) and Cu(III) coordinated species. Theoretical XANES spectra have been calculated by means of the MXAN code. The good agreement between theoretical and experimental XANES data collected for DK13, suggests that the refined structure, at least in the first coordination shell around Cu, is a good approximation of the DK13/Cu(III) coordination species present at strongly alkaline pH. All the data are consistent with a slightly distorted pyramidal CuN(4) unit, coming from the peptide bonds. Surprisingly, the His side-chains seemed not involved in the final, stable, Cu(III) scaffold.

The effect of the [Na/(Na+K)] ratio on Fe speciation in phonolitic glasses

Abstract Natural iron-bearing sodic phonolitic melts represent an extreme compositional range of the effect of the [Na/(Na+K)] ratio on the geochemical behavior of Fe in volcanic systems. Yet phonolitic melts have not been well investigated. The glasses studied here have been synthesized from liquids equilibrated over a range of oxygen fugacity conditions [log10(fO₂) from -0.68 to -11] to elucidate the role of the alkali ratio in influencing the local environment around both divalent and trivalent Fe. In this study, the Fe K-edge XAS spectra (XANES and EXAFS) have been employed, to constrain the Fe structural role (oxidation state, coordination number, bond distances) in phonolitic glasses as a function of synthesis temperature (T), [Na/(Na+K)] ratio (= 0.0, 0.25, 0.5, 0.75, 1.0) and redox state. We verify that at constant oxygen fugacity, the [Na/(Na+K)] ratio has a strong effect on the Fe3+/ (Fe2++Fe3+) ratio. The results obtained are parameterized and discussed in terms of the contrasting effects of T, fO₂, and alkali ratio.

The [4]Fe3+–O distance in synthetic kimzeyite garnet, Ca3Zr2[Fe2SiO12]

A synthetic kimzeyite analogue (Ca 3 Zr 2 [Fe 2 SiO 12 ]) has been analysed by powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) in order to determine the [4] Fe 3+ –O distance, for which only few data are available. The XRD-determined cell parameter ( a 0 = 12.625 ± 0.001 A) is consistent with those found in the literature for synthetic samples of similar composition. Also interatomic distances ( = 2.515 A, = 2.093 A, = 1.774 A, as determined by Rietveld refinement) are in keeping with structural data of natural samples when taking into account chemical differences of the samples examined. Due to the large Fe content of the tetrahedral site, the distance is unusually long compared with those of garnets where tetrahedra are occupied solely by Si, it is instead consistent with structural data for natural kimzeyite. The XANES data indicate the presence of trivalent Fe in tetrahedral coordination. The EXAFS-derived distance (1.85 ± 0.01 A) is in agreement with the few literature data available for [4] Fe 3+ , i.e . tetra-ferriphlogopite ( = 1.86 ± 0.01 A), rodolicoite ( = 1.825 A) and silicate glasses (Fe–O = 1.85 ± 0.01 A and 1.84 ± 0.02 for phonolitic and rhyolitic glasses, respectively). In view of the large size difference between FeO 4 and SiO 4 tetrahedra a further Rietveld structural refinement was performed assuming a splitting of the oxygen position, resulting in two distinct oxygen sites (O A and O B with fractional oxygen occupancies of 2/3 and 1/3) at 1.845 and 1.606 A distance, respectively, from the tetrahedral cation. Although there are still open questions on the distribution of FeO 4 and SiO 4 tetrahedra and on how the structure accommodates the size difference of these two tetrahedra, this study provides a direct determination of the [4] Fe 3+ –O distance for which only few data are available in the literature.

Competition between two redox states in silicate melts: An in-situ experiment at the Fe K-edge and Eu L3-edge

Abstract The understanding of redox equilibria as well as the knowledge of the elemental distribution in magmatic melts are of fundamental importance to constrain the genesis of magmas. In particular, the partitioning of trace elements (e.g., Eu) has demonstrated to be a useful tool for estimating the redox conditions in Earth and planetary materials. However, for a more complete comprehension of Eu in silicate melts, information regarding the effects of temperature (T), redox conditions, compositions, and the possible interference of other multivalent elements is still lacking. Here we provide new data on the oxidation states of two commonly coexistent multivalent elements (Eu and Fe) in melts, acquired by “in situ” dispersive X-ray absorption spectroscopy experiments at high temperatures and at different oxygen fugacity conditions. This work, for the first time, shows the possibility to monitor in real-time the behavior and valence variations of two elements under varying environmental conditions (like T and redox state).

North American microtektites are more oxidized than tektites

Abstract Iron oxidation states and coordination numbers have been determined by micro-X‑ray absorption near edge spectroscopy (XANES) on the cores of a large group of microtektites from the Australasian, Ivory Coast, and North American (NA) tektite strewn field. The North American microtektites used in this study have been collected from five sites at different distances from the source crater; most have SiO2 content between 70 and 80 wt%. Accurate analysis of the pre-edge peak energy position and integrated area allowed determination of Fe3+/(Fe2++Fe3+) ratios on all samples with an estimated error of ±0.05. Microtektites from the Australasian and Ivory Coast strewn fields show low values of the Fe3+/(Fe2++Fe3+) ratios, in fair agreement with tektites from the same strewn field. In contrast, microtektites from the North American strewn fields show a wide range of Fe3+/(Fe2++Fe3+) ratios from 0.02 to ca. 0.61. Comparison of Fe oxidation state data with chemical composition do not show any relation between Fe3+/(Fe2++Fe3+) ratios and Na, Ca, or K contents, thus suggesting that the high-Fe oxidation states are not the consequence of sea-water alteration. The difference between the Fe oxidation state of tektites and microtektites from the North American strewn fields suggests that some factors in the formation of the North American microtektites were different than for the North American tektites and for microtektites in the other strewn fields. Previous Fe oxidation state data on NA tektites strongly suggest that the wide range in Fe oxidation state we found on NA microtektites is not related to lateral heterogeneity of the target rocks. Despite a correlation between microtektite oxidation state and distance from the source crater, we maintain that Fe oxidation state is not related only to the microtektite droplet flight distance. This is in keeping with the fact that no significant variations in the Fe oxidation state have been found in microtektites from the Australasian strewn field, even for Australasian microtektites recovered in Antarctica. The Fe oxidation state in North American microtektites could be explained by interaction of melt droplets with a H2O-rich vapor plumes generated during the impact. These data point out that some difference must exist between the thermal histories of microtektites and tektites from the NA strewn field. Moreover, microtektites from the NA strewn field show also distinctively higher oxidation states than those from Ivory Coast or the Australasian strewn fields.

Europium structural role in silicate glasses: Reduction kinetics at low oxygen fugacity

This study is focused on the determination of the geochemical behaviour of europium (Eu) in a set of synthetic silicate glasses with composition relevant for the Earth Science and ranging from basaltic to granitic composition. The samples have been characterized through Eu LIII-edge by X-ray Absorption Spectroscopy (XAS) and the measurements have been performed in fluorescence mode at the ESRF (Grenoble, F). Eu LIII-edge XANES analysis allowed to obtain a semi-quantitative assessment of the Eu2+/(Eu2+ + Eu3+) redox ratio. Kinetics of europium reduction at low oxygen fugacity (IW buffer) has been studied on samples equilibrated at different times. Data obtained from kinetic experiments clearly show that glasses of basaltic composition reach equilibrium values of the Eu2+/(Eu2+ + Eu3+) ratio after 6 h at 1400 ° C, whereas glasses of granitic composition reach equilibrium after 60 h at 1400 ° C. Knowledge of Eu reduction kinetics is an absolute prerequisite for any study of Eu oxidation state at low oxygen fugacity.