Single crystal Mössbauer studies of 1M biotite

Electric field gradient (EFG) measurements for Fe²⁺ in the cis- and trans-sites of a large single crystal of trioctahedral biotite of the lM polytype are described. Attempts to apply thickness and polarisation corrections are outlined. The measured EFG's are distinctly rhombic: = 0.28 and 0.42 respectively for the cis- and transsites and the EFG principal directions lie approximately along the crystallographic axes. For the cis-site V _y lies close to the normal to the basal plane while for the transsite lies close to this direction.     

Mössbauer studies of marine and fresh water manganese nodules

Practically identical Mössbauer spectra have been obtained for 40 ferromanganese nodules from a wide variety of marine and fresh-water locations. None of the nodules examined contains more than one weight percent Fe²⁺, so no more than a few percent of the total iron in these nodules can be Fe²⁺. Most of the iron is present as Fe³⁺ in paramagnetic or superparamagnetic oxide phases, although hysteresis loops show the presence of small amounts of ferromagnetic phases not detected by the Mössbauer technique.     

Electric-field gradient and mean-squared-displacement tensors in hedenbergite from single-crystal Mössbauer milliprobe measurements

We report Mössbauer milliprobe measurements on small single-crystals of a magnesium-rich hedenbergite, approximate composition CaFe_0.54Mg_0.46 (SiO₃)₂, in which each of the electric-field gradient and mean-squared displacement tensors for Fe²⁺ in the M1 site of the crystal are precisely determined. Each tensor has in common, as required of crystal symmetry, the twofold axis of the monoclinic unit cell, but the principal directions of the two tensors in the perpendicular plane are non-coincident. The mean-squared displacements determined in the Mössbauer experiment exceed those determined from the X-ray vibration ellipsoids for Fe²⁺/M1 by a factor of 1.6; the anisotropy in the mean-squared displacement tensor from the Mössbauer measurements exceeds that from X-ray by a factor of around 5. The ramifications of these differences are discussed.     

Ligand field analysis of the magnetic susceptibilities and Mössbauer spectra of ilvaite,a mixed valence iron mineral

The thermal characteristics of magnetic susceptibilities and their anisotropies of single crystal of ilvaite, a mixed valence iron sorosilicate, have been analysed in the light of Ligand Field (LF) theory using a minimum number of approximations. The electronic energy pattern and the corresponding wavefunctions were obtained from best fitting of these experimental results with the corresponding theoretical values. These results were next used to calculate the thermal characteristic of the quadrupole splitting E _Q in ilvaite. It was found that the values of E _Q are reasonably close to those reported from Mössbauer studies. The present analysis suggest appreciable molecular overlap between the orbits of the ligand electrons and those of the Fe²⁺ atoms, the overlap being prominent along the chain direction as also observed from electrical conductivity measurements.     

Meteoritic Fe-Ni alloys: A review of 57Fe Mössbauer spectroscopy studies

Discovered by Rudolph L. Mössbauer in 1957, the Mössbauer effect (i.e. gamma-resonance spectroscopy) is the phenomenon of the emission or absorption of a gamma ray without loss of energy due to recoil of the nucleus and without thermal broadening. This technique has been applied to many science fields (e.g., physics, chemistry, geology, biology), since it provides information about the nuclear and electronic properties of materials. In this paper, a review of works focusing on the application of ⁵⁷Fe Mössbauer spectroscopy study of the meteoritic Fe-Ni system will be reported.     

57Fe Mössbauer spectroscopy of tektites

Tektite glasses are investigated using ⁵⁷Fe Mössbauer spectroscopy. Room temperature spectra analysis is performed using two complementary analytical methods based on two-dimensional distributions of both isomer shift and quadrupole splitting. No a priori correlation between the two hyperfine parameters is considered. The first method, based on a shape independent distribution, provides the justification for the Gaussian distribution shape used in the second method. No ferric iron contribution is evidenced by Mössbauer spectra analysis in these samples, although several criteria are used. Ferrous iron sites are shown to be continuously distributed between four- and five-fold co-ordinated sites.     

