Fe-bearing Glasses Research Articles

The unique speciation of iron in calc-alkaline obsidians

Obsidians found in silicic domes and rhyolitic tuffs are an ideal system to understand the formation conditions of calc-alkaline felsic magmas. The Optical Absorption and Electron Paramagnetic Resonance (EPR) spectroscopic data presented in this study demonstrate that Fe-speciation in obsidians is different from that observed in other geological glasses (e.g., basaltic glasses) or in synthetic Fe-bearing silicate glasses. The presence of unusual symmetric Fe2+ octahedral sites explains the characteristic black to brown color of obsidian glasses, much distinct from that of synthetic Fe-bearing glasses. The Fe3+ sites partly occur in superparamagnetic domains, as evidenced by EPR data. The dramatic intensification of the optical absorption in the visible range indicates the presence of Fe-rich domains with Fe2+-Fe3+ and/or Fe2+-Ti4+ Inter-Valence Charge Transfer (IVCT) processes. These data are consistent with the presence of Fe-rich, mixed valence clusters, located outside the glass structure. The largest of these domains contribute to the black color of obsidians because they cause a background absorption extending throughout the visible spectrum as observed in magnetite. This unusual Fe-speciation reflects the last events of the magmatic history during emplacement and cooling, indicating that obsidians cannot be considered as a quenched witness of the initial magma.

Fe-oxidation state in alkali-trisilicate glasses - A Raman spectroscopic study

This study focuses on analysis of local structural arrangements of ferric and ferrous iron in binary alkali-silicate networks, and the effect of changing Fe2+/Fe3+ ratio on the glass structure, investigated by Raman spectroscopy. Three alkali trisilicate glasses, Li2Si3O7, Na2Si3O7 and K2Si3O7, were synthesized with 4.4–6.0wt% iron oxide, to form three series with the same nominal composition but changing Fe2+/Fe3+ ratio. Structural analyses of the high-wavenumber envelope in Raman spectra of Fe-free alkali-trisilicate analogues have been useful in identifying structural species in the Fe-bearing glasses. Peak fitting of the high-wavenumber region between ca. 820 and 1200cm−1 indicates that the increase in ferric iron content is associated with a considerable increase in the Q2 species while the abundance of Q4- and Q3-decreases. However, the most prominent change with the increase in the ferric content is the rise of the peak centred at ca. 980cm−1 that we identify as related to the vibration of Fe3+-O-Si linkage, regardless of the coordination geometry around ferric cation. We have observed a roughly linear trend in the change of the Fe3+-related Raman peak area with the increase in the ferric iron content for all three alkali trisilicate series, with increasing slope from K to Li. Nonetheless, there are discrete differences in the dependence due to composition of the glass. Based on the results of this study, the obtained linear calibration trend can be used only for an approximate determination and not as a routine quantification of ferric content.

Valence and metal/silicate partitioning of Mo: Implications for conditions of Earth accretion and core formation

To better understand and predict the partition coefficient of Mo at the conditions of the deep interior of Earth and other terrestrial planets or bodies, we have undertaken new measurements of the valence and partitioning of Mo. X-ray absorption near edge structure (XANES) K-edge spectra for Mo have been measured in a series of Fe-bearing glasses produced at 1 bar and higher PT conditions. High pressure experiments have been carried out up to 19 GPa in order to better understand the effect of pressure on Mo partitioning. And, finally, a series of experiments at very low fO2 conditions and high Si content metallic liquids has been carried out to constrain the effect of Si on the partitioning of Mo between metallic liquids and silicate melt. The valence measurements demonstrate that Mo undergoes a transition from 4+ to 6+ near IW−1, in general agreement with previous 1 bar studies on FeO-free silicate melts. High pressure experiments demonstrate a modest pressure dependence of D(Mo) metal/silicate and, combined with previous results, show a significant decrease with pressure that must be quantified in any predictive expression. Finally, the effect of dissolved Si in Fe-rich metallic liquid is to decrease D(Mo) significantly, as suggested by previous work in metallurgical systems. The effect of pressure, temperature, oxygen fugacity, metallic liquid composition, and silicate melt composition can be quantified by using multiple linear regression of available experimental data for Mo. Our XANES results show that Mo will be 4+ at conditions of core formation, so only experiments carried out at fO2 of IW−1 and lower were used in the regressions. Application of predictive expressions to Earth accretion shows that D(Mo) decreases to values consistent with an equilibrium scenario for early Earth core–mantle. The Mo content of the primitive upper mantle (PUM) can be attained by metal–silicate equilibrium involving S-, C-, and Si-bearing metallic liquid, and peridotite silicate melt along the peridotite liquidus near 45 GPa and 3600 °C, late in the accretion process. This conclusion is insensitive to late giant impacts unless the degree of equilibration is very low (<5%).

