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 spectroscopy on the three principal sections of a synthetic fayalite sample in the antiferromagnetic state

The present work reports Mössbauer investigations for several temperatures below T _N on fayalite single crystal sections cut perpendicularly to the crystallographic a and b-axis (Pnma). The previously detected correspondence between the c-component of the magnetic moment on M1 from neutron diffraction and our own Mössbauer measurements published elsewhere are confirmed for the other principal sections to a large extent. Small humps in the angular dependence of two components of the internal magnetic field H(0) on T below T=23 K are in good agreement with magnetometric and calorimetric data published elsewhere; a reinterpretation of single reflection neutron data has been possible by our results. Moreover, the axes of the electric field gradient (efg) are oriented within the crystallographic axes for the M1-site at low temperatures. The violation of symmetry on the M2 position as a result of our previous investigations could be confirmed for the section ⊥ a, but not with respect to b. A possible explanation in terms of saturation effects of large line intensities at the expense of the small ones is given in the context.     

Powder- and single crystal Mössbauer spectroscopy on synthetic fayalite

Mössbauer measurements on synthetic iron orthosilicate Fe₂SiO₄ (fayalite) were carried out in the antiferromagnetic spin state below T _N 65 K. The Mössbauer parameters isomer shift , inner magnetic field H(0), angle between H(0) and the z-component of the electric field gradient (efg), quadrupole splitting QS and asymmetry parameter were determined as a function of temperature. These parameters could be attributed to the two crystallographic sites M1 and M2.The smaller isomer shift on M1 with respect to M2 displays the more covalent character of the Fe-O bond on M1, which is supported by previous neutron diffraction experiments. H(0) shows a Brillouin-type behaviour with different fields on the two crystallographic sites (stronger on M1) and a small discontinuity at T = 23 K which corresponds with previous magnetic measurements. The quadrupole splitting is equal on both sites within error bars, in agreement with previous theoretical results and in contradiction to previous Mössbauer refinements published elsewhere.     

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.     

Mössbauer milliprobe studies of small mineral samples with a silicon drift detector

Analysis of ⁵⁷Fe transmission Mössbauer spectra collected on a system where the proportional counter has been replaced with a silicon drift detector (SDD) to test milliprobing of mineral samples is described. In the region of the 14.4 keV Mössbauer line the detector has about 70% efficiency and is capable of delivering spectroscopic information with a high energy resolution and high counting rate. Satisfactory results are obtained from a phase analysis of mixtures of olivine and ilmenite in the proportion 97:3, 99:1 wt%, where in the latter case 2.4 μg of Fe³⁺ in the form of hematite was found in the ilmenite. New perovskite-type minerals (Pb_1.33Ba_0.67Fe₂O₅, Pb_1.33Sr_0.67Fe₂O₅ and Pb_1.33Ba_0.33Sr_0.33Fe₂O₅), synthesised by a combustion method, were studied by X-ray diffraction and Mössbauer spectroscopy as well. The advantage of the system with SDD compared to a conventional Mössbauer spectrometer equipped with a proportional counter as a detector is demonstrated for the perovskite samples. The Mössbauer set-up with the silicon drift detector may be successfully used for a wide range of materials containing a negligible amount of iron.     

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.     

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.     

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 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.     

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.     

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)     

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.     

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.     

A Mössbauer study of pseudotachylytes: redox conditions during seismogenic faulting

The redox conditions during frictional melting provide information on the physical and chemical conditions during seismic slip in the crust. Here we examine frictional melts from five localities by analyzing host rocks and corresponding pseudotachylytes using Mössbauer spectroscopy. The faults examined are located at South Mountain, Arizona; Fort Foster, Maine (two localities); Long Ridge fault, North Carolina; and the Homestake shear zone, Colorado. The main iron-bearing phases in the pseudotachylytes are phyllosilicates (biotite, muscovite and clays) and iron oxides (magnetite and hematite) and minor pyrite. The ferrous/ferric ratios of the phyllosilicates in the host rocks are the same as those in the pseudotachylytes, except for the hematite-bearing pseudotachylyte from the Long Ridge fault, which is more oxidized. The magnetites in the host rocks and the corresponding pseudotachylytes have different ferric and ferrous iron distributions, which is attributed to different cation chemistry, rather than redox conditions. With the exception of the South Mountain locality, the ferric/ferrous ratios of the micas are interpreted to record the primary redox state of the pseudotachylyte melt as the calculated oxygen fugacities are consistent with magnetite and hematite equilibria. Pyrite-bearing pseudotachylytes plot ~0–1 log₁₀ units above the fayalite-magnetite-quartz (FMQ) buffer. Magnetite-bearing pseudotachylytes plot ~2–4 log₁₀ units above the FMQ buffer, and hematite-bearing pseudotachylytes plot 3.5 log₁₀ units above the hematite-magnetite (HM) buffer. Hematite-bearing pseudotachylytes, together with previous oxygen isotope data, are inferred to represent melting in the presence of externally derived pressurized water. Other localities are inferred to represent melting under rock-buffered, closed system, conditions. If the localities studied are representative of seismogenic faulting, the calculated oxygen fugacities indicate that, in the system C–O–H–S, H₂O and CO₂ should be the dominant fluid species. This is the first detailed study of the redox state of pseudotachylytes.     

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.     

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.     

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.