Mössbauer and molecular orbital study of chlorites

The different Fe²⁺ lattice sites in iron-rich chlorites have been characterized by Mössbauer spectroscopy and molecular orbital calculations in local density approximation. The Mössbauer measurements were recorded at 77?K within a small velocity range (±3.5?mm?s^?1) to provide high energy resolution. Additionally, measurements were recorded in a wider velocity range (±10.5?mm?s^?1) at temperatures of 140, 200, and 250?K in an applied field (7?T) parallel to the γ-beam. The zero-field spectra were analyzed with discrete Lorentzian-shaped quadrupole doublets to account for the Fe²⁺ sites M1, M2, and M3 and with a quadrupole distribution for Fe³⁺ sites. Such a procedure is justified by the results obtained from MO calculations, which reveal that different anion (OH^?) distributions in the first coordination sphere of M1, M2, and M3 positions have more influence on the Fe²⁺ quadrupole splitting than cationic disorder. The spectra recorded in applied field were analyzed in the spin-Hamiltonian approximation, yielding a negative sign for the electric field gradient (efg) of Fe²⁺ in the M1, M2, and M3 positions. The results of the MO calculations are in quantitative agreement with experiment and reveal that differences in the quadrupole splittings (ΔE _Q ), their temperature dependence and in the isomer shifts (δ) of Fe²⁺ in M1, M2, and M3 positions can theoretically by justified. Therefore, the combined Mössbauer and MO investigation shows that the three Fe²⁺ lattice sites in the chlorites investigated here can be discriminated according to their ΔE _Q -δ parameter pairs. With the calculated average iron-oxygen bond strength, the MO study provides an explanation for the observed trend that the population of the three lattice sites by Fe²⁺ increases according to the relation M1?相似文献   

Evaluation of ferrous and ferric Mössbauer fractions

The Mössbauer fractions f for various ferrous- and/or ferric-containing oxides and oxyhydroxides, silicates and carbonates were evaluated from the experimental temperature dependence of their center shifts, using the Debye approximation for the second-order Doppler shift. It is concluded that ferrous ions exhibit a lower fraction as compared to ferric ions. Using standard mixtures of -Fe₂O₃ with selected Fe²⁺ or Fe³⁺ compounds, it is found that the calculated Fe³⁺ f values are somewhat overestimated with respect to those of Fe²⁺. Possible explanations for this shortcoming are discussed and it is suggested that a different temperature dependence of the intrinsic isomer shift is the most likely reason. This suggestion is corroborated by analyses of hematite and hedenbergite data which are available for temperatures up to 900 K and 800 K respectively.     

Voltammetric characterization of Boda Albitic Claystone. Comparison with previous Mössbauer spectroscopy results

We used ‘Voltammetry of MicroParticles’ to investigate electrochemically samples from the Boda Albitic Claystone, a potential site for deep underground disposal of nuclear waste in Hungary. The samples were collected near the surface and at a depth of 1050 m. The electrochemical characterization in acidic media allowed the identification of Fe²⁺ and Fe³⁺ ions associated with clays, hematite and pyrite. The cathodic detection of pyrite does not raise special difficulties, but the anodic one is complex and depends on the time of the samples exposure to the atmosphere. This work completes a previous Mössbauer spectroscopy study by clarifying the presence, or absence, of extremely small amounts of pyrite in some samples.     

Ferric iron content of mineral inclusions in diamonds from George Creek, Colorado determined using Mössbauer spectroscopy

The Mössbauer milliprobe allows the determination of Fe³⁺/ΣFe in samples as small as 50?μm. For the first time this technique is applied to a suite of diamonds of eclogitic paragenesis, where three garnet and five clinopyroxene inclusions in diamonds from George Creek, Colorado have been analysed. For garnet Fe³⁺/ΣFe ranges from 0–7%, while values for clinopyroxene range from 8–14%. These results are consistent with the low oxygen fugacity conditions implied by the presence of the inclusions in diamond.     