Near-infrared spectra of ferrous mineral mixtures and methods for their identification in planetary surface spectra

Iron-bearing minerals are a major component of planetary surfaces, and many can be identified by their characteristic absorption bands in the near-infrared (NIR). Here we present laboratory NIR spectra of a wide range of common Fe-bearing minerals (e.g., olivines, pyroxenes), glasses, and mineral/glass mixtures. We then use this suite of spectra to evaluate the effects of mixtures on mineral detection methods, including olivine and pyroxene spectral indices developed for the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard Mars Reconnaissance Orbiter. We find that although these indices can be compromised by minerals with atypical compositions, mineral mixtures, and the presence of other ferrous minerals, these issues can generally be mitigated by visual inspection of the spectra. However, a special case occurs when the mineral or mixture in question is spectrally indistinguishable from a more common mineral. In particular, we show that spectra of high-calcium pyroxene mixed with Fe-bearing glass can be virtually indistinguishable from common Fe-bearing olivine compositions. This effect, combined with the fact that Fe-bearing glass is generally much more difficult to detect than other ferrous minerals, may be causing glass occurrences on planetary surfaces to be underreported. In support of this hypothesis, we use Mars Express OMEGA observations to show that previous olivine detections in the north polar sand sea on Mars are actually more consistent with local mixing of glass and pyroxene. To address these issues, we present an alternative ferrous mineral identification method based on the position and shape of the 1 and 2μm iron absorption bands, which are sensitive to mineralogy, composition, and mineral mixtures in planetary surface spectra, including glass and mixtures with glass. Using Chandrayaan-1 Moon Mineralogy Mapper (M3) observations of Aristarchus Crater on the Moon, we show that these band parameters can reveal subtle spectral variations and can produce mineralogical maps at an exceptional level of detail.

Valles Marineris dune fields as compared with other martian populations: Diversity of dune compositions, morphologies, and thermophysical properties

Planetary dune field properties and their bulk bedform morphologies relate to regional wind patterns, sediment supply, climate, and topography. On Mars, major occurrences of spatially contiguous low-albedo sand dunes are primarily found in three major topographic settings: impact craters, high-latitude basins, and linear troughs or valleys, the largest being the Valles Marineris (VM) rift system. As one of the primary present day martian sediment sinks, VM holds nearly a third of the non-polar dune area on Mars. Moreover, VM differs from other regions due to its unusual geologic, topographic, and atmospheric setting. Herein, we test the overarching hypothesis that VM dune fields are compositionally, morphologically, and thermophysically distinct from other low- and mid-latitude (50°N–50°S latitude) dune fields. Topographic measurements of dune fields and their underlying terrains indicate slopes, roughnesses, and reliefs to be notably greater for those in VM. Variable VM dune morphologies are shown with topographically-related duneforms (climbing, falling, and echo dunes) located among spur-and-gully wall, landslide, and chaotic terrains, contrasting most martian dunes found in more topographically benign locations (e.g., craters, basins). VM dune fields superposed on Late Amazonian landslides are constrained to have formed and/or migrated over >10s of kilometers in the last 50My to 1Gy. Diversity of detected dune sand compositions, including unaltered ultramafic minerals and glasses (e.g., high and low-calcium pyroxene, olivine, Fe-bearing glass), and alteration products (hydrated sulfates, weathered Fe-bearing glass), is more pronounced in VM. Observations show heterogeneous sand compositions exist at the regional-, basinal-, dune field-, and dune-scales. Although not substantially greater than elsewhere, unambiguous evidence for recent dune activity in VM is indicated from pairs of high-resolution images that include: dune deflation, dune migration, slip face modification (e.g., alcoves), and ripple modification or migration, at varying scales (10s–100sm2). We conclude that VM dune fields are qualitatively and quantitatively distinct from other low- and mid-latitude dune fields, most readily attributable to the rift’s unusual setting. Moreover, results imply dune field properties and aeolian processes on Mars can be largely influenced by regional environment, which may have their own distinctive set of boundary conditions, rather than a globally homogenous collection of aeolian sediment and bedforms.

