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.     

Low temperature magnetism and Mössbauer spectroscopy study from natural goethite

In this work a magnetic characterization was made of natural goethite from Burkina Faso, Africa, by using low temperature magnetization curves, hysteresis loops, Mössbauer spectroscopy at room temperature and 4.2 K, and AC susceptibility from 10 to 400 K. The samples are from two distinct geological sites that underwent different weathering processes. All measurements point to the occurrence of typical high coercivity goethite. Through Mössbauer spectroscopy sample BL44, from Gangaol, northeast Burkina Faso showed relaxation effects due to a wide distribution of grain size, including superparamagnetism threshold. AC susceptibility also supports this interpretation. The sample BL50 from Bonga in Burkina Faso is associated with lateritic Ni and in addition to goethite this sample also contained magnetite, as determined by Verwey transition in low temperature measurements as well as a small content of hematite identified by Mössbauer spectroscopy.     

Mineralogy, 57Fe Mössbauer spectra and magnetization of chalcolithic pottery

Three chalcolithic pottery sherds, paint removed from the surface of each sherd, and an unheated red pigment (Tell-Halaf culture, Turkey) were analysed within the frame of archaeometric studies using mineralogical methods, ⁵⁷Fe Mössbauer spectroscopy, magnetization and rotational hysteresis data. From mineralogical results, the individual minerals forming the cores of the sherds were determined. It was found that the sherds are lime-rich. High temperature X-ray analysis on comparable Ca-rich material showed that the established composition is consistent with a firing temperature of 750-950°C. Apart from the pigment, each Mössbauer spectrum of Fe-bearing components consists of dominating paramagnetic doublets, arising mostly from silicate phases, and of a six-line pattern with reduced intensity, due to ferri- and/or antiferromagnetic Fe-oxide phases. For three samples, an Fe³⁺ silicate component of the spectra is clearly dominating, which points to oxidizing conditions during firing. For the others Fe²⁺ and Fe³⁺ components occur in about equal intensities. For the pigment, the magnetic sextet is of similar intensity to the Fe³⁺ silicate component. From magnetic analysis of ferrimagnetic phases it follows that a low percentage of particles of solid solutions -Fe₂O₃ – Fe₃O₄ exist, probably in part 0.1 m in diameter. The ferrimagnetic particles of at least one paint are probably covered by a thin layer of hematite as found from rotational hysteresis data. An attempt is made to draw conclusions from the experimental results, regarding the firing conditions of the sherds and paints.     

Electrical resistivity and 57Fe Mössbauer spectra of Fe-bearing calcic amphiboles

Electrical resistivity and ⁵⁷Fe Mössbauer spectra are reported for three calcic amphiboles with different Fe concentrations. AC measurements (20?Hz–1?MHz) were performed, applying impedance spectroscopy between 100 and 785?°C in an N₂ gas atmosphere. It was found that up to three semiconducting charge transport processes can be distinguished, which in part changed slightly when several runs were carried out to higher temperatures. The extrapolated DC resistivity is much smaller for an amphibole with high Fe content than for the two with lower Fe concentrations. The derived activation energies are between ～0.48 and ～1.06?eV. For temperatures ≤600?°C the results are compatible with a charge transport mechanism due to electron hopping between Fe²⁺ and Fe³⁺. Above 600?°C, dehydrogenation and/or beginning amphibole decomposition obviously alter the conduction mechanism. From Mössbauer spectra it was established that in all amphibole samples Fe²⁺ and Fe³⁺ are simultaneously present. Mössbauer parameters were derived by fitting the observed spectra to models taking the occupation of various M sites into account.     

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 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.     

A time-minimizing hybrid method for fitting complex Mössbauer spectra

The present study includes results of the application of a program consisting of a genetic algorithm routine and a conventional refinement part (“hybrid method”) for the evaluation of Mössbauer spectra published elsewhere. It is shown that the saving of total evaluation time compared with conventional refinement routines is very high due to the rapid finding of adequate starting parameters. Contrary to a recently published work on a similar topic our algorithm provides solutions of the combined interaction Hamiltonian with a minimum of conventional input data.     