Advanced analyses of 57Fe Mössbauer data of alumino-silicate glasses

⁵⁷Fe Mössbauer spectra of iron bearing alumino-silicate glasses are analysed by two complementary methods (SID and x-VBF) especially adapted for the analysis of disordered systems by taking into account distributions of hyperfine Mössbauer parameters. Qualitative and quantitative information about the oxidation state of iron are obtained as well as information about the distribution of local environments of iron. The possibility to separate the signal of ferric iron from that of ferrous iron allows to derive precise redox ratio in favourable cases but also to analyse more sharply the different contributions to Mössbauer spectra. Using two different glass series (feldspar composition, haplo-tonalitic composition), the characteristics of the two methods are described and employed to study the effect of composition, water incorporation and oxidation state on the glass structure. Optical absorption spectroscopy is used to support the interpretation of the Mössbauer spectra in case of the feldspar glasses.     

Characterization of a chalcopyrite from Brazil by Mössbauer spectroscopy and other physicochemical techniques

We studied a chalcopyrite from a Cu ore deposit in Rio Grande do Sul, Brazil, by Mössbauer spectroscopy at room temperature and 110 K. Supporting methods to check for sample purity and to characterize further sample properties were slow-scanning X-ray powder diffraction and optical and microprobe analyses of polished sections of selected grains. Chemical analyses obtained using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer showed the sample to consist of homogeneous and essentially stoichiometric chalcopyrite (CuFeS₂). Mössbauer spectra taken at both the above temperatures consist of asymmetric magnetically ordered patterns with unequal intensities of the line pairs 1–6 and 2–5, pointing to the existence of non-equivalent or multiple Fe sites. Least-squares fittings evidenced that the resonance intensity ratio of subspectrum with lower quadrupole shift (indicative of a more symmetric environment) to that with higher shift is 69:31, at room temperature, and 68:32, at 110 K. Rietveld refinement of the XRD data indicates the existence of tetragonal [a = 0.52855(1) and c = 1.0412(1) nm] and cubic [a ₀ = 0.5273(2) nm] modifications in a proportion of 74:26, in good agreement with the Mössbauer data. The saturation magnetization of the sample was 32.7 J/(T kg), confirming the oxidation state of Fe as trivalent and pointing to little to no spin canting.     

Mössbauer spectroscopic study of iron in Victorian brown coal

The forms of non-pyritic Fe in a suite of Victorian brown coals have been determined by ⁵⁷Fe Mössbauer analysis. The dominant Fe phase is a poorly-ordered ferric oxyhydroxide with a magnetic ordering temperature of (530 ± 50) K and particle size of approximately 50 Å. Upon exposure of the coal to air, this phase slowly crystallises to goethite. Most of the remaining Fe occurs as a high-spin Fe(II) species attributed to dissolved and hence mobile, Fe(II) humate, which precipitates as the ferric oxyhydroxide to an extent determined by the pH. A third species, present in a much lower concentration, appears to exhibit a transition from low-spin to high-spin Fe(II) as water is removed from the coal.     

Optical and Mössbauer spectroscopy of iron in rock-forming silicates

The inner nebula out to ~3 A.U. was depleted in volatile elements that included potassium and manganese at a very early stage of solar-system history. The inner planets and many meteorites inherited this element signature, the cause of which probably was early violent solar activity. Because of this evidence for elemental depletions correlated with volatility, one might also expect to find examples of fractionation, particularly among lower mass elements. Here we discuss the search for such effects among the isotopes of K, Mg, Si, and Ca in a wide variety of terrestrial, lunar, and meteoritic samples. We examine examples of vaporization without isotope fractionation, and a comparison of the effects expected between distillation and condensation. Effects attributable both to evaporation and condensation are observed in refractory inclusions (CAIs) in meteorites and reflect localized events in the early nebula. However, the lack of isotopic fractionation that is observed among a wider variety of presolar-system materials rules out the general operation of Rayleigh-type fractionation on primitive solar-nebular material. We conclude with a discussion of volatileelement behavior during the giant Moon-forming impact that shows that the material in the Moon was not subjected to Rayleigh-type distillation.     

