Article

A furnace design for XANES spectroscopy of silicate melts under controlled oxygen fugacities and temperatures to 1773 K

July 2003
Journal of Synchrotron Radiation 10(Pt 4):332-6

Source
PubMed

Authors:

A controlled-atmosphere furnace has been constructed for X-ray absorption spectroscopy experiments under imposed oxygen fugacities at temperatures up to 1773 K. The use of the furnace is demonstrated in a study of the oxidation state of Cr in a basaltic silicate melt (mid-ocean ridge basalt) by K-edge XANES spectroscopy. This is the first time the Cr(2+)/Cr(3+) ratio has been identified directly in an Fe-bearing melt. At typical terrestrial oxygen fugacities around half the Cr is present as Cr(2+), even though this oxidation state has never been identified in a terrestrial material and only Cr(3+) is observed after quenching to a glass. Cr(2+) oxidizes to Cr(3+) on cooling in the presence of Fe(3+) according to the electron exchange reaction Cr(2+) + Fe(3+) --> Cr(3+) + Fe(2+). This illustrates the importance of the in situ determination of metal oxidation states in melts.

The effects of temperature and pressure on the oxidation state of chromium in silicate melts

Article

Full-text available

Apr 2021
CONTRIB MINERAL PETR

The oxidation state of Cr, Cr2+/ΣCr (where ΣCr = Cr2+ + Cr3+ = 0.35 wt%), in Fe-free silicate glasses quenched from melts equilibrated as a function of pressure to 3.5 GPa at 1500 °C, and as a function of temperature to 1500 °C at atmospheric pressure, and at oxygen fugacities (fO2, in log units relative to the quartz-fayalite-magnetite (QFM) buffer) between ∆QFM = − 1 and − 2, was determined by XANES spectroscopy. Increasing temperature stabilises Cr2+ and increasing pressure stabilises Cr3+. A general expression for Cr3+/Cr2+ in silicate melts was derived: log(Cr3+/Cr2+) = 1/4(∆QFM + 8.58 − 25,050/T + 940P/T − 0.02P) + 9770/T − 7.69 + 6.22Λ + (900P − 172P2)/T, where P is pressure in GPa, T is temperature in K, and Λ is the optical basicity of the composition. This equation reproduces 213 Cr2+/ΣCr values reported here and in the literature with an average ΔCr2+/ΣCr of 0.02. A MORB melt at 1400 °C and QFM is predicted to have Cr2+/ΣCr ~ 0.35 at the surface but Cr2+/ΣCr ~ 0 at a depth of ~ 60 km. Although Cr2+ is an important oxidation state in silicate melts it is not preserved at low temperatures due to an electron exchange reaction with Fe3+.

X-ray Absorption Fine Structure (XAFS) Studies of Oxide Glasses—A 45-Year Overview

Article

Full-text available

Jan 2018

X-ray Absorption Fine Structure (XAFS) spectroscopy has been widely used to characterize the short-range order of glassy materials since the theoretical basis was established 45 years ago. Soon after the technique became accessible, mainly due to the existence of Synchrotron laboratories, a wide range of glassy materials was characterized. Silicate glasses have been the most studied because they are easy to prepare, they have commercial value and are similar to natural glasses, but borate, germanate, phosphate, tellurite and other less frequent oxide glasses have also been studied. In this manuscript, we review reported advances in the structural characterization of oxide-based glasses using this technique. A focus is on structural characterization of transition metal ions, especially Ti, Fe, and Ni, and their role in different properties of synthetic oxide-based glasses, as well as their important function in the formation of natural glasses and magmas, and in nucleation and crystallization. We also give some examples of XAFS applications for structural characterization of glasses submitted to high pressure, glasses used to store radioactive waste and medieval glasses. This updated, comprehensive review will likely serve as a useful guide to clarify the details of the short-range structure of oxide glasses.

The oxidation state of europium in silicate melts as a function of oxygen fugacity, composition and temperature

Article

Jul 2015
CHEM GEOL

Europium LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses and melts equilibrated over a range of oxygen fugacities (fO2s, from -14 to + 6 logarithmic units relative to the quartz-fayalite-magnetite, QFM, buffer) and temperatures (1250 – 1500 °C). Eu3 +/ΣEu (where ΣEu = Eu2 + + Eu3 +) values were determined from the spectra with a precision of ± 0.015. Eu3 +/ΣEu varies systematically with fO2 from 0 to 1 over the range studied, increases with decreasing melt polymerisation and temperature, and can be described by the empirical equation: Eu3 +/ΣEu = , where Λ is the optical basicity of the melt and T is the temperature in K. Eu3 +/ΣEu in glasses and melts equilibrated at the same conditions are in excellent agreement for Fe-free systems. For Fe-bearing compositions the reaction Eu2 + + Fe3 + = Eu3 + + Fe2 + occurs during quenching to a glass and the high temperature value of Eu3 +/ΣEu is not preserved on cooling; in situ measurements are essential for determining Eu3 +/ΣEu in natural melts.

3. X-ray Absorption Near-Edge Structure (XANES) Spectroscopy

Chapter

Dec 2014

Mineral‐Melt Partitioning of Redox‐Sensitive Elements

Chapter

Sep 2021

Elements with variable valence state (i.e., redox‐sensitive) often show contrasting mineral/melt partition coefficients as a function of oxygen fugacity ( f O 2 ) in magmatic systems. This is because trace‐element incorporation into crystal lattices depends on the charge, size, and crystal‐field stabilization energy of atoms, all of which differ greatly between oxidized and reduced species of the same element. This has two critical implications: (i) petrologic/geochemical modeling of partitioning behavior of redox‐sensitive trace‐elements in magmatic systems requires some knowledge of their oxidation state; and (ii) the oxidation state of magmatic systems may be inferred from partitioning relations of redox‐sensitive trace elements preserved in mineral and melt phases of rapidly cooled magmas. The advantage of this oxybarometric approach is that mineral/melt partitioning relations are not sensitive to late‐stage degassing, charge‐transfer on quenching, or surficial alteration. In this chapter we discuss the theoretical treatment of experimental mineral/melt partitioning data of redox‐sensitive trace elements, and review aspects concerning the partitioning behavior of well‐known redox‐sensitive elements, including transition metals (Ti, V, Cr, Fe), rare earth elements (Ce, Eu), U, and siderophile elements (Mo, W, Re, and platinum group elements) under planetary magmatic f O 2 conditions.

Trace-element geochemistry of diamond-hosted olivine inclusions from the Akwatia Mine, West African Craton: implications for diamond paragenesis and geothermobarometry

Article

Full-text available

Nov 2019
CONTRIB MINERAL PETR

Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al < 2.2), they are derived from a harzburgitic mantle source, and syngenetic garnets are without exception subcalcic G10 garnets. For harzburgitic olivines that cannot be identified this way, Na and Ca contents can be used: olivine inclusions with < 60 µg/g Na and Na/Al < 0.7 are all harzburgitic, whereas those with > 300 µg/g Ca or > 60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m² conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt = 1115–1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P–T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the refertilisation processes that have affected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting first. Trivalent cations appear to behave differently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na–M³⁺ exchange. In contrast, Al partitioning in harzburgites is not significantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al–Al exchange.

The oxidation state of iron in Mid-Ocean Ridge Basaltic (MORB) glasses: Implications for their petrogenesis and oxygen fugacities

Article

Full-text available

Dec 2018
EARTH PLANET SC LETT

The oxidation state of a basaltic liquid may be obtained from the average valence state of Fe in its quenched glass. Suitable glasses are widely available from the basalts erupted at Mid-Ocean Ridges (MORB). Measurements of Fe³⁺/∑Fe, where ∑Fe=Fe²⁺+Fe³⁺, by XANES spectroscopy of a globally representative sample of these MORB glasses are in good agreement with the most recent literature values determined by wet chemistry, but have improved precision, with an expected standard deviation of ±0.01 in Fe³⁺/∑Fe for each independent measurement. These precise data allow the geochemical controls on MORB redox systematics at both global and local scales to be evaluated. At the global scale, the relationship between log[Fe2O3] and [MgO] shows that Fe³⁺ behaves like a lightly incompatible element (LICE) during the crustal evolution of MORB, with its incompatibility between those of the redox-insensitive elements Li and In. The variability of Fe³⁺ about this global trend is also consistent with that of redox-insensitive elements of similar incompatibility, implying no external buffering of oxygen fugacity fO2. The variability of Fe³⁺ is anti-correlated with the variabilities of Sr and Na, which are compatible in plagioclase, but is positively correlated with the variabilities of elements that partition preferentially into clinopyroxene and/or olivine rather than plagioclase, showing that its variability is controlled at least partly by varying ratios of plagioclase to clinopyroxene and olivine during the crustal evolution of MORB. Deviations of Fe³⁺ concentrations from the global trend are, like those of other LICE, anticorrelated with the deviations from their global trends of the very incompatible elements (VICE). Extrapolation of the global trend to an assumed parental melt at 10.4 wt% [MgO] gives [Fe2O3]o, the concentration of Fe2O3 in the global average MORB parental melt, of 0.6 wt%, which is consistent with 15–20% partial melting of a spinel lherzolite source with 0.2 to 0.3 wt% Fe2O3. This estimate of source Fe2O3 agrees with the estimate for fertile upper mantle lherzolite deduced from measurements of Fe³⁺/∑Fe and total Fe in the minerals of spinel peridotite xenoliths from the lithosphere. The fO2 of the MORB glasses at 1 bar may be calculated from their measured Fe³⁺/∑Fe using a new parameterization of 478 experimental data from the literature with <60 wt% SiO2. The need for the new parameterization arises from recent experimental studies, which, among other aspects, are consistent with the ideal stoichiometry governing the thermodynamic relationship between Fe³⁺/Fe²⁺ and fO2, namely Fe³⁺/Fe²⁺ ∝ (fO2)0.25. The parameterization gives: log10⁡(Fe³⁺/Fe²⁺)=0.25ΔQFM−1.35+0.034[Na2O]+0.044[K2O]+0.023[CaO]−0.18[P2O5] where ΔQFM is the difference between the fO2 of the silicate melt and the quartz–fayalite–magnetite buffer in log10 units, logfO2(QFM) =8.58−25050/T, relative to the conventional standard state of pure O2 at 10⁵ Pa, T is in K, and [Na2O] etc., are the concentrations of the oxide components in weight percent. The global average ΔQFM recorded by MORB glasses in the compositional range of 5 to 10 wt% MgO is +0.2 ±0.3, with a small but resolvable increase with decreasing MgO. There is no evidence that Fe³⁺/∑Fe systematics in MORB are influenced by interactions with other polyvalent elements like S or Cr.

