A Mössbauer spectroscopic study of the iron redox transition in eastern Mediterranean sediments

Fe cycling at two sites in the Mediterranean Sea (southwest of Rhodes and in the North Aegean) has been studied, combining the pore water determination of nutrients, manganese, and iron, citrate-bicarbonate-dithionite (CDB) and total sediment extractions, X-ray diffraction, and ⁵⁷Fe Mössbauer spectroscopy (MBS). At the Rhodes site, double peaks in the CDB-extractable Mn and Fe profiles indicate non-steady-state diagenesis. The crystalline iron oxide hematite, identified at both sites by room temperature (RT) MBS, appears to contribute little to the overall Fe reduction. MBS at liquid helium temperature (LHT) revealed that the reactive sedimentary Fe oxide phase was nanophase goethite, not ferrihydrite as is usually assumed. The pore water data at both sites indicates that upon reductive dissolution of nanophase goethite, the upward diffusing dissolved Fe²⁺ is oxidized by Mn oxides, rather than by nitrate or oxygen. The observed oxidation of Fe²⁺ by Mn oxides may be more common than previously thought but not obvious in sediments where the nitrate penetration depth coincides with the Mn oxide peak. At the Rhodes site, the solid-phase Fe(II) increase occurred at a shallower depth than the accumulation of dissolved Fe²⁺ in the pore water. The deeper relict Mn oxide peak acts as an oxidation barrier for the upward diffusing dissolved Fe²⁺, thereby keeping the pore water Fe²⁺ at depth. At the North Aegean site, the solid-phase Fe(II) increase occurs at approximately the same depth as the increase in dissolved Fe²⁺ in the pore water. Overall, the use of RT and cryogenic MBS provided insight into the solid-phase Fe(II) gradient and allowed identification of the sedimentary Fe oxides: hematite, maghemite, and nanophase goethite.     

The effect of As, Co, and Ni impurities on pyrite oxidation kinetics: An electrochemical study of synthetic pyrite

Synthetic pyrite crystals doped with As, Co, or Ni, undoped pyrite, and natural arsenian pyrite from Leadville, Colorado were investigated with electrochemical techniques and solid-state measurements of semiconducting properties to determine the effect of impurity content on pyrite’s oxidation behavior. Potential step experiments, cyclic voltammetry, and AC voltammetry were performed in a standard three-electrode electrochemical cell setup. A pH 1.78 sulfuric acid solution containing 1 mM ferric iron, open to atmospheric oxygen, was chosen to approximate water affected by acid drainage. Van der Pauw/Hall effect measurements determined resistivity, carrier concentration and carrier mobility.The anodic dissolution of pyrite and the reduction of ferric iron half-reactions are taken as proxies for natural pyrite oxidation. Pyrite containing no impurities is least reactive. Pyrite with As is more reactive than pyrite with either Ni or Co despite lower dopant concentration. As, Co, and Ni impurities introduce bulk defect states at different energy levels within the band gap. Higher reactivity of impure pyrite suggests that introduced defect levels lead to higher density of occupied surface states at the solid-solution interface and increased metallic behavior. The current density generated from potential step experiments increased with increasing As concentration. The higher reactivity of As-doped pyrite may be related to p-type conductivity and corrosion by holes. The results of this study suggest that considering the impurity content of pyrite in mining waste may lead to more accurate risk assessment of acid producing potential.     

Redox potential measurements and Mössbauer spectrometry of Fe adsorbed onto Fe (oxyhydr)oxides