Electronic and magnetic properties of a natural aegirine as observed from its Mössbauer spectra

A natural sample of aegirine, ideally NaFeSi₂O₆, has been studied by transmission Mössbauer spectroscopy in the range 4.2–480?K. At selected temperatures, a longitudinal external field of 60?kOe was applied to the absorber. The sample was observed to order magnetically at 11±1?K. The paramagnetic Mössbauer spectra (MS) show the presence of ～10% Fe²⁺ in the M1 sites of the clinopyroxene structure. These MS have been decomposed into four quadrupole doublets: two minor ones for Fe²⁺ on M1 sites, a dominant one due to Fe³⁺ on M1 sites, and a second ferric component, with a contribution of ～3% and attributable to the tetrahedral sites. Two possibilities concerning the origin of the two distinct Fe²⁺ (M1) doublets are discussed. They are respectively based on inter-valence charge transfer and on the existence of distinct Fe²⁺ orbital configurations at the two M1 sites. Neither of the two models could be firmly excluded. The asymmetry parameter η of the electric field gradient at the Fe³⁺ (M1) sites is close to 1.0 and the quadrupole splitting within 0.34±0.01?mm/s at all temperatures. The MS at 4.2?K shows an asymmetric hyperfine-field distribution for Fe³⁺, with a maximum-probability field of 468?kOe. The maximum-probability field for Fe²⁺ is found to be 220?kOe. The shape of the applied-field MS at 4.2?K implies a static antiferromagnetic ordering and was successfully interpreted by a bidimensional distribution of the magnitude and orientation of the hyperfine field. Finally, the temperature variations of the respective centre shifts and quadrupole splittings could be explained on the basis of existing theoretical models.     

Effect of γ-irradiation on clays and organoclays: a Mössbauer and XRD study

The interaction of γ-rays with smectite clays induces noticeable changes in the structure and physicochemical properties of the minerals. For sodium-montmorillonite, Mössbauer spectra show that γ-irradiation causes a partial reduction of trivalent iron to the divalent state due to hydrogen radicals production from the radiolysis of interlayer water. The XRD results show no change in the interlayer space upon irradiation and therefore the radiolysis of interlayer water causes no measurable changes in the architectural organization of the interlayer environment. Intercalation of simple organic molecules (ethylene glycol, tert-butanol and tetraalkylammonium salts) causes partial oxidation of structural divalent iron and affects the Mössbauer parameters of the M₁ and M₂ ferric components. Irradiation of ethylene glycol- or tert-butanol-clay composites indicates reduction of trivalent iron to the divalent state. XRD data show that the irradiation of clay-ethylene glycol complex causes collapse of the initial double layer of glycol molecules to a single layer complex. Finally, XRD results show that the effects of γ-irradiation on clay-tetraalkylammonium complexes depend upon the chain length of the organic cations.     

Evaluation of ferrous and ferric Mössbauer fractions. Part II

Mössbauer fractions f are reported for various ferrous- and/or ferric-containing oxides, hydroxides, silicates, and phosphates to extend the list previously reported by De Grave and Van Alboom (1991). The f fractions were evaluated from the experimental temperature dependencies of their center shifts, assuming the Debye model for the lattice vibrations. For most Fe²⁺ sites the characteristic Mössbauer or lattice temperatures Θ_M are in the range 300–400 K, while those for Fe³⁺ sites are close to or exceed 500 K, implying significantly higher f fractions for Fe³⁺ than for Fe²⁺, in particular at room temperature. A correlation between Θ_M and the coordination type, or, for a given valence state and coordination type, between Θ_M and the mineral type is, however, not obvious.     

Mössbauer study of the thermal decomposition of lepidocrocite and characterization of the decomposition products

