Iron In Silicate Glasses Research Articles

Calibration of Fe XANES for high-precision determination of Fe oxidation state in glasses: Comparison of new and existing results obtained at different synchrotron radiation sources

Micro-X-ray absorption near-edge structure (m-XANES) spectroscopy has been used by several recent studies to determine the oxidation state and coordination of iron in silicate glasses. Here, we present new results from Fe m-XANES analyses on a set of 19 Fe-bearing felsic glasses and 9 basaltic glasses with known, independently determined, iron oxidation state. Some of these glasses were measured previously via Fe XANES (7 rhyolitic, 9 basaltic glasses; Cottrell et al. 2009), while most felsic reference glasses (12) were analyzed for the first time. The main purpose of this study was to understand how small changes in glass composition, especially at the evolved end of silicate melt compositions occurring in nature, may affect a calibration of the Fe m-XANES method.

Absence of pressure-induced electron spin-state transition of iron in silicate glasses upon compression

Upon compression, silicate melts in the Earth’s interior are expected to be subject to successive structural transitions with multiple densification mechanisms that are distinct from those of their crystalline analogs. Experimental verification of this phenomenon remains a major target of glass-melt studies. Early studies of Fe spin-state transitions in silicate glasses under compression used synchrotron X-ray emission spectroscopy (XES) to develop seemingly irreconcilable interpretations that vary from complete transitions to low spin states at high pressure to a prevalence of high spin states. In an effort to reconcile the controversy, Mao et al. (February–March 2014 issue of American Mineralogist ) showed that the XES data must be properly handled with a correct reference spectrum for each spin state and suggested that the subtle effect of pressure-induced broadening in the spectra be considered. Based on the new analyses, they concluded that no pressure-induced spin-state transitions exist in iron-bearing silicate glasses at high pressure, relevant to Earth’s deep lower mantle. Thanks to several series of experimental studies, the elusive spin state in glasses under compression is being revealed, rendering the spin state of iron among the best understood for glasses at high pressure.

Redox Equilibrium of Iron in Silicate Glasses

The effect of some additives on the equilibrium of valence forms of iron is investigated. It is demonstrated that tin and carbon more significantly than fluorine reduce iron. The redox potential of the glass matrix affects the equilibrium between difference valence forms of iron much less than additives correcting spectral characteristics of glass.

Iron in silicate glasses: a systematic analysis of pre-edge, XANES and EXAFS features

A large number (67) of silicate glasses containing variable amounts of iron oxide were studied by high-resolution XANES spectroscopy at the Fe K-edge to determine an accurate method to derive redox information from pre-edge features. The glass compositions studied mimic geological magmas, ranging from basaltic to rhyolitic, dry and hydrous, with variable quench rates. The studied glasses also include more chemically simple calco-sodic silicate glass compositions. The Fe contents range from 30 wt.% to less than 2000 ppm. For most of the series of composition studied, the pre-edge information varies linearly with redox, even under high-resolution conditions. The average coordination of Fe(II) is often similar to its Fe(III) counterpart except in highly polymerized glasses because of the strong influence exerted by the tetrahedral framework on iron's sites. Natural volcanic glasses (from various volcanoes around the world) show similar variations. The average coordination of Fe(II) is often comprised between 4.5 and 5. Fe(III) shows larger variations in coordination (4 to 6, depending on composition). Bond valence models are proposed to predict the average coordination of Fe based on composition. Molecular dynamics simulations (Born–Mayer–Huggins) potentials were carried out on some compositions to estimate the magnitude of disorder effects (both static and thermal) in the XAFS analysis. XANES calculations based on the MD simulations and FEFF 8.2 show large variations in the local structures around Fe. Also, 5-coordinated Fe(III) is found to be an important moiety in ferrisilicate glasses. For Fe(II), discrepancies between glass and melt are larger and are related to its greater structural relaxation at Tg. Also, a strong destructive interference between network formers and modifiers explain the relatively weak intensity of the next-nearest neighbors contributions in the experimental spectra.