The redox properties of Fe^II adsorbed onto a series of Fe^III (oxyhydr)oxides (goethite, lepidocrocite, nano-sized ferric oxide hydrate (nano-FOH), and hydrous ferric oxide (HFO)) have been investigated by rest potential measurements at a platinum electrode, as a function of pH (−log₁₀[H⁺]) and surface coverage. Using the constant capacitance surface complexation model to describe Fe^II adsorption onto these substrates, theoretical values of the suspension redox potential (E_H) have been computed, under the assumption that Fe^II adsorption occurs at crystal growth sites of the substrate surface. Good agreement between calculated and experimental E_H values is observed for nano-FOH and HFO, however the redox potentials measured for lepidocrocite and goethite are significantly more oxidizing than predicted. Mössbauer spectroscopic analysis of ⁵⁷Fe^II adsorbed onto HFO and goethite shows that in both cases the adsorbed ⁵⁷Fe^II is incorporated into the crystal structure of the substrate, in broad agreement with the thermodynamic model, but is almost completely oxidized to ⁵⁷Fe^III. The mechanism by which the adsorbed ⁵⁷Fe^II is oxidized is not resolved in this work, but is thought to be due to electron transfer to the substrate, rather than a net oxidation of the suspension. The disagreement between experimental and calculated rest potential measurements in the goethite and lepidocrocite systems is thought to be due to the poor electrochemical equilibration of these suspensions with the platinum electrode, rather than a failure of the thermodynamic model. The model developed for the redox potential of adsorbed Fe^II allows direct assessment of the reactivity of this species towards oxidized pollutants.     

Ferric iron in orthopyroxene: a Mössbauer spectroscopic study

Ferric iron solid solution in synthetic orthopyroxene has been studied along the joins MgSiO₃-Al₂O₃ · Fe₂O₃ and MgSiO₃-Fe₂O₃. The partially reduced synthetic orthopyroxenes showed that major incorporation of ferric iron can only occur together with a concomitant incorporation of Al. Maximum solid solution of ferric iron along the join MgSiO₃-Fe₂O₃ was found to be only 0.63 wt% Fe₂O₃ at 1000°C and 2 kb total pressure. From the observed Mössbauer parameters octahedral ferric iron can be assigned to the MI position in orthopyroxene. Incorporation of Fe³⁺ and/or Al will increase the disorder of Fe²⁺ and Mg between the M1 and M2 sites, which is also observed in a ferric iron-containing aluminous orthopyroxene of metamorphic origin. In the assemblage orthopyroxene + sillimanite + quartz the ferric iron content of orthopyroxene is directly related to oxygen fugacity.     

The effect of As, Co, and Ni impurities on pyrite oxidation kinetics: Batch and flow-through reactor experiments with synthetic pyrite

Pyrite samples synthesized with As, Co, or Ni impurities and without added impurities were oxidized in batch and mixed flow-through reactors in the presence of 1 mM ferric iron, at pH 2. Six samples from each dopant population were used to provide a statistically robust comparison; two natural samples from Leadville, CO (major impurities Pb, As, Bi, Ag, Zn) and Elba, Italy (Co, As) were also included. In each experiment, three reaction progress variables were monitored: ferric iron, ferrous iron, and sulfate. The pyrite samples with impurities have average oxidation rates that are faster than the undoped samples, with As- and Co-doped pyrite having the highest rates. As, Co, and Ni were released to solution in accordance with their concentrations in the solid samples. As concentrations in the batch reactor experiments tended to remain constant, in contrast to Co and Ni, which increased over time. Initial rates, calculated from the batch reactor experiments, were faster than the steady-state rates calculated from the mixed flow-through reactor experiments. Apparent rates calculated using sulfate were faster than apparent rates calculated using ferric and ferrous iron, reflecting oxidation of ferrous iron in solution by dissolved oxygen. The results imply that impurities in pyrite do contribute to its reactivity, in agreement with studies using electrochemical methods. Oxidation rate differences among pyrite samples with different impurities are probably too small to warrant explicit consideration in environmental modeling applications, but are important to understanding pyrite oxidation mechanisms and semiconducting properties.     

An electrochemical study of pyrite oxidation in the presence of carbon coatings in cyanide medium

