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1.
A combined polarized optical absorption and 57Fe Mössbauer spectroscopy study of inhomogeneous, Fe and Ti-bearing terrestrial hibonite (Madagascar) has been carried out. Mössbauer data were also obtained on synthetic material prepared under different fo2 inconditions. A strong band at 5400 cm-1 in the near-infrared spectra is attributed to spin-allowed d-d transitions of Fe2+ occupying tetrahedral sites within the spinel blocks of the hibonite crystal structure. There is agreement with the Mössbauer results, showing that ferrous iron orders onto a single, low-coordinated crystallographic site. Ferric iron is distributed over several positions, but shows strongest preference for the large bipyramidal site located outside the spinel blocks. The colour and pleochroism of hibonite in thin section is related to a prominent UV absorption edge, and several broad absorption bands in the visible spectrum ascribed to charge-transfer transitions involving Fe2+, Fe3+ and Ti4+.  相似文献   

2.
γ LiAlO2 doped with Fe3+ in the tetrahedral site has been examined by extended X-ray absorption fine structure (EXAFS) analysis, and Mössbauer and optical spectroscopy. The isomer shift (IS) is ?0.026 mm/s (Fe-Pd); the quadrupole splitting (QS) is 0.62 mm/s. Anisotropic optical absorption is prominent at ～391, 452, and 463 nm. The K-edge absorption spectrum shows a prominent absorption near 7,113 ev typical of tetrahedrally coordinated Fe3+.  相似文献   

3.
The influence on the spinel structure of Fe3+ → Cr substitution was studied in flux-grown synthetic single crystals of the magnesiochromite–magnesioferrite (MgCr2O4–MgFe2O4) solid solution series. Samples were analysed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption and Mössbauer spectroscopy. With the exception of iron-poor samples (3–12 mol-% MgFe2O4), optical absorption and Mössbauer spectra show that iron occurs almost exclusively as trivalent Fe in the present samples. A very intense and broad absorption band at ca 7,800 cm?1 dominates the optical absorption spectra of samples with higher Fe-contents. The appearance of this band is related to a distinct structural disorder of Fe3+ and a development of magnetic ordering as demonstrated by Mössbauer spectra. Profound composition-related changes are observed in the Mössbauer spectra, which are magnetically unsplit in the range 2–41 mol-% magnesioferrite, but become magnetically split in the range 59–100 mol-% magnesioferrite. Structural parameters a 0 and M–O increase with magnesioferrite content and inversion degree, while u and T–O decrease. Our study confirms the previously reported (Lavina et al. 2002) influence of Fe3+ at the M site on T–O bond lengths in the spinel structure.  相似文献   

4.
The Mössbauer spectra of five samples of Fe x O with compositions in the range 1.00>x>0.95 have been recorded at 298 K and 4.2 K. The spectrum of Fe x O at 298 K consists of an asymmetric doublet which was fitted to one Fe2+ singlet, two Fe2+ doublets and 1 Fe3+ singlet. The Mössbauer parameters vary consistently with the increasing density of defects as x decreases. The Mössbauer spectrum of Fe x O at 4.2 K consists of a large number of unresolved lines. The data were fitted to a series of singlets to enable the rough calculation of quantities relating to the mean Fe2+ and Fe3+ environments. The results of the fits to the 298 K spectra are briefly discussed in terms of a physical model for the defect structure of Fe x O.  相似文献   

5.
Mössbauer spectra were recorded at multiple temperatures between 80 and 293 K to study the nature of Fe3+ in Fe0.05Mg0.95SiO3 perovskite that had been synthesised in a multianvil press at 1650 °C and 25 GPa at its mimimum stability limit. The Mössbauer data were fitted to a model with quadrupole splitting distributions (Fe2+) and Lorentzian lineshapes (Fe3+ and Fen+). The centre shift data were fitted to a Debye model with the following results: ΘM (Fe2+)=365±52 K and ΘM (Fe3+)=476±96 K. Hyperfine parameter data for Fe3+ suggest occupation of the octahedral site only. The average valence seen by the Mössbauer effect in rapid electron exchange that occurs between Fe2+ and Fe3+ is calculated from the hyperfine parameters to be 2.50±0.07. Correction of area fractions for site-dependent recoil-free fractions gives a value for Fe3+/∑Fe of 9.4±1.4%, which is independent of temperature. A perovskite phase of similar composition synthesised in the multianvil press at higher oxygen fugacity gives a value for Fe3+/∑Fe of 16±3%, where Fe3+ appears to occupy both sites in the perovskite structure.  相似文献   