The dissolution mechanism of sulphur in hydrous silicate melts. II: Solubility and speciation of sulphur in hydrous silicate melts as a function of fO2

Raman and X-ray absorption spectroscopy (XANES) measurements on a series of experimentally synthesised, sulphur (S)-bearing, hydrous silicate glasses were used to determine the S-speciation and S-oxidation state as a function of glass composition and oxygen fugacity (fO2) and to decipher the dissolution mechanism of S in silicate melts. Synthesised glasses include soda-lime (SLG), K2Si4O9 (KSG), albite and trondhjemite (TROND) compositions. A series of SLG and KSG glasses, doped with small quantities of Fe, was also studied in order to determine the effect of Fe/S on the S solubility. The experiments were performed in internally heated (IHPV) and cold seal (CSPV) pressure vessels at 200MPa, 1000 and 850°C and a range of fO2 from log fO2=QFM−2.35 to QFM+4 (QFM is quartz–fayalite–magnetite oxygen buffer).The systematic correlation of features in Raman and XANES spectra allows the identification of at least four different S-species in the glasses depending on fO2 and Fe/S of the system. In XANES spectra of Fe-free glasses SH−, H2S and SO42− are visible as peaks at 2466, 2471.8 and 2482eV, respectively. In Raman spectra peaks at 2574 and 990cm−1 indicate the presence of HS bonds and SO42−, respectively, but SH− and H2S can not be distinguished using a Raman spectroscopy. In Fe-bearing glasses FeS bonding is identified at 2469eV in the case of XANES and at 298, 372 and 420cm−1 in the case of Raman spectra. The intensities of peaks related to SH bonding systematically decrease and the intensities of peaks related to FeS bonding systematically increase with increasing Fe/S in both the XANES and the Raman spectra indicating that in the presence of Fe, FeS bonding is preferred over SH bonding. The total S solubility at sulphur saturation in the Fe-free melts is a function of the degree of melt polymerisation and it increases with increasing NBO/T (from 0.03 to 1.91wt.% S). The S2− species are more soluble than the S6+ species in contrast to previously studied Fe-bearing “natural” compositions.The change from S2− to S6+ is observed at log fO2=QFM−1 to QFM+1 which is ~1.5 log unit lower than the range of fO2 previously reported for Fe-rich compositions indicating that Fe influences not only the speciation but also the oxidation state of S in silicate melts at given redox conditions. The natural implications are that S6+ in Fe-poor magmas can be stable at lower fO2 than previously predicted and, hence, S6+ may act as an oxidising agent in the mantle wedge by successively oxidising Fe2+ to Fe3+ via the reaction H2SO4+9FeO=FeS+4Fe2O3+H2O. For the silicate melt generated in the mantle wedge and containing about 10wt.% total FeO, the change in the Fe3+/ΣFe ratio from 0.1 to 0.2 will correspond to an increase in the log fO2 from QFM−0.5 to QFM+1.5 and will require only 1000–3000ppm S extracted from subducted slab.