A XANES Determination of the Oxidation State of Chromium in Silicate Glasses

Article

May 2004

Cr K-edge X-ray absorption near-edge structure (XANES) spectra were recorded for silicate glasses of various compositions in the system CaO + MgO + Al2O3 + SiO2 ± TiO2, quenched from melts equilibrated as a function of oxygen fugacity (fo2) at 1400 °C. The spectra vary systematically with fo2 (logfo2 ∼0 to -16) indicating changes in the Cr oxidation state. The intensity of a shoulder on the absorption edge (attributed to the 1s → 4s transition) was quantified using the corresponding peak in the XANES derivative spectrum and used to determine Cr2+/ΣCr. The resulting Cr2+/ΣCr values are in agreement with the theoretical dependence on logfo2 suggesting that the 1s → 4s feature is diagnostic of Cr2+ in a silicate glass and σ(Cr2+/ΣCr) = ± 0.015. The Cr2+/ΣCr ratio for a given logfo2 changes with the glass composition which may reflect the ability of the melt to accommodate the Jahn-Teller distorted coordination geometry which stabilizes Cr2+·Cr2+/ΣCr varies between ∼ 0.3 and 0.8 over the logfo2 range bounded by the Ni/NiO and Fe/FeO fo2 buffers, suggesting that Cr2+ may be important in natural melts even though this oxidation state has never been identified in a terrestrial material. The development of a methodology for determining Cr2+/ΣCr from XANES spectra of quenched glasses is an essential precursor to in situ experiments on Fe-bearing silicate melts at high temperature.

X-ray Absorption Near-Edge Structure (XANES) Spectroscopy

Article

Full-text available

Feb 2014

The previous Reviews in Mineralogy volume on spectroscopic methods (Vol. 18 Spectroscopic Methods in Mineralogy and Geology, Frank C. Hawthorne, ed. 1988), contained a single chapter on X-ray absorption spectroscopy which reviewed aspects of both EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near-Edge Structure) (Brown et al. 1988, Chapter 11) However, since publication of that review there have been considerable advances in our understanding of XANES theory and applications. Hence EXAFS and XANES have been separated into their own individual chapters in the current volume. In this chapter we endeavor to bring the reader up to date with regard to current XANES theories, as well as, introducing them to the common applications of the technique in mineralogy, geochemistry and materials science. There have been several reviews of XANES (cf., Brown et al. 1988, Brown and Parks 1989, Manceau et al. 2002, Brown and Sturchio 2002, Mottana 2004, Rehr and Ankudinov 2005, de Groot 2001, 2005, and papers therein). In this chapter on XANES it is not our intention to provide a comprehensive review of all the XANES studies since 1988 but to summarize what X-ray edges are commonly investigated and what one can expect to be able to extract from the data. The reader is also advised to read the chapters in this volume on analytical transmission electron microscopy by Brydson et al. (2014, this volume) where (core level) electron energy loss (EELS) spectroscopy is discussed, and by Lee et al. (2014, this volume) on X-ray Raman spectroscopy (XRS), as these techniques provide element specific information similar to XANES. X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation is a well-established technique providing information on the electronic, structural and magnetic properties of matter. In XANES, a photon is absorbed and an electron is excited from a core state to an empty state. To excite an electron in a given core-level, the photon energy has to be equal or higher than the binding energy of this core-level. This gives rise to the opening of a new absorption channel when the photon energy is scanned. The energy of an absorption edge therefore corresponds to the core-level energy, which is characteristic for each element, making XANES an element-selective technique. In addition to the XANES region, at higher energies the extended X-ray absorption fine structure (EXAFS) region is found. The spectral shape in the near-edge region is determined by electronic density of states effects and gives mainly information about the electronic properties and the local geometry of the absorbing atom. The EXAFS region is dominated by single scattering events of the outgoing electron on the neighboring atoms, providing mainly information about the local geometric structure around the absorbing site. In this chapter we will focus on XANES.

Spinel-free and spinel-poor dunite veins crosscutting the Wadi Rajmi ophiolite chromitite (northern Oman ophiolite)

Article

May 2013

Peculiar dunitic veins almost or totally free of spinels crosscut a podiform chromitite ore body in the Wadi Rajmi, northern Oman ophiolite. They probably originated from a komatiitic melt which was oversaturated in Fo≤94 olivines and which evolved to precipitate simultaneously both chromian spinels, with Cr# ranging from 0.6 to 0.8, and Fo91-93 olivines. The absence or the low modal amounts of spinels are possibly governed by a Cr-undersaturation state of the involved melt which crystallized under relatively low cooling rates to generate the spinel-free and the spinel-poor dunites. A shallow and highly depleted mantle source for this komatiitic melt was envisaged during a converging tectonic regime, initiated earlier in the dynamic history of the Oman ophiolite.

XANES study of the oxidation state of Cr in lower mantle phases: Periclase and magnesium silicate perovskite

Article

May 2007

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 °C) at reducing oxygen fugacity conditions (∼iron-wüstite buffer IW to IW - 2), and on Cr:MgSiO3 perovskite with 0.5 wt% Cr2O3.57Fe Mössbauer 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 Cr2+, 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 Cr2+ content reached at most 12.5% but results from Mössbauer spectroscopy combined with chemical composition data suggest that some Cr2+ oxidized during cooling through the reaction Cr2+ + Fe3+ → Cr3+ + Fe2+. In iron-free Cr:MgO, the Cr2+ content is much higher and reaches ∼40%. In Cr:MgSiO3 perovskite with 0.006 Cr pfu (similar to estimated lower mantle abundance), chromium is mainly divalent.

The effect of Cr2O3 on the partial melting of spinel lherzolite in the system CaO-MgO-Al2O3-SiO 2-Cr2O3 at 1.1 GPa

Article

Aug 2004
J PETROL

Xi Liu

Investigating cerium redox changes between aluminosilicate glass and melt: A multispectroscopic approach

Article

Mar 2024

Redox control of glasses is paramount both to their fusion process and to obtaining the desired properties of high technological glasses. However, the link between melting parameters, such as temperature, furnace atmosphere, or quenching rate, and the redox state of the final products is poorly understood. In this work, in situ x-ray absorption near-edge structure (XANES) data at Ce L3-edge data were acquired at high temperatures on cerium-containing sodium aluminosilicate glasses, allowing the determination of thermodynamic constants necessary to predict the cerium redox state over a wide temperature range (900–1500 °C). The results obtained were compared to the Raman spectra of samples quenched at different temperatures. Our findings demonstrate that the quench performed was fast enough to block the cerium oxidation state, meaning the redox measured at room temperature is representative of a high temperature state. This was further verified by room temperature Raman spectroscopy, where a relationship was found between the spectra and melting conditions. Wet chemical analysis, XANES at Ce L3-edge, Raman spectroscopy, and optical absorption spectroscopy were successfully used to determine the redox state of cerium in aluminosilicates.

How to Measure the Oxidation State of Multivalent Elements in Minerals, Glasses, and Melts?