Mössbauer and infrared spectrometry of lizardite-1T from Monte Fico, Elba

A well crystallized and homogeneous specimen of lizardite from Monte Fico, Elba, Italy, has been studied by Mössbauer and Fourier transform infrared (FTIR) spectrometries. One of the aims was the determination of the oxidation state and the distribution of iron in the structure of this reference sample. Mössbauer data indicate the presence of octahedral ferrous iron, octahedral ferric iron and tetrahedral ferric iron (59.9, 31.3 and 8.8% of total iron, respectively). The existence of only one octahedral site, previously suggested by X-ray structure refinement, is confirmed. The occurrence of tetrahedrally coordinated iron is indicated also by FTIR spectrometry, in particular by the presence of an absorption band at 790 cm^–1. Based also on new electron microprobe data, the improved crystal chemical formula for lizardite from Monte Fico is: (Mg_2.74Fe²⁺ _0.10Fe³⁺ _0.05Al_0.11)_Σ=3.00 ?· (Si_1.94Al_0.05Fe³⁺ _0.01)_Σ=2.00O_5.05(OH)_3.95.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

A structural,magnetic, and Mössbauer spectral study of several Na–Mn–Fe-bearing alluaudites

The synthesis and the chemical, structural, magnetic, and Mössbauer spectral characterization of three synthetic alluaudites, Na₂Mn₂Fe(PO₄)₃, NaMn Fe₂(PO₄)₃ and Na₂MnFe^IIFe^III(PO₄)₃, and a natural sample with the nominal composition of NaMn Fe₂(PO₄)₃, collected in the Buranga pegmatite, Rwanda, are reported. All four compounds have the expected alluaudite monoclinic C2/c structure with the general formula [A(2)A(2)][A(1)A(1)A(1)₂]M(1)M(2)₂(PO₄)₃ in which manganese(II) is on the M(1) site and manganese(II), iron(III) and, in some cases, iron(II) on the M(2) site. The X-ray structure of Na₂Mn₂Fe(PO₄)₃ also indicates a partially disordered distribution of Na^I and Mn^II on the M(1) and A(1) crystallographic sites. All four compounds are paramagnetic above 40 K and antiferromagnetically ordered below. Above 40 K the effective magnetic moments of NaMnFe₂(PO₄)₃ and Na₂MnFe^IIFe^III(PO₄)₃ are those expected of high-spin manganese(II) and iron(III) with the ⁶A_1g electronic ground state and high-spin iron(II) with the ⁵T_2g electronic ground state. In contrast, the effective magnetic moment of Na₂Mn₂Fe(PO₄)₃ is lower than expected as a result of enhanced antiferromagnetic exchange coupling by the manganese(II) on the M(2) site. The Mössbauer spectra of all four compounds have been measured from 4.2 to 295 K and have been found to be magnetically ordered below 40 K for Na₂Mn₂Fe(PO₄)₃ and 35 K for the remaining compounds. The Mössbauer spectra of Na₂Mn₂Fe(PO₄)₃ exhibit the two expected iron(III) quadrupole doublets and/or magnetic sextets expected for a random distribution of manganese(II) and iron(III) ions on the M(2) site. Further, the Mössbauer spectra of Na₂MnFe^IIFe^III(PO₄)₃ exhibit the two iron(II) and two iron(III) quadrupole doublets and/or magnetic sextets expected for a random distribution of iron(II) and iron(III) on the M(2) site. Surprisingly, the synthetic and natural samples of NaMnFe₂(PO₄)₃ have 19 and 10% of iron(II) on the M(2) site; apparently the presence of some iron(II) stabilizes the alluaudite structure through the reduction of iron(III)–iron(III) repulsion. The temperature dependence of the iron(II) quadrupole splitting yields a 440 to 600 cm^–1 low-symmetry component to the octahedral crystal field splitting at the M(2) site. The iron(II) and iron(III) hyperfine fields observed at 4.2 K are consistent with the presence of antiferromagnetic ordering at low temperatures in all four compounds.     

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.     

Advances of ferrous and ferric Mössbauer recoilless fractions in minerals and glasses

Mössbauer spectroscopy has been used widely to characterize the ferric (Fe³⁺) and ferrous (Fe²⁺) proportions and coordination of solid materials. To obtain these accurately, the recoilless fraction is indispensible. The recoilless fractions (f) of iron-bearing minerals, including oxides, oxyhydroxides, silicates, carbonates, phosphates and dichalcogenides, and silicate glasses were evaluated from the temperature dependence of their center shifts or absorption area with the Debye model approximation. Generally, the resolved Debye temperature (θ_D) of ferric iron in minerals, except dichalcogenides, through their center shifts ranging from 400 to 550 K, is significantly larger than ferrous iron ranging from 300 to 400 K, which is consistent with the conclusion from previous work. The resolved f (Fe³⁺)_RT with the center shift model (CSM) ranges from 0.825 to 0.925, which is larger than that obtained for f(Fe²⁺)_RT, which ranges from 0.675 to 0.750. Meanwhile, the θ_D and f resolved from temperature-dependence of absorption are generally lower than from center shifts, especially for ferric iron. The significant difference between f(Fe³⁺) and f(Fe²⁺) indicates the necessity of recoilless fraction correction on the Fe³⁺/(Fe³⁺+Fe²⁺) resolved from Mössbauer spectra.