The effect of redox state on the local structural environment of iron in silicate glasses: a combined XAFS spectroscopy, molecular dynamics, and bond valence study

A series of 27 silicate glasses of various compositions containing 0.2–2 at.% iron were synthesized at various oxygen fugacity values. The glasses were examined using X-ray absorption fine structure (XANES) spectroscopy at the Fe K-edge in order to determine iron oxidation state and first-neighbor coordination number. Spectral information extracted from the pre-edge region and principal component analysis (PCA) of the XANES region, together with a spectral inversion, were used to derive the end-member spectral components for Fe(II) and Fe(III). Linear trends in the pre-edge features were observed for most compositional series of the glasses examined as a function of Fe(II)/Fe(III) content. These linear trends are believed to be due to the similarity of average coordination numbers for both Fe(II) and Fe(III) end-members in each series. This result is consistent with model simulations of the XANES region and molecular dynamics (MD) simulations for the two end-member compositions which also show that Fe(II) and Fe(III) have similar average coordination numbers. These simulations also suggest the presence of five-coordinated Fe(III) in the melt phase. Based on a bond valence analysis of these MD simulations, a simple model is proposed to help predict the speciation of iron in oxide and silicate glasses and melts.

Oxidation state of iron in silicate glasses and melts: a thermochemical model

The acid and base dissociation constants of FeO and Fe 2O 3 components in silicate melts are defined in terms of observed relationships between atomistic properties of dissolved oxides (nephelauxetic parameters, electronegativity, fractional ionic character of the bond) and polymerization constants in simple systems. These constants are obtained from the Toop–Samis model depicting the Gibbs free energy of mixing of binary MO–SiO 2 melts, which is coupled with the amphoteric treatment of altervalent dissolved oxides. Model parameterization is carried out on the basis of the extended set of data concerning thermodynamic activity of FeO in melts buffered by equilibrium with pure iron metal and a gaseous phase and on the various measurements of bulk redox state of iron in chemically complex melts at various T, fO 2 conditions. Dissociation constants are related to thermodynamic parameters of the main dissolved species (O 2−, Fe 2+, Fe 3+, FeO 2 −) without any significant error progression. As an ancillary result, thermochemical calculations allow to quantify to some extent the systematic errors in the Fe II/Fe III bulk redox ratio arising from the utilization of Mössbauer spectroscopy on quenched melts and glasses.

Redox and clustering of iron in silicate glasses

Soda–lime–silica glasses (70 mol% SiO 2:15 mol% Na 2O:15 mol% CaO) containing 0.1–5 mol% Fe 2O 3 have been investigated using optical absorption spectroscopy, Mössbauer spectroscopy, and wet chemical analysis. Iron redox ratios measured by these different techniques are in agreement. Samples were annealed at 550°C for 1 h in air. As the iron content increases more of the iron ions are in adjacent sites and this manifests itself as changes in the optical and Mössbauer spectra. For example an absorption peak in the visible region which appears with increasing iron content can be assigned to Fe 2+–O 2−–Fe 3+ interactions. Mössbauer parameters indicate that, as Fe 2+ and Fe 3+ ions cluster, their co-ordination becomes more tetrahedral although the iron concentration has no effect on the Fe 2+/ΣFe ratio in the glasses studied. Estimates of the proportion of Fe 3+ ions in adjacent sites have been made using Mössbauer data.

Ferric-ferrous equilibria in Na 2O-FeO-Fe 2O 3-SiO 2 melts: Effects of analytical techniques on derived partial molar volumes

A published comparison (MYSEN et al., 1985a) between Mössbauer spectroscopy and wet chemistry applied to silicate glasses containing large concentrations of total iron (> 14 wt% Fe 2O 3) indicates a large, systematic discrepancy in the determination of FeO between the two techniques, with Mössbauer results typically lower in the ferrous component. This may be accounted for by the common assumption that ferric and ferrous iron in silicate glasses have equivalent Mössbauer absorption efficiencies at room temperature, which has been shown to be invalid for several crystalline materials ( Van Loef, 1966; Grant et al., 1967; Sawatzky et al., 1969; Andersen et al., 1975). Although this assumption has been demonstrated to be valid for glasses with low concentrations of total iron (DYAR et al., 1987), the approximation breaks down when total iron concentrations approach 14 wt% Fe 2O 3. As a consequence, measurements of thermodynamic properties on iron-bearing silicate liquids may take quite different values if Mössbauer spectroscopy rather than wet chemistry is used to determine the ferric and ferrous concentrations in quenched iron-rich glasses. Accordingly, we have measured ferrous iron concentrations in 65 Na 2O-FeO-Fe 2O 3-SiO 2 glasses quenched from melts equilibrated in air between 926 and 1583°C using a colorimetric wet chemical method. These data were used to derive a symmetric, regular solution model for ferric-ferrous equilibria which has a standard error of 0.36 (2δ) in the prediction of wt% FeO. Additional liquids with initial compositions close to the stoichiometry of acmite (NaFe 3+Si 2O 6) but which crossed the perferruginous join due to sodium loss during high-temperature equilibration (1420–1540°C) are significantly reduced relative to other Na 2O-FeO-Fe 2O 3-SiO 2 liquids. A re-interpretation of the density data of Dingwell et al. (1988) based upon these wet chemical ferric-ferrous data demonstrates that the partial molar volume of Fe 2O 3 is independent of composition in the Na 2O-FeO-Fe 2O 3-SiO 2 system with a derived value of 41.78 ± 0.41 cc/ mol at 1400°C. This is equivalent within error to the value presented by Lange and Carmichael (1987) of 42.13 ± 0.28 cc/ mol derived from both 4-component synthetic and 9-component natural melts and thereby indicates that the partial molar volume of Fe 2O 3 is independent of composition over a compositional range relevant to magmas.