Book

Sep 2021

An Experimental-XANES Investigation of Cr Valence Systematics in Basaltic Liquids and Applications to Modeling Cr2+/ Σ Cr Evolution in Crystallizing Basaltic Magma Systems

Article

Jan 2021
GEOCHIM COSMOCHIM AC

Harvesting quantitative fO2 information from synchrotron μ-XANES measurements of Cr²⁺/ΣCr in olivine phenocrysts requires robust thermodynamic or empirical models that can accurately predict Cr²⁺/ΣCr in basaltic liquids as a function of fO2, temperature, and liquid chemistry. We present the results from a combined experimental-XANES study designed to illuminate how evolving liquid chemistry and decreasing temperature influence the equilibrium Cr²⁺/ΣCr ratios in crystallizing basaltic liquids. The Cr valence dataset produced from these experiments was fit with a symmetric regular solution model; this fitting generated a model equation that predicts the Cr²⁺/ΣCr in basaltic magmas. Using MELTS in conjunction with the newly calibrated Cr valence model, we calculated the Cr²⁺/ΣCr of a tholeiitic liquid undergoing isobaric equilibrium crystallization at 2.5 °C intervals. The modeling results indicate that Cr²⁺/ΣCr evolves dynamically in crystallizing liquids composition and may be partially decoupled from bona fide changes in magmatic fO2. These calculations suggest that the increasing iron content of the residual liquid is the most influential factor influencing the Cr²⁺/ΣCr of the residual liquid. Olivine normative tholeiitic liquids following a liquid line of descent parallel to an oxygen fugacity buffer curve experience significant decreases in the equilibrium Cr²⁺/ΣCr of their residual liquids. Our modeling also demonstrates that Cr²⁺/ΣCr values preserved in early olivine phenocrysts indeed reflect the magmatic fO2 conditions under which they grew; however, the effects of magmatic fO2 are also superimposed on the underlying influences of temperature and melt chemistry.

In situ High-Temperature Experiments

Article

Full-text available

Feb 2014

Solubility of platinum-arsenide melt and sperrylite in synthetic basalt at 0.1 MPa and 1200°C with implications for arsenic speciation and platinum sequestration in mafic igneous systems

Article

Full-text available

May 2017
GEOCHIM COSMOCHIM AC

To better understand the Pt-As association in natural magmas, experiments were done at 1200 °C and 0.1 MPa to measure the solubility of Pt and Pt-arsenide phases (melt and sperrylite, PtAs2), as well as to determine the oxidation state, and identify evidence for Pt-As complexing, in molten silicate. Samples consisting of synthetic basalt contained in chromite crucibles were subject to three experimental procedures. In the first, platinum solubility in the synthetic basalt was determined without added arsenic by equilibrating the sample with a platinum source (embedded wire or bead) in a gas-mixing furnace. In the second, the sample plus a Pt-arsenide source was equilibrated in a vacuum-sealed fused quartz tube containing a solid-oxide oxygen buffer. The third approach involved two steps: first equilibrating the sample in a gas-mixing furnace, then with added arsenide melt in a sealed quartz tube. Oxygen fugacity was estimated in the latter step using chromite/melt partitioning of vanadium. Method two experiments done at high initial arsenic activity (PtAs melt + PtAs2), showed significant loss of arsenic from the sample, the result of vapour transfer to newly-formed arsenide phases in the buffer. Method three experiments showed no loss of arsenic, yielding a uniform final distribution in the sample. Analyses of run-product glasses from experiments which did not show arsenic loss reveal significant increase in arsenic concentrations with fO2, varying from ∼10 ppm (FMQ-3.25) to >10,000 ppm (FMQ + 5.5). Despite very high arsenic loadings (>1000 ppm), the solubility of Pt is similar in arsenic-bearing and arsenic-free glasses. The variation in arsenic solubility with fO2 shows a linear relationship, that when corrected for the change in the activity of dissolved arsenic with the melt ferric/ferrous ratio, yields a solubility-fO2 relationship consistent with As³⁺ as the dissolved species. This result is confirmed by X-ray absorption near edge structure (XANES) determination on run-product glasses. Levels of arsenic required for Pt-arsenide saturation are 50–500 ppm over the fO2 range of most terrestrial basalts (FMQ to FMQ-2), >100× higher than the arsenic concentrations typical of such magmas, indicating significant enrichment of arsenic is required if Pt-arsenide saturation is to occur. In contrast, the level of dissolved Pt required to saturate in sperrylite is >8× lower than for pure Pt, suggesting that arsenic enrichment could lead to Pt removal at concentrations much less than required for pure metal saturation.

Mutual interaction of redox pairs in silicate melts: Equilibria involving metallic phases

Article

Full-text available

Mar 2016

Alexander Borisov

Mutual interaction of redox pairs resulting in the origin or decomposition of a metal phase is analyzed based on the concept of intrinsic oxygen fugacity. Reactions are suggested that may be responsible for the synthesis (or decomposition) of submicrometer-sized metal nuggets when experimental melts are quenched and can explain the possible crystallization of Pt—Fe alloys when basaltic melts cool and elucidate the nature of color of Au-bearing ruby glass. Possible disproportionation reactions of certain lower oxides into a higher oxide and respective metal in cooled silicate melts are discussed.

Cerium Oxidation State in Silicate Melts: Combined fO2, Temperature and Compositional Effects

Article

Jul 2015
GEOCHIM COSMOCHIM AC

To quantify the relative proportions of Ce3+ and Ce4+ in natural magmas, we have synthesized a series of Ce doped glasses ranging in composition from basalt to rhyolite (±H2O) at 0.001 and 1GPa, under fO2 conditions varying from FMQ -4.0 to FMQ +8.4, and temperatures from 1200 to 1500°C. The ratio in the experimental run products was determined both potentiometrically and in situ, using Ce M4,5-edge X-ray absorption near-edge structure (XANES) spectroscopy. For a given melt composition, the change in ratio with fO2 follows the trend predicted from the reaction stoichiometry assuming simple oxides as melt species. In addition to fO2, melt composition and water content have been found to be secondary controls on Ce4+/Ce3+, with more depolymerized melts and hydrous compositions favoring the stabilization of Ce3+. The Ce4+/Ce3+ ratio can be expressed through the equation,log=logfO2+-0.8990(±0.0805)-3.856(±0.083)·xH2O-3.889(±0.037)where T is in Kelvin, NBO/T is the proportion of non-bridging oxygen to tetrahedrally coordinated cations, and xH2O is the mole fraction (calculated using molecular oxides, e.g. Al2O3, Na2O) of water dissolved in the melt. A recent study conducted by Burnham and Berry (2014, Chemical Geology) investigating Ce oxidation state in silicate melts using Ce L3-edge XANES, equilibrated under a subset of the conditions investigated here, showed a similar dependence of on T and fO2, however, melt composition was found to have the opposite effect, with decreasing melt polymerization resulting in an increased abundance of Ce4+. This apparent discrepancy likely arises from the presence of alkalies and H2O in the compositions presented in this study, which were absent in the study of Burnham and Berry (2014). Our results indicate that even at relatively low oxygen fugacity, trace amounts of Ce4+ will be present in most terrestrial igneous systems, suggesting that Ce partitioning could be a sensitive indicator of fO2.

Contribution to the study of the structural environment of chromium in glasses.

Article

Full-text available

Jun 2009

Olivier Villain

The aim of the thesis is to study the structural environment of chromium in glasses, mainly by X-Ray absorption spectroscopy and optical absorption spectroscopy (NIR-visible-UV). An innovative method has been developped for the determination of the relative proportion of CrIII and CrVI by XANES. The major part of the results concerns the evolution of the local structure around CrIII. EXAFS shows no significant modification of both the coordinence and the Cr-O distance with respect to glass composition. In the same time, optical absorption properties (crystal field splitting, Racah parameter, bandwidth and band intensity, additional interference structures) depend strongly on the nature of network formers and other cations, thus pointing out a modification of Cr site distribution, of Cr site distorsion and of the covalency of the Cr-O bond. In silicate glasses, optical parameters are sensitive to the nature of alkaline cations (and secondarily to alkaline earth cations). This preference of chromium for alkaline cations can be explained by a bond valence model and is indicative of a non random environment of chromium within glass structure. Consequently, chromium cannot constitute a suitable probe of the structure of silicate glasses. In borate glasses, optical absorption properties are sensitive to Na/B ratio and are therfore related to the change of boron coordination (3B/4B). In borosilicate glasses, optical properties depend on both Na/B and Si/B ratios. The use of optical absorption spectroscopy both at high and low temperatures shows that the thermal expansion of Cr site does not follow macroscopic dilatation measured on the same samples.

The effect of silica contents on Pd, Pt and Rh solubilities in silicate melts: An experimental study

Article

Full-text available

Jun 2011
EUR J MINERAL

The effect of silica contents on Pt, Pd and Rh solubilities in CaO–MgO–Al2O3–SiO2 melts was investigated at air condition. In a pseudobinary system diopside-anorthite eutectic (DA)–silica at 1450 °C, the maximum solubility of Pd (391 ppm) was observed in melts with 55 wt% SiO2. In more basic and more silicic melts Pd solubility is lower, being 338 ppm at 50 wt% SiO2 and 316 ppm at 70 wt% SiO2. In contrast, Pt and Rh solubilities in these melts systematically decrease with increasing silica, from 8.9 and 35.2 ppm, respectively, in the DA composition down to 4.0 and 21.7 ppm in melts with 70 wt% SiO2. The results on Pt solubility provide a new support to the role that PtFe alloys precipitation may play during melt evolution from basaltic to silicic compositions.In silica-free CaO-Al2O3 melts the solubility of Pt and Rh at 1550 °C was found to be extremely high (230 and 319 ppm, respectively). Adding silica up to 50 wt% results in dramatic decrease in Pt and Rh solubilities (to 5.5 and 29 ppm, respectively).Our results also demonstrate that the contents of trace level impurities in experimental charges (such as W, Mo and alkalis) originating from furnace contamination from earlier experimental runs decrease by two orders of magnitude with melt SiO2 contents increasing from 0 to 70 wt%. The ratio of a contaminant concentration in two different melts from a single run is approximately inversely proportional to the ratio of the contaminant activity coefficients in these melts.