The lepidocrocite (-FeOOH) to maghemite (-Fe₂O₃), and the maghemite to hematite (-Fe₂O₃) transition temperatures have been monitored by TGA and DSC measurements for four initial -FeOOH samples with different particle sizes. The transition temperature of -FeOOH to -Fe₂O₃ and the size of the resulting particles were not affected by the particle size of the parent lepidocrocite. In contrast, the -Fe₂O₃ to -Fe₂O₃ transition temperature seems to depend on the amount of excess water molecules present in the parent lepidocrocite. Thirteen products obtained by heating for one hour at selected temperatures, were considered. Powder X-ray diffraction was used to qualify their composition and to determine their mean crystallite diameters. Transmission electron micrographs revealed the particle morphology. The Mössbauer spectra at 80 K and room temperature of the mixed and pure decomposition products generally had to be analyzed with a distribution of hyperfine fields and, where appropriate, with an additional quadrupole-splitting distribution. The Mössbauer spectra at variable temperature between 4.2 and 400 K of two single-phase -Fe₂O₃ samples with extremely small particles show the effect of superparamagnetism over a very broad temperature range. Only at the lowest temperatures (T55 K), two distributed components were resolved from the magnetically split spectra. In the external-field spectra the m_I=0 transitions have not vanished. This effect is an intrinsic property of the maghemite particles, indicating a strong spin canting with respect to the applied-field direction. The spectra are successfully reproduced using a bidimensional-distribution approach in which both the canting angle and the magnetic hyperfine field vary within certain intervals. The observed distributions are ascribed to the defect structure of the maghemites (unordered vacancy distribution on B-sites, large surface-to-bulk ratio, presence of OH^- groups). An important new finding is the correlation between the magnitude of the hyperfine field and the average canting angle for A-site ferric ions, whereas the B-site spins show a more uniform canting. The Mössbauer parameters of the two hematite samples with MCD₁₀₄ values of respectively 61.0 and 26.5 nm display a temperature variation which is very similar to that of small-particle hematites obtained from thermal decomposition of goethite. However, for a given MCD the Morin transition temperature for the latter samples is about 30 K lower. This has tentatively been ascribed to the different mechanisms of formation, presumably resulting in slightly larger lattice parameters for the hematite particles formed from goethite, thus shifting the Morin transition to lower temperatures.Senior Research Associate, National Fund for Scientific Research (Belgium)     

Activation energy of annealed metamict gadolinite from 57Fe Mössbauer spectroscopy

Gadolinite, REE₂FeBe₂Si₂O₁₀, is commonly metamict. ⁵⁷Fe Mössbauer annealing studies of fully metamict gadolinite from Ytterby, Sweden, have been completed in argon atmosphere from 873 to 1473 K. This technique has rarely been employed in studies of metamict minerals. Changes in the experimental parameters of Mössbauer spectra are sensitive indicators of the thermal recrystallization process of metamict gadolinite and revealed two stages of the structural recovery: a major stage from 873 to 1073 K and a slower recovery stage from 1133 to 1473 K. These observations are confirmed by X-ray powder diffraction. In relation to the first stage, the exponential behaviour of the changes in the Mössbauer parameters can be used for deriving the activation energy E _a of the recrystallization process. The calculated value E _a =1.97 eV in argon atmosphere explains the common occurrence of gadolinite in the fully or partially metamict state. Results of Mössbauer spectroscopy suggest that the recrystallization of metamict gadolinite is a displacive transition that involves rotation and translation of SiO₄ and BeO₄ to their normal positions associated with removal of OH groups from the structure.     

High-pressure single-crystal X-ray diffraction and synchrotron Mössbauer study of monoclinic ferrosilite

The phase and spin transitions in single-crystal monoclinic ferrosilite, FeSiO₃, were investigated using X-ray diffraction and Mössbauer spectroscopy up to lower-mantle pressures and room temperature in a helium pressure medium. Using single-crystal X-ray diffraction, we measured the equation of state of ferrosilite up to about 43 GPa. We observed a P2₁/c-to-C2/c phase transition between 1.5 and 1.7 GPa and a phase transition from C2/c to a distinct P2₁/c structure between 30 and 34 GPa. With time-domain Mössbauer spectroscopy, we determined the hyperfine parameters of ferrous iron up to 95 GPa. The phase transitions were correlated with discontinuities in Mössbauer spectral features. We observed the onset of high-spin-to-low-spin transitions in the M1 and M2 sites at ～37 GPa and ～74 GPa, respectively. Understanding the electronic structure of iron in a well-characterized single crystal of ferrosilite may help interpret the behavior of iron in complex dense silicate phases.