6.
(Mg,Fe)(Si,Al)O3 perovskite samples with varying Fe and Al concentration were synthesised at high pressure and temperature at varying conditions of oxygen fugacity using a multianvil press, and were characterised using ex?situ X-ray diffraction, electron microprobe, Mössbauer spectroscopy and analytical transmission electron microscopy. The Fe3+/ΣFe ratio was determined from Mössbauer spectra recorded at 293 and 80?K, and shows a nearly linear dependence of Fe3+/ΣFe with Al composition of (Mg,Fe)(Si,Al)O3 perovskite. The Fe3+/ΣFe values were obtained for selected samples of (Mg,Fe)(Si,Al)O3 perovskite using electron energy-loss near-edge structure (ELNES) spectroscopy, and are in excellent agreement with Mössbauer data, demonstrating that Fe3+/ΣFe can be determined with a spatial resolution on the order of nm. Oxygen concentrations were determined by combining bulk chemical data with Fe3+/ΣFe data determined by Mössbauer spectroscopy, and show a significant concentration of oxygen vacancies in (Mg,Fe)(Si,Al)O3 perovskite.  相似文献   

7.
Magnetization, susceptibility and Mössbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of theT 2g triplet of 1,120K. Predominant exchange interactions in the iron-rich samples are ferromagnetic withJ=1.2 K, as for other trioctahedral ferrous minerals. A peak in the susceptibility of thuringite occurs atT m=5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the Mössbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4 Å?1). A factor favouring this random ferromagnetic ground state over the planar antiferromagnetic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illustrate how long range ferromagnetic order may be destroyed by vortices forming around the Fe3+-Fe3+ pairs.  相似文献   

8.
Three natural lawsonites from Syke Rock, Mendocino Co., Reed Ranch, Marin Co., and Blake Gardens, Sonoma Co., all from the Coast Range Region in California, were studied by 57Fe Mössbauer spectroscopy, electron microprobe analysis, and X-ray powder diffraction. The samples contain about 0.6, 1.0, and 1.4 wt% of total iron oxide, respectively. 57Fe Mössbauer spectra are consistent with the assumption that high-spin Fe3+ substitutes for Al in the octahedrally coordinated site. The Mössbauer spectrum of lawsonite from Syke Rock exhibits a second doublet with 57Fe hyperfine parameters typical for octahedrally coordinated high-spin Fe2+. Electronic structure calculations in the local spin density approximation yield quadrupole splittings for Fe3+ in quantitative agreement with experiment indicating, however, that substitution of Al by Fe3+ must be accompanied by local distortion around the octahedral site. Model calculations also reproduce the room temperature hyperfine parameters of ferrous high-spin iron assuming the substitution of Ca by Fe2+. However, it cannot be excluded that Fe2+ may occupy a more asymmetric site within the microstructural cavity occupied by Ca and a H2O molecule.  相似文献   

9.
We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg0.88Fe0.09)(Si0.94Al0.10)O3 between 1 bar and 100 GPa and at 300 K, using diamond cells and synchrotron Mössbauer spectroscopy techniques. At pressures below 12 GPa, our Mössbauer spectra can be sufficiently fitted by a “two-doublet” model, which assumes one ferrous Fe2+-like site and one ferric Fe3+-like site with distinct hyperfine parameters. The simplest interpretation that is consistent with both the Mössbauer data and previous X-ray emission data on the same sample is that the Fe2+-like site is high-spin Fe2+, and the Fe3+-like site is high-spin Fe3+. At 12 GPa and higher pressures, a “three-doublet” model is necessary and sufficient to fit the Mössbauer spectra. This model assumes two Fe2+-like sites and one Fe3+-like site distinguished by their hyperfine parameters. Between 12 and 20 GPa, the fraction of the Fe3+-like site, Fe3+/∑Fe, changes abruptly from about 50 to 70%, possibly due to a spin crossover in six-coordinate Fe2+. At pressures above 20 GPa, the fractions of all three sites remain unchanged to the highest pressure, indicating a fixed valence state of iron within this pressure range. From 20 to 100 GPa, the isomer shift between the Fe3+-like and Fe2+-like sites increases slightly, while the values and widths of the quadruple splitting of all three sites remain essentially constant. In conjunction with the previous X-ray emission data, the Mössbauer data suggest that Fe2+ alone, or concurrently with Fe3+, undergoes pressure-induced spin crossover between 20 and 100 GPa.  相似文献   