The determination of sulfate and sulfide species in hydrous silicate glasses using Raman spectroscopy

Raman spectroscopy is used to identify the sulfur speciation (sulfur valence state) in “technical” iron-free soda-lime and potassium-silicate glasses and in “natural” glass compositions such as basalt, andesite, and rhyodacite. The presence of sulfate (S6+) is marked in Raman spectra of oxidized synthetic and natural glasses by bands at ~990 and ~1000 cm-1, respectively. The presence of sulfide (S2-) in the reduced technical glasses is marked in Raman spectra by a band at 2574 cm-1 indicating that S2- is present as HS- in these Fe-free glasses. Such a band is absent in the Raman spectra of reduced basaltic, andesitic, and rhyodacitic glasses. However, an additional band at ~400 cm-1 appears in the Raman spectra of the reduced S-bearing “natural” glasses when compared to that of S-free reduced “natural” reference glasses indicating that S2- is most likely complexed with Fe in these glasses. Thus, the dissolution mechanism of S2- appears to be different in Fe-free and Fe-bearing glasses and S2- is dissolved as either HS--species or Fe-S complexes, respectively. The data shown here demonstrate the potential of Raman spectroscopy in identifying the sulfur valence state in silicate glasses. In addition, S2- is dissolved as completely different complexes when comparing “technical” iron-free and “natural” iron-bearing, hydrous silicate glass compositions.

DIRECT LABORATORY ANALYSIS OF SILICATE STARDUST FROM RED GIANT STARS

We performed combined focused ion beam/transmission electron microscopy studies to investigate the chemistry and structure of eight presolar silicate grains that were previously detected by NanoSIMS oxygen isotope mapping of the carbonaceous chondrite Acfer 094. The analyzed presolar silicates belong to the O isotope Groups I/II (17O-enriched and 18O-depleted) and therefore come from 1–2.5 M☉ asymptotic giant branch stars of close-to-solar or slightly lower-than-solar metallicity. Three grains are amorphous, Mg-rich, and show a variable, but more pyroxene-like composition. Most probably, these grains have formed under circumstellar low-temperature conditions below the crystallization temperature. Three grains are Fe-bearing glasses similar to the "glass with embedded metal and sulfides" (GEMS) grains found in interplanetary dust particles. However, two of the meteorite GEMS grains from this study lack comparatively large (≳20 nm) Fe-rich inclusions and have sulfur contents <1 at.%, which is different than observed for the majority of GEMS grains. These grains likely condensed under strong non-equilibrium conditions from an Si-enriched gas. One olivine is characterized by a crystalline core and an amorphous, more Fe-rich rim, which is probably the result of interstellar medium sputtering combined with Mg removal. The detection of another olivine with a relatively high Fe content (Mg# 0.9) shows that circumstellar crystalline silicates are more Fe-rich than astrophysical models usually suggest. The overall predominance of olivine among the crystalline silicate stardust population compared to pyroxene indicates preferential formation or survival of this type of mineral. As pyroxene is indeed detected in circumstellar outflows, it remains to be seen how this result is compatible with astrophysical observations and experimental data.

Entropy and structure of oxidized and reduced iron-bearing silicate glasses

The influence of ferrous and ferric iron on the low-temperature heat capacity and vibrational entropy of silicate glasses has been determined by adiabatic calorimetry. Two pairs of samples based on sodium disilicate and calcium Tschermak molecule compositions have been studied. Along with previous data for another Fe-bearing glass, these results have been used to complement the available set of composition independent partial molar relative entropies of oxides in silicate glasses with S 298 − S 0 values of 56.7 and 116 J/mol for FeO and Fe 2O 3, respectively. The calorimetric data indicate that the fraction of fivefold coordinated Al is significant in the CaO–“FeO”–Al 2O 3–SiO 2 system and that association of Ca 2+ and Na + with Fe 3+ in tetrahedral coordination for charge compensation does not entail significant changes in coordination for these two cations. At very low temperatures, however, the heat capacity is no longer an additive function of composition because of unexpectedly high positive deviations from Debye laws. These anomalies are stronger for the reduced than the oxidized glasses and considerably larger than for iron-free glasses, but their origin cannot be established from the present measurements.