Luminescence of radiation-oxidized iron in silicate glasses and vitreous silicas

Luminescence of radiation-oxidized iron in silicate glasses and vitreous silicas

Iron-soda-silica glasses: Preparation, properties, structure

The role of iron in silicate glasses is not yet completely clear. The system appears complicated because the iron ions may occupy both coordination sites O h and T d. By means of optical and EPR spectroscopy and by considering results from other techniques, we find that in our Na 2O·2SiO 2 glasses, these ions behave both as network modifiers and formers, even if Fe 3+ is in the majority in T d sites and Fe 2+ is in the majority in O h sites. When the concentration of Fe 3+ rises we have the formation of Fe 3+ clusters at the expense of Fe 3+ isolated ions. Also the presence of interactions between Fe 2+ and the groupings formed by Fe 3+ is demonstrated. In very reduced samples some iron is present as metal.

Iron in silicate glasses of granitic composition: a M�ssbauer spectroscopic study

57Fe Mossbauer spectra of natural glasses (pumices and obsidians) and of synthetic glasses of granitic composition have been analyzed. — Ferric iron is found in tetrahedral coordination if enough M+-cations are available to balance the charge of both M+Fe3+O2 and M+AlO2 complexes. In other compositions the ratio of tetrahedrally to octahedrally coordinated Fe3+ depends on the ratio of mono-to divalent cations. — Ferrous iron occurs in two distinctly different octahedral sites. The existence of these sites can be attributed to different anionic units adjacent to Fe2+. The degree of polymerization of these units is reflected in the quadrupole splitting. The anionic units adjacent to Fe2+ are depolymerized for increasing mean Z/r2 of the network modifiers, which do not stabilize M3+ in the tetrahedra by local charge balance. — Increasing pressure diminishes the geometric differences between these types of ferrous iron-oxygen-octahedra, which gives rise to a more even distribution of Fe2+ among these sites and thereby to an ordering in the network of melts.

Conductivity, Mössbauer and electron spin resonance of iron in silicate glass

Conductivity, Mössbauer Spectroscopy and Electron Spin Resonance (ESR) techniques are used to study the role of Fe in Calcium Silicate glasses. Results show that, in addition to Fe 2+ and Fe 3+ in 4 and 6-coordinated environments, α-Fe 2O 3 and magnetite-like particles are precipitated as the Fe 2O 3 content is gradually increased from 5 to 30 m/o. These results are confirmed by both Mössbauer and ESR techniques and qualitatively correlated to changes in the D.C. conductivity of the glasses.

Optical absorption due to tetrahedral and octahedral ferric iron in silicate glasses

The optical absorption spectra of commercial soda-lime-silicate glasses is strongly influenced by the presence of ferrous and ferric iron. The absorption spectrum for iron arises from charge transfer and ligand field absorptions due to ferrous and ferric iron in octahedral and/or tetrahedral coordination. The present study was undertaken to use optical absorption spectra (300–2500 nm) to measure the levels of iron and other colorants in commercial glasses. In accord with Mossbauer analyses, the optical data from this work indicates that octahedral, as well as tetrahedral, ferric iron exists in soda-lime-silicate glasses. The observed behavior of the ligand field and charge transfer absorptions suggests that the ratio of octahedral to tetrahedral ferric iron changes with iron redox. Absorption in the ultraviolet had previously been explained by a combination of both ferrous and ferric charge transfer. The present study indicates that charge transfer absorption due to ferrous iron and both octahedral and tetrahedral ferric iron must be taken into account in order to explain the ultraviolet absorption of soda-lime-silicate glasses.