The effect of titanium and phosphorus on ferric/ferrous ratio in silicate melts: An experimental study

Article

Full-text available

Dec 2013

The effect of TiO2 and P2O5 on the ferric/ferrous ratio in silicate melts was investigated in model silicate melts at air conditions in the temperature range 1,400-1,550 degrees C at 1-atm total pressure. The base composition of the anorthite-diopside eutectic composition was modified with 10 wt % Fe2O3 and variable amounts of TiO2 (up to 30 wt %) or P2O5 (up to 20 wt %). Some compositions also contained higher SiO2 concentrations to compare the role of SiO2, TiO2, and P2O5 on the Fe3+/Fe2+ ratio. The ferric/ferrous ratio in experimental glasses was analyzed using a wet chemical technique with colorimetric detection of ferrous iron. It is shown that at constant temperature, an increase in SiO2, TiO2, and P2O5 content results in a decrease in the ferric/ferrous ratio. The effects of TiO2 and SiO2 on the Fe3+/Fe2+ ratio was found to be almost identical. In contrast, adding P2O5 was found to decrease ferric/ferrous ratio much more effectively than adding silica. The results were compared with the predictions from the published empirical equations forecasting Fe3+/Fe2+ ratio. It was demonstrated that the effects of TiO2 are minor but that the effects of P2O5 should be included in models to better describe ferric/ferrous ratio in phosphorus-bearing silicate melts. Based on our observations, the determination of the prevailing fO(2) in magmas from the Fe3+/Fe2+ ratio in natural glasses using empirical equations published so far is discussed critically.

Calibration of an Empirical Thermometer and Oxybarometer based on the Partitioning of Sc, Y and V between Olivine and Silicate Melt

Article

Full-text available

Feb 2013
J PETROL

The evolution of basaltic magmas depends on their redox state, hence oxygen fugacity, but there is increasing evidence that this intensive thermodynamic variable may be less well understood in basalts than commonly supposed. The redox state of terrestrial basalts has to a large extent been inferred from the Fe3+/Fe2+ ratios of their quenched glasses. However, this quantity appears to be significantly affected during late and post-eruptive processes in magmatic systems (e.g. by degassing, charge-transfer reactions of redox-variable species, and alteration), so that the degree to which the Fe3+/Fe2+ ratios preserved in basaltic glasses reflect the oxidation state of the magma at high temperature is unclear. Because olivine is the first silicate mineral to crystallize from primitive basaltic liquids on cooling following decompression, the equilibrium partitioning relations preserved in olivine phenocrysts in basalts are, in principle, less disturbed by these late and post-eruptive processes and, therefore, may better reflect the high-temperature (pre-eruptive) conditions of the magma. Here we calibrate an oxybarometer based on the strong sensitivity of the partitioning of vanadium between olivine and silicate melt to oxygen fugacity. Our empirical parameterization, calibrated over a range of redox conditions between four log10 units above and below the quartz–fayalite–magnetite (QFM) oxygen buffer, takes into account the effects of temperature, olivine composition (i.e. Mg/Fe ratios) and melt composition (namely the activities of CaO, SiO2, AlO1·5, NaO0·5 and KO0·5), and allows oxygen fugacity determinations to within ∼0·25 log10 units. We also explore the sensitivity of the exchange partitioning of Sc and Y between olivine and melt to temperature as a geothermometer. Our calibration indicates that this geothermometer allows temperature to be estimated to within 15°C, but precision is strongly dependent on the Sc and Y measurements in olivine and melt.

In situ High-Temperature Experiments

Chapter

Full-text available

Feb 2014

When Frank Hawthorne (1988) edited the Reviews in Mineralogy volume on "Spectroscopic Methods in Mineralogy and Geology," all the experiments presented had been performed at room pressure and room temperature because, at that time, vibrational and X-ray techniques were already quite difficult at ambient conditions so more sophisticated sample environments were not a priority. However, it has now become somewhat easier to perform experiments in situ at high temperatures (HT), high pressures (HP) or under combined high temperature and pressure (HP-HT). These types of experiments are becoming routine on crystals, glasses and liquids (see Shen and Wang 2014, this volume). High-temperature experiments are important because most of the physical properties of high-temperature liquids, such as magmas and melts, are related to their atomic structure. Consequently, it is important to probe the local environment of the atoms in the sample under the conditions noted above (e.g., HT). However, at very high temperatures 1200 C) it is difficult to use conventional furnaces because of a number of experimental difficulties associated with their use: temperature regulation, thermal inertia and spatial obstruction of the sample Due to the progress made in the development of lasers and X-ray, neutron and magnetic sources it is now possible to perform experiments in situ at HT, HP and HT-HP on samples of millimeter or micron size.

François Garin: Pioneer work in catalysis through synchrotron radiation

Article

Full-text available

Nov 2013
CR CHIM

Dominique Bazin

Starting from the late seventies, the progressively increased availability of beamlines dedicated to X-ray absorption spectroscopy allowed the execution of experiments in chemistry. In this manuscript, I describe the contribution of François Garin at the frontier of heterogeneous catalysis and synchrotron radiation. Working at LURE as a scientific in charge of a beamline dedicated to X-ray absorption spectroscopy during almost twenty years and thus, having the opportunity to discuss with research groups working in heterogeneous catalysis in Europe as well as in the United States, it was quite easy to show that his work is clearly at the origin of current research in heterogeneous catalysis, not only in France, but in different synchrotron radiation centres.

The Ti4+/Ti3+ ratio of magmatic melts: Application to the problem of the reduction of lunar basalts

Article

Full-text available

Jul 2012

Alexander Borisov

Using published experimental data an expression was derived for the Ti4+/Ti3+ ratio as a function of temperature, oxygen fugacity, and melt composition. The equation can be used to estimate Ti3+ content in lunar basaltic melts. It was shown that the Ti3+ content in melts is probably no higher than the Fe3+ content even under the reduced conditions typical of lunar magmas. Trivalent Ti can lead to some decrease in $f_{O_2 }$ during melt cooling under closed-system conditions, but it cannot reduce Fe2+ in melt to metal, because it will be completely consumed by Fe3+ reduction to Fe2+. The presence of additional reducers, such as Cr2+, can be favorable for the formation of metal during melt cooling.

Photoreduction of iron by a synchrotron X-ray beam in low iron content soda-lime silicate glasses

Article

May 2013
CHEM GEOL

The redox of alkaline-earth silicates with iron concentrations ranging from 150 to 5000 ppm was measured using X-ray absorption spectroscopy at the K-edge of iron, in fluorescence mode. These measurements were carried out at three beam lines with different characteristics: FAME (BM30) and DUBBLE (BM26) at ESRF (Grenoble, France) and XAFS2 at LNLS (Campinas, Brazil). In those conducted at BM26, we observed X-ray photoreduction of iron in soda-lime silicate glasses, and we studied it systematically as a function of the iron content of the samples. The photoreduction due to the X-ray beam has a critical impact on the redox measurements at room temperature for glasses with iron contents below 1000 ppm. Results show that a constant amount of Fe3 + seems to be reduced by the beam, regardless of the total iron content within the sample. A tentative explanation for this phenomenon is proposed. The photoreduction effect is reversible and can be prevented by making the measurements at a temperature of 450–500 °C, instead of room temperature. Using this new measurement protocol, the redox state at equilibrium with air of a family of glasses with iron contents ranging from 150 to 5000 ppm has been analysed and compared to values obtained by wet chemistry analyses and additional results by Densem and Turner (1938). Wet chemical analysis, commonly used nowadays, does not show any particular variation of the redox with iron content. XANES results indicate a more complex behaviour where the redox increases as the amount of iron decreases, which is in closer agreement with the early results by Densem and Turner (1938) obtained with a different wet chemical analysis protocol no longer used.

Mutual Interaction of Redox Pairs in Silicate Melts: V5+/V4+/V3+/V2+ Tetrad and Other Equilibria.

Article

Full-text available

Jul 2013
PETROLOGY+

Alexander Borisov

Mutual interaction of redox pairs was evaluated using the concept of intrinsic oxygen fugacity in a melt. A “rose of $f_{O_2 } $ trends” was proposed for the visualization of possible directions of mutual oxidation/reduction of redox pairs for a given melt composition. Missing thermodynamic data were estimated for vanadium redox pairs in sodium disilicate melt, and the V5+/V4+/V3+/V2+ tetrad was quantitatively described. The redistribution of vanadium in different valence states was estimated during quenching to glass of model iron-free and natural iron-bearing melts.