The anodic and cathodic behaviour of pyrite with clay and different carbon coatings of activated carbon, graphite and carbonaceous matter in cyanide medium was investigated using the potentiodynamic method. The presence of clay coating did not change the polarisation curve appearance for either the anodic oxidation of pyrite or the cathodic reduction of oxygen or the potential of the current plateau, but only decreased the plateau current especially at a higher coating thickness. The presence of the carbon coatings marginally shifted the rest potential for pyrite to a more anodic position and slightly changed the polarisation curve appearance for pyrite oxidation. The current density for pyrite oxidation largely increased in the presence of the carbon coatings, the potential at the plateau shifted to more cathodic positions, and the plateau width became smaller. These effects became more noticeable at a higher coating thickness. The activated carbon, graphite and carbonaceous matter coatings performed similarly in affecting pyrite oxidation at a similar thickness. The carbon coatings significantly increased the limiting current densities for oxygen reduction on pyrite, and the limiting current plateau became steeper at a higher coating thickness. The carbon coatings increased the limiting current density for oxygen reduction to a similar extent at a low coating thickness, but increased to varied extents at a higher coating thickness. The carbon coatings also greatly increased the cathodic current density for gold reduction on pyrite. The enhancement of pyrite oxidation and oxygen or gold reduction on pyrite by the carbon coatings was likely attributed to the electrochemical interaction between pyrite and the carbon materials with electron-rich surfaces and high conductivity. The presence of the carbon coatings significantly increased the oxidation of pyrite in aerated cyanide solutions and the preg-robbing of pyrite especially at a higher coating thickness.     

The crystal chemistry of pumpellyite: An X-ray Rietveld refinement and 57Fe Mössbauer study

Pumpellyite of the general formula W₈X₄Y₈-Z₁₂O_56-n(OH)_n contains Fe, Al and Mg in two crystallographically different octahedral sites. Three different pumpellyite samples covering the known compositional field from Al- to Fe-rich have been studied to determine the valence state and intracrystalline partitioning of the Fe cations between the two independent octahedral sites. Fe⁺² and Fe⁺³ cation partitioning is interpreted on the basis of results obtained by ⁵⁷Fe Mössbauer spectroscopy at 293 and 77 K and from Rietveld structure analysis performed on powder X-ray diffraction data. Pumpellyite from low-grade metamorphic rocks typically contains a majority of iron in the Fe⁺³ oxidation state, which is found in the smaller and less symmetrical octahedral Y-site. Fe⁺² was also present in all pumpellyite samples studied and it is located in the larger and more symmetrical octahedral X-site.     

Characterization of Fe(III) (hydr)oxides in arsenic contaminated soil under various redox conditions by XAFS and Mössbauer spectroscopies

Characterization of Fe(III) (hydr)oxides in soils near the Ichinokawa mine was conducted using X-ray absorption fine structure (XAFS) and Mössbauer spectroscopies, and the structural changes were correlated with the release of As into pore-water. The Eh values decreased monotonically with depth. Iron is mainly present as poorly-ordered Fe(III) (hydr)oxides, such as ferrihydrite, over a wide redox range (from Eh = 360 to −140 mV). Structural details of the short-range order of these Fe(III) (hydr)oxides were examined using Mössbauer spectroscopy by comparing the soil phases with synthesized ferrihydrite samples having varying crystallinities. The crystallinity of the soil Fe (hydr)oxides decreased slightly with depth and Eh. Thus, within the redox range of this soil profile, ferrihydrite dominated, even under very reducing conditions, but the crystalline domain size, and, potentially, particle size, changed with the variation in Eh. In the soil–water system examined here, where As concentration and the As(III)/As(V) ratio in soil water increased with depth, ferrihydrite persisted and maintained or even enhanced its capacity for As retention with increased reducing conditions. Therefore, it is concluded that As release from these soils largely depends on the transformation of As(V) to As(III) rather than reductive dissolution of Fe(III) (hydr)oxide.     

Iron in silicate glasses of granitic composition: a Mössbauer spectroscopic study

⁵⁷Fe Mössbauer spectra of natural glasses (pumices and obsidians) and of synthetic glasses of granitic composition have been analyzed. — Ferric iron is found in tetrahedral coordination if enough M⁺-cations are available to balance the charge of both M⁺Fe³⁺O₂ and M⁺AlO₂ complexes. In other compositions the ratio of tetrahedrally to octahedrally coordinated Fe³⁺ depends on the ratio of mono-to divalent cations. — Ferrous iron occurs in two distinctly different octahedral sites. The existence of these sites can be attributed to different anionic units adjacent to Fe²⁺. The degree of polymerization of these units is reflected in the quadrupole splitting. The anionic units adjacent to Fe²⁺ are depolymerized for increasing mean Z/r ² of the network modifiers, which do not stabilize M³⁺ in the tetrahedra by local charge balance. — Increasing pressure diminishes the geometric differences between these types of ferrous iron-oxygen-octahedra, which gives rise to a more even distribution of Fe²⁺ among these sites and thereby to an ordering in the network of melts.     