10.
Ilvaite samples from six different localities in Japan are found to be members of a solid-solution series varying from Ca(Fe2+,Fe3+)2Fe2+(OH)O Si2O7 to approaximately Ca(Fe2+,Fe3+)2Fe 0.5 2+ Mn 0.5 2+ (OH)O Si2O7, and have been studied by Mössbauer spectrometry and magnetic measurements. The variation in intensity of Mössbauer doublets confirms that Mn substitutes for Fe2+ in the M(B) cation site. An temperatures decreasing from 300 K to 4K, an abrupt change in the reciprocal mass magnetic susceptibility, 1/x g, occurs about 120 K; 1/x g depends linearly upon temperature above 120 K. This change, which is characterized by an unusual mode of decrease in 1/x g, has been interpreted based on Mössbauer spectra at 80 K: the spectra of Fe2+ and Fe3+ in the M(A) site show Zeeman splitting, whereas those of Fe2+ in the M(B) site do not show the effect. This Mössbauer evidence suggests that magnetic spins of Fe in M(A) are in an ordered state, very likely of antiparallel coupling, whereas those of Fe in M(B) are randomly oriented, showing that below 120 K ilvaite has two different magnetic states for Fe ions. As there is a line of evidence that the spins of Fe in M(B) would take an ordered state at extremely low temperatures, ilvaite magnetism may be regarded as basically antiferromagnetic. The magnetic spins of Fe in M(A) and M(B) undergo magnetic transitions at different specific temperatures, thus giving as a whole unusual features of magnetism.  相似文献   

11.
Natural alexandrite Al2BeO4:Cr from Malyshevo near Terem Tschanka, Sverdlovsk, Ural, Russia, has been characterized by 57Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine oxidation state and location of iron. The sample contains 0.3 wt% of total iron oxide. The 57Fe Mössbauer spectrum can be resolved into three doublets. Two of them with hyperfine parameters typical for octahedrally coordinated high-spin Fe3+ and Fe2+, respectively, are assigned to iron substituting for Al in the octahedral M2-site. The third doublet is attributed to Fe3+ in hematite. Electronic structure calculations in the local spin density approximation are in reasonable agreement with experimental data provided that expansion and/or distortion of the coordination octahedra are presumed upon iron substitution. The calculated hyperfine parameters of Fe3+ are almost identical for the M1 and M2 positions, but the calculated ligand-field splitting is by far too large for high-spin Fe3+ on M1.  相似文献   

12.
The Mössbauer spectra of one chromite at 298 K and one chromite at 298, 200, 170, 140 and 90 K have been analyzed in this study. A Voigt-based quadrupole splitting distribution (QSD) method was used to analyze the spectra. The tetrahedral site Fe2+ and the octahedral site Fe3+ quadrupole splitting distributions (QSDs) were obtained from the Mössbauer spectra of chromites, and the multiple tetrahedral site Fe2+ Gaussian QSD components and the large widths σ Δ of the Gaussian QSD components of the tetrahedral site Fe2+ QSDs for chromites were attributed to next-nearest neighbor effects. In addition, temperature dependences of the isomer shift and the quadrupole splitting were presented and discussed. Comparisons between the Mössbauer parameters for thickness-corrected folded spectra and raw-folded spectra of chromites were made, and the results show that the two sets of the Mössbauer parameters and ratios of ferric to total iron as well as χ2 are very close to each other. This is because of the small absorber thickness of chromites in this study. Comparisons between the Mössbauer parameters of chromites obtained using the Voigt-based QSD method and a Lorentzian doublet method were also made. The results show that there are some differences between the two sets of the Mössbauer parameters and ratios of ferric to total iron, but not significant. However, much larger χ2 were obtained when the Lorentzian doublet method was used to fit the spectra of chromites. This indicates that the Voigt-based QSD method is more adequate to analyze the Mössbauer spectra of chromites from the point of view of statistics.  相似文献   