Structural environments around molybdenum in silicate glasses and melts. II. Effect of temperature, pressure, H2O, halogens and sulfur

Article

Full-text available

Jun 2006

The local structure around molybdenum (at a concentration of 2000 ppm) in densified silicate glasses (to 7 kbar), melts (to 1210 K), and fluid-bearing (either H2O, halogens, or sulfur) glasses was investigated by the means of X-ray absorption fine structure (XAFS) spectroscopy at the molybdenum K-edge. The spectra show that molybdate moieties [i.e., Mo(VI)O42-] are the dominant form of molybdenum in anhydrous melts and in densified glasses, with only a minor amount of tetravalent molybdenum. Also, H2O and halogens have a limited effect on the local structure of molybdenum by promoting tetravalent coordination, but they do not complex Mo. In contrast, sulfur is found to complex molybdenum at moderate oxygen and sulfur fugacities. Thio-oxo-molybdate moieties [Mo(IV,V,VI)OnSn (n = 1, 2, 3)] are observed in sulfur-bearing glasses. Thio-oxo-molybdate moieties are characterized by Mo=S2- bonds, which result in these moieties being disconnected (mobile) within the melt. These moieties also polymerize with decreasing redox state of Mo (as Mo-S units), enhancing molybdenite saturation and nucleation in the melt. A new and largely unexplored area of research involving the structure and stability of thio-molybdate moieties in magmatic systems is outlined, which can help reconcile some of the apparent discrepancies in the geochemistry of molybdenum in synthetic systems versus natural systems, in particular in systems where molybdenum partitions into the fluid phase.

The oxidation state of titanium in silicate melts

Article

Nov 2023
GEOCHIM COSMOCHIM AC

Oxygen fugacity buffering in high-pressure solid media assemblies from IW-6.5 to IW+4.5 and application to the V K -edge oxybarometer

Article

Mar 2023

Control of oxygen fugacity during high-temperature phase equilibrium experiments is required to simulate the conditions that exist in natural systems. At high pressures, oxygen fugacity may be imposed using solid buffer equilibria via the classic “double capsule” technique. This design becomes untenable, however, at temperatures above the melting points of commonly used noble metal capsule materials and/or where buffer assemblages may alloy with the capsule or contaminate the sample. Here we introduce and test a modified double capsule approach that includes a solid metal-oxide buffer in close proximity to but separate from the sample of interest. Buffers used include (in order of most oxidized to reduced) Ni-NiO, Co-CoO, W-WO3, Fe-FeO, Mo-MoO2, Cr-Cr2O3, V-V2O3, Ta-Ta2O5, and Nb-NbO. At a fixed temperature, these buffers span a wide range—up to 10 log fO2 units. To demonstrate the buffering capacity of this double capsule approach, secondary redox equilibria and V-doped CaO-MgO-Al2O3-SiO2 system glasses were studied in experiments using the double capsule geometry. The secondary equilibria provide an independent verification of the oxygen fugacity established in the double capsule environment. The glasses proved difficult to interpret, and our results provide guidance to future efforts to utilize the glass oxybarometer at reducing conditions. Application of this modified double capsule technique to studies of V valence in MgAl2O4 spinels led to the recognition of several factors that will affect V valence in this system: temperature of equilibration, duration of experiment, and spinel bulk composition. We have synthesized V-bearing MgAl2O4 spinel at the reduced conditions of the Cr-Cr2O3, (IW-3.51), Ta-Ta2O5, (IW-5.37), and Nb-NbO buffers (IW-5.44). This spinel exhibits a very small V3+ pre-edge peak consistent with its reduced nature. The absence of evidence for V2+ suggests that MgAl2O4 spinel excludes V2+ due to the preference of V for octahedral sites. This finding is supported by DFT calculations for spinels of variable composition, and in agreement with some other indirect evidence for preference for V3+ in aluminous spinels (Bosi et al. 2016; Paque et al. 2013).

The effect of composition, temperature and pressure on the oxidation state and coordination environment of copper in silicate melts

Article

Aug 2023
GEOCHIM COSMOCHIM AC

No detectable redox exchange between sulfur and iron during rapid cooling of basalts

Article

Aug 2023
EARTH PLANET SC LETT

The Thermodynamic Controls on Sulfide Saturation in Silicate Melts with Application to Ocean Floor Basalts

Chapter

Sep 2021

H. St. C. O'Neill

A thermodynamic model to calculate the sulfide content at sulfide saturation or SCSS of basaltic and intermediate composition silicate melts has been built from four independently measurable thermodynamic entities, namely the standard state Gibbs free energy of the saturation reaction, the “sulfide capacity”, and the activities of FeO in the silicate melt and of FeS in the coexisting sulfide: ln S 2 − SCSS = Δ G FeO − FeS o / RT + ln C S 2 − − ln a FeO sil melt + ln a FeS sulf The model was calibrated for silicate melts of basic and intermediate composition from published experimental results as a function of temperature, silicate melt composition, and sulfide matte composition in the system Fe‐Ni‐Cu‐S‐O at 1 bar. The likely effects of pressure and H 2 O content on SCSS were included in an exploratory way. The model was tuned against the large dataset of S contents in OFB glasses of Jenner and O'Neill (2012), giving it a precision comparable to that of the S analyses themselves, which is ~ 5%. All but 3% the OFB glasses were found to be sulfide saturated within uncertainty; these 3% have lost S by devolatization, revealed by their low S/Se. Applying the model to other OFB datasets suggests sulfide saturation is ubiquitous, including olivine‐hosted melt inclusions proposed previously to be sulfide undersaturated. The sulfur fugacity ( f S 2 ) of undegassed Ocean Floor Basalts varies proportionally to f O 2 , with log 10 f S 2 typically within the range −0.6 to +0.4.

How to Measure the Oxidation State of Multivalent Elements in Minerals, Glasses, and Melts?

Chapter

Full-text available

Sep 2021

All natural materials and most manufactured technologically relevant materials contain a variable but consequent amount of multivalent elements, like iron. Many physicochemical properties of materials highly depend on the type of multivalent cations they contain, as well as on the concentrations and redox states of those multivalent elements. For instance, color is probably the most visible characteristic, resulting from the presence of multivalent transition elements. Other properties like viscosity and density of liquids are affected by those elements, and, as such, they can be seen as important variables for geologic and industrial phenomena. The present contribution focuses on providing an overview of the most common techniques currently in use for the investigation of multivalent elements in natural and industrial materials. It covers both destructive and non‐destructive methods, as well as element‐specific and bulk techniques.

The coordination of Cr2+ in silicate glasses and implications for mineral-melt fractionation of Cr isotopes

Article

Aug 2021
CHEM GEOL

The Cr²⁺-O and Cr³⁺-O bond lengths in CaO-MgO-Al2O3-SiO2 glasses containing ~0.3 wt% Cr with Cr²⁺/ΣCr = ~ 1 or 0 (where ΣCr = Cr²⁺ + Cr³⁺) were determined by extended X-ray absorption fine structure (EXAFS) spectroscopy to be 2.034(5) and 1.967(7) Å, respectively. The Cr²⁺-O bond length is consistent with square planar coordination and the Cr³⁺-O bond length with octahedral coordination. These bond lengths were used to calculate force constants of 1206(9) Nm⁻¹ for Cr²⁺-O and 2001(21) Nm⁻¹ for Cr³⁺-O in silicate melts. The value for Cr³⁺-O is similar to previous estimates but that for Cr²⁺-O is almost 20% lower. The force constants were used to calculate the difference in Cr isotopic composition (Δ⁵³Cr) between both olivine and melt and spinel and melt for MORB with equal amounts of Cr²⁺ and Cr³⁺ at 1150 °C. The resulting values, Δ⁵³Crol-mlt = −0.050(4) and Δ⁵³Crspl-mlt = 0.087(4), are larger by ~0.03 than those obtained using CrO bond lengths estimated from ionic radii. For Δ⁵³Crspl-mlt this difference is equivalent to a change in temperature of over 300 °C at constant Cr²⁺/ΣCr. The fractionation of Cr isotopes in ocean island basalts should be larger for fractional crystallisation in the crust than partial melting in the mantle due to the strong negative effect of pressure on Cr²⁺/ΣCr. The new bond length data for Cr²⁺-O provide an additional constraint for modelling and interpreting Cr isotope fractionation during igneous petrogenesis.