The distribution of Fe2+ and Fe3+ in iron-bearing tourmalines: A Mössbauer study

Summary Room-temperature Mössbauer spectra of five iron-bearing tourmalines were measured and analyzed. The Fe²⁺/Fe³⁺ ratio and the iron occupancy of the Y and Z positions could be assigned to all samples, with the help of two previously well characterized samples, from Mexico and Madagascar. Ferric or ferrous ions or both partially occupy the Z as well as the Y octahedra. This fact of observation is interpreted as the chemical response, during crystal growth, to the requirement of size matching for the edge-sharing Y and Z oxygen octahedra. It accounts for the inexistence of solid solution between the Mg and (Li, Al) tourmalines.

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Die Verteilung von Fe²⁺ und Fe³⁺ in eisenhaltigen Turmalinen: Eine Mössbauer-Untersuchung

Zusammenfassung Mössbauer-Spektren von fünf eisenhaltigen Turmalinen wurden gemessen und analysiert. Das Verhältnis Fe²⁺/Fe³⁺ und die Eisenverteilung konnten mit Hilfe von zwei gut identifizierten Turmalin-Kristallen von Mexiko und Madagascar für die Y-und Z-Lagen aller Exemplare bestimmt werden. Zweiwertiges sowie dreiwertiges Eisen findet sich sowohl in der Z-als auch in der Y-Lage. Da sich die Y-und Z-Oktaeder in einer gemeinsamen Kante treffen, wird diese Beobachtung als chemische Antwort des Kristalles auf die erforderte Größenanpassung der Y-und Z-Oktaeder während seines Wachstums erklärt. Die Abwesenheit der festen Lösung zwischen Dravit und Elbait kann somit erklärt werden.

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With 4 Figures     

Sites occupancy of Fe3+ in Garfield Nontronite: A spectroscopic study

New data on the structure of Garfield nontronite have been produced by the use of different spectroscopic techniques: Mössbauer spectroscopy, optical spectroscopy, X-ray absorption edges and EXAFS and NMR. The tetrahedral iron content is found to be no higher than 1 percent. All iron atoms belong to the octahedral sheet excluding the possibility of the presence of non crystallized phases. Some ambiguities remain about the coherence of the octahedral sheet because of the presence of two doublets in the Mössbauer spectrum and at least two lineshapes in NMR spectra of OH which correspond to different environments.     

A statistical model to relate pyrite oxidation and oxygen transport within a coal waste pile: case study,Alborz Sharghi,northeast of Iran

This paper presents a statistical model for predicting pyrite fraction remaining in a coal waste pile at the Alborz Sharghi coal mine located at northeast of Iran. This model calculates the fraction of pyrite remaining using mole fraction of oxygen diffused into the pore spaces of the pile and the pile depth. Comparison of the statistical outputs revealed that a second-order polynomial expression with respect to oxygen mole fraction and depth provides a better correspondence to the field measurements for the fraction of pyrite remaining with a RMSE of 0.089231. Besides, two statistical relationships have been proposed for the remaining pyrite fraction and the mole fraction of oxygen versus the pile depth. A quadratic polynomial shows the best correlation of the field measured data. The suggested models were successfully validated with the acceptable confidence levels of 92 and 90 % for remaining pyrite and oxygen using a new data set which revealed the fact that they can be applied in similar situations. Both statistical analysis and field data indicate that the pyrite oxidation process is limited to the shallower depths of the waste pile where the mole fraction of oxygen decreased rapidly.     