Nature and distribution of iron sites in a sodium silicate glass investigated by neutron diffraction and EPSR simulation

Abstract The short and medium range structure of a NaFeSi2O6 (NFS) glass has been investigated by high-resolution neutron diffraction with Fe isotopic substitution, combined with Empirical Potential Structure Refinement (EPSR) simulations. The majority (∼60%) of Fe is 4-coordinated ([4]Fe) and corresponds only to ferric iron, Fe3+, with a distance d [ 4 ] Fe 3 + - O = 1.87 ± 0.01 A. This is at variance with the 3D-structure predicted by glass stoichiometry. The existence of a majority of [4]Fe3+ sites illustrates a glass structure that differs from the structure of crystalline NaFeSi2O6, which contains only octahedral Fe3+. The EPSR modeling of glass structure shows that [4]Fe3+ is randomly distributed in the silicate network and shares corner with silicate tetrahedra. The network-forming behavior of [4]Fe3+, coupled with the presence of Na+ ions acting as charge-compensators, is at the origin of peculiar physical properties of Fe-bearing glasses, such as the increase of the elastic modulus of sodium silicate glasses with increasing Fe-concentration. Our data provide also direct evidence for 5-coordinated Fe, with an average distance d [ 5 ] Fe - O = 2.01 ± 0.01 A. This second Fe population concerns both Fe2+ and Fe3+. 5-Coordinated Fe atoms tend to segregate by sharing mainly edges. The direct structural evidence of the dual role of ferric iron in NFS glass provides support for understanding the peculiar properties of NFS glass, such as magnetic, optical, electronic or thermodynamic properties.

XANES studies of Fe-bearing glasses.

XANES studies of Fe-bearing glasses.

The Diversity of Newly Discovered Deterioration Patterns in Ancient Egyptian Pigments: Consequences to Entirely New Restoration Strategies and to the Egyptological Colour Symbolism

ABSTRACTPolished cross sections through more than 1000 pigment samples from ancient Egyptian monuments (5th Dynasty to Roman Times) were investigated in reflected light, with SEM, and with electron microprobe techniques (EMPA). These combined analytical techniques allow many insights into the painting layers and down to the substrate. Textural relations of both the primary paintings and of the deterioration products can then be delineated. They also discern the stages of the various deterioration processes and the degree of damage of the painting layers.The textural patterns of six deterioration processes not known before are discussed in detail and are documented in BSE-micrographs. Five of these processes commence in synthetic Cu-pigments (Egyptian Blue, glass pigment, and Green Frit) and Fe-bearing glass pigments. The 6th process leads to the formation of complex phosphates and is mainly exogenous. It causes severe damage to natural and synthetic pigment layers. These processes result in destruction of the primary pigments and coloured decorations.Consequences as to the colour symbolism and future treatment of befallen paintings are discussed.

Near-infrared spectroscopy of probable impact melt from three large lunar highland craters

The composition of small areas within three large lunar highland craters are compared using near-infrared, Earth-based telescopic data. Spectra from many areas in the walls and central peaks of the craters indicate the presence of crystalline components which include olivine, two types of pyroxene, and sometimes Fe-bearing feldspar. Spectra from other regions suspected of being impact melt deposits, located on the floors and walls of the three craters, have remarkably similar, yet anomalous features. These features are interpreted to indicate the presence of pyroxene, Fe-bearing glass, and Fe-bearing feldspar. Pyroxene and feldspar are believed to occur primarily in the form of lithic clasts and rapidly recrystallized impact melt. The Fe-bearing glass is interpreted as impact melt glass.