The oxidation state of chromium in basaltic silicate melts

Article

Apr 2021
GEOCHIM COSMOCHIM AC

Cr³⁺/Cr²⁺ in melts in the systems CaO-MgO-Al2O3-SiO2 ± Na2O ± K2O doped with Cr added as 0.5 wt% Cr2O3 were determined as a function of oxygen fugacity (fO2) at 1400 °C by XANES spectroscopy of their quenched glasses, using the intensity of the shoulder on the Cr K-edge due to the 1 s → 4 s transition. The addition of Na and K to the system CMAS increases Cr³⁺/Cr²⁺ at constant temperature and fO2, in good agreement with the predictions from the “ideal optical basicity”. The new results have been combined with previous results to calibrate a model for Cr³⁺/Cr²⁺ in silicate melts as a function of temperature, pressure and melt composition: log10(Cr³⁺/Cr²⁺) = 1/4 ΔQFM + 3031/T − 2.26 + (843P – 158 P²)/T + ∑cZXZ. where XZ are the mole fractions of the oxide components Z defined on the single-cation basis and ∑cZXZ = 2.00 XNaO0.5 + 1.00 (XMgO + XFe2+O) + 0.37 (XAlO1.5 + XFe3+O1.5) + 2.12 XKO0.5 + 2.44 XCaO + 3.69 XTiO2, T is temperature in K, P is pressure in GPa, and ΔQFM is the difference between the fO2 of the silicate melt and the Quartz-Fayalite-Magnetite buffer at 10⁵ Pa, given by log10fO2(QFM) = 8.58–25050/T, relative to the conventional standard state of pure O2 at 10⁵ Pa. The effect of pressure is markedly non-linear, so the model should not be extrapolated above 4 GPa. Combining this model with the similar one for Fe³⁺/Fe²⁺ in silicate melts gives: log10(Cr³⁺/Cr²⁺) = log10(Fe³⁺/Fe²⁺) + 3031/T − 0.9 + (1317P – 219 P²)/T + ∑jZXZ. where ∑jZXZ = 1.00 (XMgO + XFe2+O) + 0.37 (XAlO1.5 + XFe3+O1.5) −1.59 XKO0.5 + 0.04 XCaO + 3.69 XTiO2. High Cr²⁺/∑Cr in silicate melts is promoted by high temperature and low pressure, as well as low Fe³⁺/∑Fe. For a parental MORB melt composition at 1250 °C (1523 K), 10⁵ Pa, with Fe³⁺/∑Fe = 0.10, the model predicts Cr²⁺/ΣCr = 0.27. The effect of pressure is very large: the Cr²⁺/ΣCr in the above example would drop to 0.03 at 2 GPa and 1250 °C. The Cr²⁺ present in Fe³⁺-containing melts at magmatic temperatures decreases on cooling because of the electron exchange reaction: Cr²⁺ + Fe³⁺ = Cr³⁺ + Fe²⁺.

How to measure the oxidation state of multivalent elements in minerals, glasses and melts?

Preprint

Full-text available

Mar 2021

Effect of Sulfur Speciation on Chemical and Physical Properties of Very Reduced Mercurian Melts

Article

Jul 2020
GEOCHIM COSMOCHIM AC

The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5-4 wt.%) compared with other terrestrial planets (<0.1 wt.%), a result of high S solubility under its very low oxygen fugacity (estimated ƒO2 between IW-3 and IW-7). Due to decreasing O availability at these low ƒO2 conditions, and an abundance of S²⁻, the latter acts as an important anion. This changes the partitioning behaviour of many elements (e.g. Fe, Mg, and Ca) and modifies the physical properties of silicate melts. To further understand S solubility and speciation in reduced magmas, we have analysed 11 high pressure experiments run at 1 GPa in a piston cylinder at temperatures of 1250 to 1475 °C and ƒO2 between IW-2.5 to IW-7.5. S K-Edge XANES is used to determine coordination chemistry and oxidation state of S species in highly reduced quenched silicate melts. As ƒO2 decreases from IW-2 to IW-7, S speciation goes through two major changes. At ∼IW-2, FeS, FeCr2S4, Na2S, and MnS species are destabilized, CaS (with minor Na2S) becomes the dominant S species. At ∼ IW-4, Na2S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low ƒO2 affect the activities of SiO2, MgO and CaO in the melt, stabilizing enstatite at the expense of forsterite, and destabilizing plagioclase and clinopyroxene. These shifts cause the initial layering of Mercury’s solidified magma ocean to be enstatite-rich and plagioclase poor. Our results on S speciation at low ƒO2 are also applicable to the petrologic evolution of enstatite chondrite parent bodies and perhaps early Earth.

Synchrotron-Based X-Ray Absorption Spectroscopy for the Study of Geological Materials

Article

Mar 2020

A. N. Kravtsova

The stability of divalent Ge in silicate melts and its geochemical properties

Article

Nov 2019
CHEM GEOL

The oxidation state of Ge in silicate glasses, quenched from melts, was determined by X-ray absorption spectroscopy. The melts were equilibrated over the range of relative oxygen fugacities (fO2) from IW -3 to IW +10, where IW is the iron-wüstite oxygen buffer in logarithmic units. X-ray absorption near edge structure (XANES) spectra of the samples show that over the range in fO2 from IW -2.8 to IW + 2.4, the Ge⁴⁺/(Ge²⁺ + Ge⁴⁺) ratio increases from 0.05 to 0.95. Modelling of extended X-ray absorption fine structure (EXAFS) gives the Ge²⁺–O bond length as 1.89 ± 0.03 Å. Olivine–melt partitioning experiments were also conducted, which show that Ge²⁺ is highly incompatible, with DGe2+ol/melt < 0.005, whereas DGe4+ol/melt is ∼ 1, where D is the partition coefficient. The geochemical properties of Ge during the magmatic differentiation of the Moon and other reduced rocky planets and achondrite parent bodies will therefore be entirely different to that familiar from terrestrial examples. In particular, the incompatible nature of Ge²⁺ may explain the anomalous enrichment of Ge in KREEP basalts.

The oxidation state and coordination environment of antimony in silicate glasses

Article

Jun 2019
CHEM GEOL

Antimony K-edge X-ray absorption near edge structure (XANES) spectra were recorded for Sb in glasses of five synthetic CaO-MgO-Al2O3-SiO2 (CMAS) compositions and a simplified mid-ocean ridge basalt (MORB). The glasses were quenched from melts equilibrated at 1300 °C, atmospheric pressure, and oxygen fugacities (fO2) from logfO2 = –9 to 0 (ΔQFM from −1.7 to 7.3, where ΔQFM is the fO2 in log units relative to the quartz-fayalite-magnetite buffer), and 1400 °C, 1.0 GPa and logfO2 = +4.7 (ΔQFM+11.0). Comparison with model compounds (Sb, Sb2O3, β-Sb2O4, MnSb2O4 and Zn7Sb2O12) indicates that the oxidation state of Sb changes from Sb³⁺ to Sb⁵⁺ over the range of fO2 investigated. Using the spectra of the most reduced and oxidised glasses as end-members the oxidation state of Sb, Sb⁵⁺/ΣSb (where ΣSb = Sb³⁺ + Sb⁵⁺), was determined from the spectra of other samples by linear combination fitting. At terrestrial fO2s nearly all Sb occurs as Sb³⁺ in these silicate melts. The stability of Sb⁵⁺ was found to be positively correlated with the CaO content and optical basicity of the melts. Extended X-ray absorption fine structure (EXAFS) spectra were recorded for glasses prepared at pressures from 1.0 to 3.0 GPa, for which Sb⁵⁺/ΣSb = 0 and 1, and for a synthetic Sb-bearing sample of anorthite. The Sb³⁺O and Sb⁵⁺O bond lengths in the glasses are similar (1.938(4) and 1.964(5) Å, respectively) and suggest trigonal pyramidal coordination of Sb³⁺and octahedral Sb⁵⁺. The Sb³⁺O bond length did not change with pressure. Anorthite (CaAl2Si2O8) was found to readily incorporate Sb as Sb⁵⁺ on a tetrahedral site.

In situ high-temperature EXAFS measurements on radioactive and air-sensitive molten salt materials

Article

Full-text available

Jan 2019
J SYNCHROTRON RADIAT

The development at the Delft University of Technology (TU Delft, The Netherlands) of an experimental set-up dedicated to high-temperature in situ EXAFS measurements of radioactive, air-sensitive and corrosive fluoride salts is reported. A detailed description of the sample containment cell, of the furnace design, and of the measurement geometry allowing simultaneous transmission and fluorescence measurements is given herein. The performance of the equipment is tested with the room-temperature measurement of thorium tetrafluoride, and the Th—F and Th—Th bond distances obtained by fitting of the EXAFS data are compared with the ones extracted from a refinement of neutron diffraction data collected at the PEARL beamline at TU Delft. The adequacy of the sample confinement is checked with a mapping of the thorium concentration profile of molten salt material. Finally, a few selected salt mixtures (LiF:ThF 4 ) = (0.9:0.1), (0.75:0.25), (0.5:0.5) and (NaF:ThF 4 ) = (0.67:0.33), (0.5:0.5) are measured in the molten state. Qualitative trends along the series are discussed, and the experimental data for the (LiF:ThF 4 ) = (0.5:0.5) composition are compared with the EXAFS spectrum generated from molecular dynamics simulations.