The oxidation of pyrite at pH 7 in the presence of reducing and nonreducing Fe(III)-chelators

The oxidation rate of pyrite at pH 7, 25°C and at constant partial pressure of oxygen (0.21 and 0.177 atm) was measured in the presence of the Fe(III)-chelators NTA, oxalate, leucine, EDTA, citrate, IDA and the Fe(III)-reductant ascorbic acid. With the exception of leucine and EDTA, non-reducing Fe(III)-chelators increased the oxidation rate relative to the reference state of formation of the Fe(OH)₂⁺ complex at pH 7. The rate increase was proportional to the logarithm of the conditional stability constant of the ligands for the complexation of Fe³⁺. No effect on the oxidation rate was observed in the presence of EDTA, which shifted the redox potential of the redox couple Fe²⁺/Fe³⁺ to a value below that in the absence of any ligand at pH 7. Ascorbic acid decreased the pyrite oxidation rate by a factor of 5 at ascorbic acid concentrations between 10⁻⁴ and 10⁻² mol L⁻¹. Comparison of the rate constants for the oxidation of ascorbic acid by surface bound Fe(III) in the absence and presence of pyrite shows that the pyrite surface accelerates this reaction by a factor of 10. The oxidation of both pyrite and ascorbic acid is of fractional order with respect to ascorbic acid (HAsc): r_py=0.55 c(HAsc)^−0.35 r_HAsc=3.6 c(HAsc)^0.59. Both the results from experiments with Fe(III)-chelating ligands and the Fe(III)-reductant, suggest a very efficient interference in the electron cycling between Fe(II) and Fe(III) at the pyrite surface. The interference seems to be mainly related to the reductive side of the iron cycling. It is therefore concluded that the electron transfer between ferric iron and pyritic sulfur limits the pyrite oxidation rate at pH 7.     

Natural attenuation of TCE, As, Hg linked to the heterogeneous oxidation of Fe(II): an AFM study

Hydrous ferric oxide (HFO) colloids formed, in strictly anoxic conditions upon oxidation of Fe²⁺ ions adsorbed on mineral surface, were investigated under in situ conditions by contact mode atomic force microscopy (AFM). Freshly cleaved and acid-etched large single crystals of near endmember phlogopite were pre-equilibrated with dissolved Fe(II) and then reacted with Hg(II), As(V) and trichlorethene (TCE)-bearing solutions at 25 °C and 1 atm. HFO structures are found to be of nanometer scale. The As(V)–Fe(II) and Hg(II)–Fe(II) reaction products are round (25 nm) microcrystallites located predominantly on the layer edges and are indicative of an accelerated Fe(II) oxidation rate upon formation of Fe(II) inner sphere surface complexes with the phyllosilicate edge surface sites. On the other hand, TCE–Fe(II)–phlogopite reaction products are needle-shaped (45 nm long) particles located on the basal plane along the Periodic Bond Chains (PCBs) directions. Experiments with additions of sodium chloride confirm the importance of the Fe(II) adsorption step in the control of the overall heterogeneous Fe(II)–TCE electron transfer reaction.

Kinetic measurements at the nanomolar level of Hg° formed upon reduction of Hg(II) by Fe(II) in presence of phlogopite particles provide further convincing evidence for reduction of Hg(II)_aq coupled to the oxidation of Fe(II) adsorbed at the phlogopite–fluid interface, and indicate that sorption of Fe(II) to mineral surfaces enhances the reduction rate of Hg(II) species. The Hg(II) reduction reaction follows a first-order kinetic law. Under our experimental conditions, which were representative of many natural systems, 80% of the mercury is transferred to the atmosphere as Hg° in less than 2 h.

The reduction of a heavy metal (Hg), a toxic oxyanion (arsenate ion) and a chlorinated solvent (TCE) thus appear to be driven by the high reactivity of adsorbed Fe(II). This is of environmental relevance since these three priority pollutants are that way reductively transformed to a volatile, an immobilizable and a biodegradable species, respectively. Such kinetic data and reaction pathways are important in the evaluation of natural evaluation scenarios, in the optimization of Fe(II)/mineral mixtures as reductants in technical systems, and in general, in predicting the fate and transport of pollutants in natural systems.     