13.
The Mössbauer absorption spectra of arfvedsonite are composed of three quadrupole doublets which are ascribed to Fe2+ in M1 and M2 sites and to Fe3+ in M2 sites. The relative intensities of the resonances are a measure of the distribution of iron at the different sites, but it is necessary to correct for a difference between the recoil-free fractions. At room temperature [Fe2+] seems detected with an efficiency of only about 85% of that of [Fe3+]. Results of [Fe2+]/[Fe3+] determinations by Mössbauer spectroscopy and by wet chemical analysis of a series of arfvedsonite samples, separated from various rocks from the Ilimaussaq intrusion, south Greenland, are compared and agree reasonably well.  相似文献   

14.
Over thirty samples of natural Ti-bearing amphiboles with Ti- and Fe-contents ranging from 0.111 to 0.729 atom per formula unit (a.p.f.u.) and from 0.479 to 2.045 a.p.f.u., respectively, were studied by means of optical absorption spectroscopy and microprobe analysis. Thirteen samples were also studied by Mössbauer spectroscopy. A strong pleochroic absorption edge, causing the dark brown colours of Ti-bearing amphiboles, is attributed to ligand-metal and metal-metal charge transfer transitions involving both iron and titanium ions (O2?→ Fe3+, Fe2+, O2?→ Ti4+ and Fe2+ + Ti4+→ Fe3+ + Ti3+). A broad intense Y-polarized band ～22?000?cm?1 (ν1/2?≈?3700?cm?1) in spectra of two low iron amphiboles with a relatively low Fe3+/Fetotal ratio, both from eclogite-like rocks in kimberlite xenoliths, was attributed to electronic Fe2+(M3) + Ti4+(M2)→Fe3+(M3)+Ti3+(M2) IVCT transitions. The IVCT bands of other possible ion pairs, involving Ti4+ and Fe2+ in M2 and M1, M4 sites, respectively, are presumed to be at higher energies, being obscured by the absorption edge.  相似文献   

15.
We report Mössbauer milliprobe measurements on small single-crystals of a magnesium-rich hedenbergite, approximate composition CaFe0.54Mg0.46 (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.  相似文献   

16.
The Fe3+/Σ Fe of twenty-nine experimentally formed, iron-bearing silicate glasses has been determined by wet-chemical and Mössbauer spectroscopic methods from 5–10 mg individual splits of 20–40 mg experimental run products. The wet-chemical and Mössbauer analyses were conducted in two separate laboratories (University of California, Berkeley, and the Geophysical Laboratory, respectively). The Fe3+/Σ Fe ranges from less than 0.2 to 0.96, and the total iron oxide content of the samples, from 2.2 to 34.7 wt %, added as Fe2O3. The interlaboratory comparison shows 70% of the Fe3+/ΣFe analyses from the two methods within the quoted uncertainties (±1 σ) of each other and 83% of the analyses within ±2 σ of each other. Replicate analyses in the current data set result in variations within ±1 σ. These uncertainties are similar to those obtained from several hundred Fe3+/Σ Fe analyses of reequilibrated natural rock and simple system compositions carried out with identical analytical methods in the two laboratories. There is no systematic bias in the results from either of the two techniques. The Fe3+/Σ Fe of silicate glasses can be analyzed, therefore, with equal confidence by either the wet-chemical or the Mössbauer spectroscopic method.  相似文献   