Vanadium, Sulfur, and Iron Valences in Melt Inclusions as a Window into Magmatic Processes: A Case Study at Nyamuragira Volcano, Africa

Article

Feb 2018
GEOCHIM COSMOCHIM AC

This study describes microscale sulfur (S), vanadium (V), and iron (Fe) K-edge X-ray absorption near edge structure (µ-XANES) spectroscopy measurements on olivine-hosted melt inclusions (MI) preserved in tephras (1986 and 2006) and lavas (1938 and 1948) erupted from Nyamuragira volcano (D.R. Congo, Africa). The S, V, and Fe spectroscopic data are used to constrain the evolution of oxygen fugacity (fO2) and sulfur speciation for the entrapped melts. Melt inclusions from lavas show evidence of post-entrapment crystallization and were thus reheated prior to µ-XANES analysis. The MI from tephra show no evidence of post-entrapment crystallization and were, therefore, not reheated. Sulfur, V, and Fe µ-XANES results from 1938, 1948, and 2006 eruptive materials are all similar within analytical uncertainty and provide similar average calculated melt fO2's based on XANES oxybarometry. However, olivine-hosted MI from the 1986 tephras yield significantly different S, V, and Fe XANES spectra when compared to MI from the other eruptions, with disagreement between calculated fO2's from the three valence state oxybarometers beyond the uncertainty of the calibration models. Their V µ-XANES spectra are also significantly more ordered and yield more reduced average V valence. The S µ-XANES spectra display a significantly more intense low-energy spectral resonance, which indicates differences in Fe-S bonding character, and greater variability in their measured sulfate content. These V and S spectroscopic features are best explained by crystallization of sub-micrometer magnetite and sulfide crystallites within the 1986 inclusions. The sensitivity of XANES spectroscopy to short-range order allows these crystallites to be recognized even though they are not easily detected by imaging analysis. This shows that V and S µ-XANES are potentially highly sensitive tools for identifying the presence of volumetrically minor amounts of spinel and sulfide within inclusions extracted from rapidly-cooled samples of tephra. Additionally, the observation that rehomogenized 1938 and 1948 inclusions from lava yield similar S, V, and Fe XANES spectra to the 2006 inclusions from tephra may be an encouraging indication that rehomogenization appears to have enabled the successful recovery of their pre-eruptive fO2, despite their complex post-eruptive histories.

The in situ determination of redox states in silicate melts

Conference Paper

Mar 2004
LITHOS

In Situ Characterisation of Model UK Nuclear Waste Glasses by X-ray Absorption Spectroscopy Under Process Conditions

Article

Feb 2011
Mater Res Soc Symp Proc

The local co-ordination environment of Zr in a model alkali borosilicate glass, of relevance to nuclear waste immobilisation, was studied by in situ X-ray absorption spectroscopy between 25-1060°C. Analysis of Zr K-edge XANES spectra, in comparison with those of well characterised standards, demonstrated, for the first time, the reversible transformation of ZrO 6 to ZrO7 co-ordination polyhedra at high temperature. This observation was rationalised on the basis of the combined effects of network modifier cation diffusion and thermal expansion.

The effect of oxygen fugacity, melt composition, temperature and pressure on the oxidation state of cerium in silicate melts

Article

Jan 2013
CHEM GEOL

Ce LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses prepared over a range of oxygen fugacities (fO2s, from − 10 to + 11 in logarithmic units relative to the quartz-fayalite-magnetite, QFM, buffer), temperatures (1300–1500 °C), and pressures (1 atm and 1 GPa). The oxidation state ratio of Ce, Ce4 +/∑ Ce (where ∑ Ce = Ce3 ++ Ce4 +), was determined from the spectra allowing the relationships between Ce4 +/∑ Ce and fO2, temperature, pressure and melt composition to be determined. Ce4 +/∑ Ce varied systematically with fO2 from 0 to ~ 0.8 over the range of conditions studied. Ce4 + is stabilised relative to Ce3 + by less polymerised compositions and lower temperatures, while pressure appears to have almost no effect (possibly stabilising Ce4 +). Ce4 + in an Fe2 +-bearing melt is not preserved on cooling to a glass due to the reaction Ce4 ++ Fe2 + = Ce3 ++ Fe3 +. Ce4 +/∑ Ce in natural melts is exceedingly small but may be recorded in the mineral zircon as an increased abundance of Ce relative to the other rare earth elements, which occur exclusively in the trivalent state. The magnitude of this Ce anomaly has considerable potential as an oxy-barometer.

The effect of f(O2) on the partitioning and valence of V and Cr in garnet/melt pairs and the relation to terrestrial mantle V and Cr content

Article

Aug 2011

Chromium and vanadium are stable in multiple valence states in natural systems, and their distribution between garnet and silicate melt is not well understood. Here, the partitioning and valence state of V and Cr in experimental garnet/melt pairs have been studied at 1.8-3.0 GPa, with variable oxygen fugacity between IW-1.66 and the Ru-RuOâ (IW+9.36) buffer. In addition, the valence state of V and Cr has been measured in several high-pressure (majoritic garnet up to 20 GPa) experimental garnets, some natural megacrystic garnets from the western United States, and a suite of mantle garnets from South Africa. The results show that Cr remains in trivalent in garnet across a wide range of oxygen fugacities. Vanadium, on the other hand, exhibits variable valence state from 2.5 to 3.7 in the garnets and from 3.0 to 4.0 in the glasses. The valence state of V is always greater in the glass than in the garnet. Moreover, the garnet/melt partition coefficient, D(V), is highest when V is trivalent, at the most reduced conditions investigated (IW-1.66 to FMQ). The V{sup 2.5+} measured in high P-T experimental garnets is consistent with the reduced nature of those metal-bearing systems. The low V valence state measured in natural megacrystic garnets is consistent with f{sub Oâ} close to the IW buffer, overlapping the range of f{sub Oâ} measured independently by Fe{sup 2+}/Fe{sup 3+} techniques on similar samples. However, the valence state of V measured in a suite of mantle garnets from South Africa is constant across a 3 log f{sub Oâ} unit range (FMQ-1.8 to FMQ-4.5), suggesting that the valence state of V is controlled by the crystal chemistry of the garnets rather than f{sub Oâ} variations. The compatibility of V and Cr in garnets and other deep mantle silicates indicates that the depletion of these elements in the Earth's primitive upper mantle could be due to partitioning into lower mantle phases as well as into metal.

First-row transition elements, Y and Ga in kimberlite and lamproite: Applications to diamond prospectivity and petrogenesis

Article

Oct 2008

Tyson C. Birkett

Whole-rock magmatic compositions of hypabyssal-facies samples from on-craton kimberlites predict diamond prospectivity. Within a database of some 100 kimberlites worldwide, 88% of occurrences are correctly assigned using Ti, Cr, Ni and Y whole-rock concentrations. Prospective occurrences show lower Ti, Y, Ga, Mn, Fe, Sc, Zn, V and Cu, and higher Ni and Cr than non-prospective, otherwise similar rocks. Magnesium, more abundant in prospective occurrences, can also be used to distinguish these two groups, although to a lesser extent. Similar differences, excepting Ti, are observed between diamondiferous (prospective) and non-diamondiferous (non-prospective) lamproites. These distinctions result from differences in both groundmass and suspended-load compositions of these rocks. Differences in oxygen fugacity attending the genesis and evolution of kimberlitic magma most likely contribute to these distinct compositions, although other factors such as diamond presence in source regions and pressure and temperature conditions probably contribute as well.

An experimental determination of the effect of pressure on the Fe3+/ Fe ratio of an anhydrous silicate melt to 3.0 GPa

Article

Feb 2006

H. St. C. O'Neill

The effect of pressure on the Fe³⁺/∑Fe ratio of an anhydrous andesitic melt was determined from 0.4 to 3.0 GPa at 1400 °C with oxygen fugacity controlled internally by the Ru + RuO2 buffer. Values of Fe³⁺/∑Fe were determined by Mössbauer spectroscopy on quenched glasses with a precision of ±0.01, one standard deviation. This precision was verified independently by XANES spectroscopy of the same samples. The XANES spectra show a systematic increase in energy and decrease in intensity of the 1s → 3d transition with increasing pressure. The results to 2.0 GPa are in good agreement with predictions from density and compressibility measurements fitted to a Murnaghan equation of state, but the datum at 3.0 GPa has higher Fe³⁺/∑Fe than predicted from the trend established by the lower-pressure data. This might be due to a coordination change in Fe³⁺ at high pressure; although there is no evidence for this in the Mössbauer spectra, such a change could account for the change in intensity of the 1s → 3d transition in the XANES spectra with pressure.

Study of crystallisation of yttrium garnets Y3R5O12 (R=Al, Ga) by high temperature EXAFS

Article

Full-text available

Apr 1997

In this paper, we present a new high temperature EXAFS device using a laser heating system and aerodynamic levitation for experiments up to 2300 degrees C. We report here the first spectra of Y K-edge far YAG at room temperature and in the liquid.

Collecting XAFS spectra at soft X-ray energies in a heated loop cell up to 1600 K

Article

Full-text available

Jun 1999

A new device to collect in-situ-, high-temperature XAFS spectra is presented which runs at low x-ray energies (K edges from Na to Cl). XAFS data is collected in the fluorescence mode (Ge solid state detector) using a heated loop that contains the sample. The actual temperature is measured using an optical pyrometer and a thermocouple (temperatures are +/- 20 K at 1500 K). The first XAFS spectra were collected at the Si K-edge, with 0.2 eV steps, on the ABS1 spectrometer at Super-AGO (LURE, Orsay). The study of the melting of a Na2Si2O5 glass is an example of the possibilities offered by this new area of research.

New X-ray absorption experimental set-up dedicated to DeNOx catalysis

Article

Jul 1999
NUCL INSTRUM METH B

In this paper, we present a complete in situ X-ray absorption reaction cell that simulates DeNOx experimental conditions. In this device, the sample is placed at high temperature under the flow of a complex mixture of reactive gases (NO, C3H6, O2, N2). Particular attention has been paid to the material and design of the sample holders in order to avoid preferential gas circulation and thus ensure a true diffusion of the reactive gases on the catalyst. Moreover, precise control of the gas flow is maintained by means of mass flow controllers, and the safety of the set-up is assured by CO, CO2 and NO detectors directly relayed to an electronic device, which is itself linked to the various mass flow controllers. Finally, the possibilities of this device are illustrated through the in situ XANES study of a Cu-ZSM-5 catalytic system.