Equilibrium iron isotope fractionation factors of minerals: Reevaluation from the data of nuclear inelastic resonant X-ray scattering and Mössbauer spectroscopy

We have critically reevaluated equilibrium iron isotope fractionation factors for oxide and sulfide minerals using recently acquired data obtained by Mössbauer spectroscopy and inelastic nuclear resonant X-ray scattering (INRXS) synchrotron radiation. Good agreement was observed in the iron β-factors of metallic iron (α-Fe) and hematite calculated using both Mössbauer- and INRXS-derived data, which supports the validity and reliability of the calculations. Based on this excellent agreement, we suggest the use of the present data on the iron β-factors of hematite as a reference.The previous Mössbauer-derived iron β-factor for magnetite has been modified significantly based on the Fe-sublattice density of states obtained from the INRXS experiments. This resolves the disagreement between naturally observed iron isotope fractionation factors for mineral pairs involving magnetite and those obtained from the calculated β-factors. The correctness of iron β-factor for pyrite has been corroborated by the good agreement with experimental data of sulfur isotope geothermometers of pyrite-galena and pyrite-sphalerite. A good correlation between the potential energy of the cation site, the oxidation state of iron and the iron β-factor value has been established. Specifically, ferric compounds, which have a higher potential energy of iron than ferrous compounds, have higher β-factors. A similar dependence of β-factors on the oxidation state and potential energy could be extended to other transition metals. Extremely low values of INRXS-derived iron β-factors for troilite and Fe₃S significantly widen the range of iron β-factors for covalently bonded compounds.     

In situ Mössbauer spectroscopy: Evidence for green rust (fougerite) in a gleysol and its mineralogical transformations with time and depth

A miniaturized Mössbauer spectrometer, adapted to the Earth’s conditions from the instrument developed for Mars space missions, has been used for the first time to study in situ variations with depth and transformations with time of iron minerals in a gleysol. The instrument is set into a PVC tube and can be moved up and down precisely (±1 mm) at the desired depth. Mössbauer spectra were obtained from 15 to 106 cm depth and repeated exactly at the same point at different times to follow mineralogical transformations with time. X-ray diffraction (XRD) and selective extraction techniques were performed on soil samples. The piezometric level of the water table was measured and the composition of the soil solution was monitored in situ and continuously, with a multiparametric and automatic probe. All the Mössbauer spectra obtained are characteristic of Fe(II)-Fe(III) green rust-fougerite, a natural mineral of the meixnerite group, that is, whose structural formula is: [Fe_{1 − x}^II Mg_y Fe_x^III (OH)_2+2y]^x+[xA, mH₂O]^x−, where x is the ratio Fe³⁺/Fe_tot. and A the intercalated anion. The name of fougerite has been formally approved by the Commission on New Minerals and Mineral Names of IMA (number 2003-057), on January 29, 2004. No other iron phases have been found by this way or by XRD. About 90% of total iron is extractible by dithionite-citrate-bicarbonate, and 60% by citratebicarbonate. In the horizons showing oximorphic properties that are in the upper part of the studied soil profile, x ratio in fougerite, deduced from Mössbauer spectra, is approximately 2/3. In the deepest horizons that show reductomorphic properties, x ratio is only 1/3. Fast mineralogical transformations were observed at well-defined points in soil, as evidenced by x ratio variations observed when Mössbauer spectra were acquired at different times at the same depth. Variations of the level of the water table and of pe and pH of the soil solution were simultaneously observed and could explain these mineralogical transformations. A ternary solid solution model previously proposed for OH-fougerite has been extended to chloride, sulphate, and carbonate green rusts to estimate the Gibbs free energies of formation of fougerite, providing for possible anions other than OH⁻ in the interlayer and for Mg substitution. Soil solutions appear as largely oversaturated with respect to OH-fougerite, either oversaturated or undersaturated to “carbonate-fougerite” and “sulphate-fougerite”, and largely undersaturated with respect to “chloro-fougerite”. Fougerite forms most likely from oversaturated solutions by coprecipitation of Fe³⁺ with Fe²⁺ and Mg²⁺. Oxidation and reduction are driven by pH and pe variations, with both long timescale variations and short duration events. Exactly as synthetic green rusts are very reactive compounds in the laboratory, fougerite is thus a very reactive mineral and readily forms, dissolves, or evolves in soils.     