17.
The mixed valence iron silicate deerite, Fe 6 2+ Fe 3 3+ O3[Si6O17](OH)5 whose crystal structure has only recently been fully determined, had been previously examined by both susceptibility measurements and Mössbauer spectroscopy. We reinterpret the Mössbauer measurements from Frank and Bunbury and show that all contradictions between the phenomena, observed in their experiments, and the mineralogical structure disappear when one accepts a thermally activated electron delocalisation associated with Fe2+ ? Fe3+ charge transfer.  相似文献   

18.
Natural barbosalite Fe2+Fe3+ 2 (PO4)2(OH)2 from Bull Moose Mine, South Dakota, U.S.A., having ideal composition, was investigated with single crystal X-ray diffraction techniques, Mössbauer spectroscopy and SQUID magnetometry to redetermine crystal structure, valence state of iron and evolution of 57Fe Mössbauer parameter and to propose the magnetic structure at low temperatures. At 298?K the title compound is monoclinic, space group P21/n, a o ?= 7.3294(16)?Å, b o ?=?7.4921(17)?Å, c o ?=?7.4148 (18)?Å, β?=?118.43(3)°, Z?=?2. No crystallographic phase transition was observed between 298?K and 110?K. Slight discontinuities in the temperature dependence of lattice parameters and bond angles in the range between 150?K and 180?K are ascribed to the magnetic phase transition of the title compound. At 298?K the Mössbauer spectrum of the barbosalite shows two paramagnetic components, typical for Fe2+ and Fe3+ in octahedral coordination; the area ratio Fe3+/Fe2+ is exactly two, corresponding to the ideal value. Both the Fe2+ and the Fe3+ sublattice order magnetically below 173?K and exhibit a fully developed magnetic pattern at 160?K. The electric field gradient at the Fe2+ site is distorted from axial symmetry with the direction of the magnetic field nearly perpendicular to Vzz, the main component of the electric field gradient. The temperature dependent magnetic susceptibility exhibits strong antiferromagnetic ordering within the corner-sharing Fe3+-chains parallel to [101], whereas ferromagnetic coupling is assumed within the face-sharing [1?1?0] and [?1?1?0] Fe3+-Fe2+-Fe3+ trimer, connecting the Fe3+-chains to each other.  相似文献   

19.
The electronic absorption spectra of Fe2+ in non-chromium beryls are examined. Fe2+ in the Al-rich six-coordinate site produces absorption bands at about 820 nm and 970 nm polarizedE‖c. Fe2+ in the channel produces bands at 820 nm (⊥c) and 2100 nm (‖c). Some blue beryls which are more intensely colored than most aquamarines, have an absorption band at ～700 nm (‖c) which is suggested to arise from an Fe2+/Fe3+ intervalence interaction. Fe2+ in both the six-coordinate site and the channel is identified in the Mössbauer spectra. The Mössbauer spectra of deep blue beryls are unusual and have not been satisfactorily explained. Color changes which accompany heating and irradiation are strongly influenced by the channel iron.  相似文献   

20.
The valence and distribution of iron in vivianite, lazulite, babingtonite, rockbridgeite, acmite, aegirine-augite, hedenbergite, and ilvaite were studied with optical and Mössbauer spectroscopy. Optically activated intervalence charge transfer between Fe2+ and Fe3+ in neighboring sites through common edges or faces is observed in all these minerals irrespective of the polymerization of the iron-oxygen polyhedra ranging from finite clusters to infinite structural units. However, a distinct decrease occurs in the energy of the corresponding optical absorption band with increasing number of Fe2+ and Fe3+ ions involved in the charge transfer process. Thermally activated electron delocalization between Fe2+ and Fe3+ occurs only if Fe2+ and Fe3+ occupy crystallographically equivalent or geometrically very similar neighboring sites which share common edges to form extended structural units such as the ribbon in ilvaite. If the Fe-O polyhedra form finite clusters of two, three, or four polyhedra (e.g., in vivianite, lazulite, and babingtonite, respectively) no thermally-activated mixed-valence states of iron are observed. In aegirine, extended regions of the M1 chain are statistically occupied by Fe2+ and Fe3+ giving rise to thermally-activated electron delocalization in addition to the intervalence band in the optical absorption spectrum. The intensity of the optical intervalence absorption has been measured in a number of systems: ? values range from 60 to 210.  相似文献   

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