Environmental cells for in situ X-ray diffraction and X-ray absorption spectroscopic studies of heterogeneous catalysts

Article

May 1995
NUCL INSTRUM METH B

Application of synchrotron radiation for in situ studies of catalysis requires construction of suitable environmental cells. We describe here examples of cells for extremes of temperature, pressure and chemical conditions. These cells are suitable for XAS and XRD and have proved reliable over a decade of use. For the first time we show in situ XAS cells for catalysis studies based on conversion electron yield (CEY) detection.

A new device for XAFS data collection up to 2000 K (or 3700 K under vacuum)

Article

Aug 1995
NUCL INSTRUM METH B

A new device to collect high- to ultra high temperature X-ray absorption fine structure (XAFS) data is presented. The experimental XAFS data is collected in the energy-dispersive mode using the heating wire technique. The X-ray beam is focused on a 500 μm ⊘ hole (drilled within a Pt90Rh10 heating wire) that contains the sample (100–300 μm thickness). The wire is heated by the Joule effect. The actual temperature is measured using an optical pyrometer and an electrical power-temperature calibration, based on 10 model compound melting points (T is ± 20 K on the average). The maximum possible temperature is that of the wire melting point (∼ 2000 K for the Pt90Rh10 alloy). However, much higher temperatures can be achieved by the use of Ir, W, Ta (etc.) wires under inert atmosphere (to prevent the wire from oxidation by air). Data collection at all temperatures is rather fast (3 s/spectrum for a 3–13 Å−1k-domain used) and can be performed every 20 K. Therefore, in situ, high temperature kinetics can also be studied as a function of time such as phase transitions, oxidation reactions or melting phenomena. The study of GeO2 polymorphs as a function of temperature will be shown to give an example of the possibilities offered by this technique.

Structural transformation in Ni-bearing Na2Si2O5 glass and melt

Article

Sep 1994

The coordination environment around Ni at minor concentration levels (2 wt. % NiO) in Na2Si2O5 glass and melt (293–1250K) has been investigated by Ni K-edge X-ray Absorption Fine Structure (XAFS) spectroscopy. X-ray Absorption Near Edge Structure (XANES) spectra indicate that both IVNi and VNi occur in the glass, whereas IVNi is dominant in the melt. The average anharmonic Ni-O distances derived from XAFS spectra of the glass at 293 K and the melt above 1150 K are 2.00 (±0.02) and 1.97 (± 0.02) Å, respectively. The Ni-O distance in the glass is consistent with a mixture of IVNi and VNi, whereas that in the melt is consistent with dominantly IVNi, suggesting a Ni-coordination change during glass melting. This major change in Ni coordination indicates significant reorganization of the melt during quenching and crystallization. These observations for a compositionally-simple melt/glass system may help explain the enrichment of Ni in early-formed igneous minerals. They also suggest the possibility of a pressure-induced coordination change around Ni in the mantle which may influence geochemical modeling of the deep Earth.

In situ XAS-study on transition metal doped metal oxides

Article

Mar 1995
PHYSICA B

We have performed in situ X-ray absorption experiments on doped metal oxides at high temperatures (T > 1273 K) and under defined oxygen activities to study the redox behavior and local environment of the dopants in these oxides. First measurements at the K-edges of iron and copper were carried out on mixed crystals like (Ni1−xFex)1−δO and (Mg1−xCux)1−δO by adjusting oxygen activities between a02 = 0.21 and 10−8 at temperatures up to 1523 K. While iron could be reduced in the NiO matrix from Fe3+ to Fe2+, indicated by an absorption edge shift to lower energies, for copper in MgO no change in the XANES or EXAFS regions was observed indicating a stable Cu-valency of + 2.

Evidence from chromium abundances in mantle rocks for extraction of picrite and komatiite melts

Article

Feb 1990
NATURE

ONE of the more important aspects of understanding the geochemical differentiation of the Earth is an accurate knowledge of the compositions of magmas produced by direct melting of the upper mantle, which are called primary magmas1. It is clear that a variety of primary magma compositions can develop within the mantle, depending on the source composition, the presence of volatiles, the extent of melting and the pressure of melt segregation1-4. Proposed primary magma compositions span a wide composition range, from basalts with ~8-ll% MgO, through picrites with ~11-20% MgO, to komatiites with ~20-28% MgO; there is no agreement as to which is the dominant type of primary magma. We suggest here that the abundances of chromium in samples of the Earth's upper mantle indicate that, of the wide variety of possible primary liquids produced in experimental studies of mantle melting, the dominant type of primary liquid actually extracted from the mantle has been picritic to komatiitic, with >~15% MgO.

The Distribution of Cr Between Basaltic Melt and Chromian Spinel as an Oxygen Geobarometer

Article

Jan 2000
CAN MINERAL

A thermodynamic model of Cr distribution between chromian spinel and basaltic melt has been developed using the Sack & Ghiorso (1991) thermodynamic model for chromian spinel and an equation analogous to that of Sack er nl. (1980) to calculate the Cr oxidation state in the melt. The model has been calibrated using published experimental data on the composition of coexisting chromian spinel and basaltic melt. The model can be used either to calculate the oxygen fugacity, f(O-2), of magmas saturated with chromian spinel, to predict the Cr content of the melt, or to calculate the oxidation state of Cr in the melt. The f(O-2) values calculated for a set of natural lavas (primitive MORB and a Hawaiian pumice) using the method show good agreement with f(O-2) values calculated from measured values of Fe3+/Fe2+ in the melt. An EXCEL spreadsheet for calculating log f(O-2) using composition of glass and chromian spinel is available from the authors upon request.

Extended X-ray-absorption fine structure studies of levitated undercooled metallic melts

Article

Nov 1996

We report the first attempt to combine electromagnetic levitation with extended x‐ray‐absorption fine structure (EXAFS) spectroscopy for the determination of nearest‐neighbor distances in the metastable state of an undercooled metallic melt. The experiments have shown that it is possible to measure EXAFS spectra for a freely suspended liquid droplet up to temperatures of 1390 °C. In particular, spectra at an undercooling of 325 °C were recorded. The spectra show certain differences between the overheated and the deeply undercooled regime. Modeling of the pair distribution function is required for correct interpretation, because asymmetric effects due to the large disorder in the hot liquid have to be considered. © 1996 American Institute of Physics.

A new furnace design for use in combined X-ray absorption and diffraction of catalysis and ceramics studies: formation from carbonate precursors of Cu, Co, Mn spinels for the oxidation of CO and the formation of PLZT, a piezoelectric ceramic

Article

May 1995
NUCL INSTRUM METH B

In order to measure combined X-ray absorption fine structure, XAFS and X-ray diffraction, XRD in-situ on a sample, which is being heated in a furnace, there are a number of constraints on the furnace design due to the geometry of the detectors around the sample. We present a new furnace design which is based on a tube furnace: primarily for ceramics or catalysis research where the sample is heated in air, but at temperatures of up to 1200°C. The design and specifications of this furnace are presented.To demonstrate the use of the high temperature furnace we have chosen a system which contains three elements, Mn, Co and Cu, whose XAFS and XRD have been measured. The sample was heated in air to 600°C and the initial collapse of the crystalline carbonates was followed by the formation of the final spinel phase. Furthermore, the structural changes also accompany changes in the oxidation states, especially of Cu and Mn. The Cu and Mn in the final product is present as a mixture of Cu 1+ and 2+ and Mn as 3+ and 4+ oxidation states. The XAFS analysis of the data also yielded information about the local coordination changes during the transformation of the precursor carbonates to the final spinel product. We also show some preliminary data from the kinetics of formation of PLZT [Pb0.92La0.08(Zr0.65Ti0.35)0.98O3], from an amorphous gel to a crystalline phase material (ABO3).

A quantitative in situ Fe K-XAFS study (T> 1270 K) on the oxidation degree of iron in (Mg1-xFex)1-δO

Article

Jun 1999

The point defect structure of a doped oxide, (Mg1-xFex)1-δO, was quantitatively determined by in situ high temperature X-ray absorption spectroscopy (1270 K<T<1470 K). Spectra were recorded under defined oxygen activity at the dopant's K-edge. The degree of oxidation a versus T, ao2 and x can be consistently described by a simple defect model. This model considers divalent iron ions (AFe-O=215.8 pm), randomly dissolved in the cation sublattice, and a defect associate between two trivalent iron ions and a cation vacancy (RFe-O=205.0 pm). Indications of trivalent ions occupying interstitial sublattice sites were found near the (Mg1-xFex)1-δO/MgFe2O 4 phase boundary.

A furnace design for XANES spectroscopy of silicate melts under controlled oxygen fugacities and temperatures to 1773 K

Abstract

No full-text available

Recommended publications

The Effect of Composition on Cr2+/Cr3+ in Silicate Melts