黔西南高砷煤中砷赋存状态的XAFS和铁的Moessbauer谱研究

利用X射线吸收精细结构分析(XAFS）和铁的穆斯堡尔谱(Moessbauer)对黔西南高砷煤中砷和铁的存在形式进行了研究。研究发现，高砷煤中的砷主要以高价砷的形式存在，也有少量以As2O3、砷黄铁矿、砷硫化物的形式存在。除1个样品中的铁全部以顺磁性针铁矿或超顺磁性的针铁矿的形式存在外，其它样品中的Fe主要是黄铁矿中的Fe，约占全铁的60％一91％；其次是黄铁钾矾中的Fe，约占全铁的9％一40％。     

Enrichment of As and S in the finest fractions of crushed rock samples — an experimental study simulating the formation of till

To simulate the behavior of As and S in the formation of till, the contents of these elements were determined in the total material, the −0.064 mm fraction and the 0.064–0.5 mm fraction of crushed rock samples. For most of the studied rock types there was a conspicious enrichment of As and a much less prominent enrichment of S in the finest fraction. Accordingly, in the formation of till the mere mechanical crushing of rock material can create enrichment of the derived till.     

The structural state of iron in oxidized vs. reduced glasses at 1 atm: A57Fe Mössbauer study

A general model for the structural state of iron in a variety of silicate and aluminosilicate glass compositions in the systems Na₂O-Al₂O₃-SiO₂-Fe-O, CaO-Al₂O₃-SiO₂-Fe-O, and MgO-Al₂O₃-SiO₂-Fe-O is proposed. Quenched melts with variable Al/Si and NBO/T (average number of nonbridging oxygens per tetrahedrally coordinated cation), synthesized over a range of temperatures and values of oxygen fugacity, are analyzed with⁵⁷Fe Mössbauer spectroscopy. For oxidized glasses with Fe³⁺/∑Fe>0.50, the isomer shift for Fe³⁺ is in the range ～0.22–0.33 mm/s and ～0.36 mm/s at 298 K and 77 K, respectively. These values are indicative of tetrahedrally coordinated Fe^3?. This assignment is in agreement with the interpretation of Raman, luminescence, and X-ray,K-edge absorption spectra. The values of the quadrupole splitting are ～0.90 mm/s (298 K and 77 K) in the Na-aluminosilicate glasses and compare with the values of 1.3 mm/s and 1.5 mm/s for the analogous Ca- and Mg-aluminosilicate compositions. The variations in quadrupole splittings for Fe³⁺ are due to differences in the degree of distortion of the tetrahedrally coordinated site in each of the systems. The values of the isomer shifts for Fe²⁺ ions in glasses irrespective of Fe³⁺/∑Fe are in the range 0.90–1.06 mm/s at 298 K and 1.0–1.15 mm/s at 77 K. The corresponding range of values of the quadrupole splitting is 1.75–2.10 mm/s at 298 K and 2.00–2.35 mm/s at 77 K. The temperature dependence of the hyperfine parameters for Fe²⁺ is indicative of noninteracting ions, but the values of the isomer shift are intermediate between those values normally attributable to tetrahedrally and octahedrally coordinated Fe²⁺. The assignment of the isomer-shift values of Fe²⁺ to octahedral coordination is in agreement with the results of other spectral studies. For reduced glasses (Fe³⁺/∑Fe≈<0.50), the value of the isomer shift for Fe³⁺ at both 298 K and 77 K increases and is linearly correlated with decreasing Fe³⁺/∑Fe in the range of \(f_{O_2 } \) between 10^?3 and 10^?6 atm when a single quadrupole-split doublet is assumed to represent the absorption due to ferric iron. The increase in value of the isomer shift with decreasing \(f_{O_2 } \) is consistent with an increase in the proportion of Fe³⁺ ions that are octahedrally coordinated. The concentration of octahedral Fe³⁺ is dependent on the \(T - f_{O_2 } \) conditions, and in the range of log \(f_{O_2 } \) between 10^?2.0 and 10^?5 a significant proportion of the iron may occur as iron-rich structural units with stoichiometry similar to that of inverse spinels such as Fe₃O₄, in addition to isolated Fe²⁺ and Fe³⁺ ions.