Mössbauer spectra of Fe x O (x>0.95)

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.     

Mössbauer and ELNES spectroscopy of (Mg,Fe)(Si,Al)O3 perovskite: a highly oxidised component of the lower mantle

(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.     

The MgCr2O4–MgFe2O4 solid solution series: effects of octahedrally coordinated Fe3+ on T–O bond lengths

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.     

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.     

Iron in hibonite: a spectroscopic study

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⁺.     

Mössbauer spectra and magnetic features of ilvaites

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.     

Distribution of Fe2+ and Fe3+ and next-nearest neighbour effects in natural chromites: comparison between results of QSD and Lorentzian doublet analysis

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.     

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.     

Influence of cation distribution on the optical absorption spectra of Fe3+-bearing spinel s.s.–hercynite crystals: evidence for electron transitions in VIFe2+–VIFe3+ clusters

Flux-grown Fe³⁺-bearing spinel s.s.–hercynite solid-solution crystals, (Mg _1-y Fe²⁺ _y )Al₂O₄ (0 < y≤ 1), have been investigated by means of electron microprobe technique and Mössbauer and electronic spectroscopy. Obtained results show that different electronic processes cause intense optical absorption bands in the near-infrared spectral region. In addition to an electronic d–d transition in single-ion ^IVFe²⁺, observed at 5200 cm^?1, intense and broad bands at 9500 and 14 500 cm^?1 are assigned to exchange-coupled pair (ECP) and intervalence charge-transfer (IVCT) transitions in ^VI Fe ^{2+ VI} Fe³⁺clusters, respectively. The net linear extinction coefficients of these bands (α) were calibrated against Fe²⁺ and Fe³⁺ concentrations and site distributions previously defined by combined microchemical, Mössbauer, and XRD structural refinement data. The following expressions were obtained: where α is measured in cm^?1 and concentrations are expressed in mol?l^?1. The present results show that optical absorption spectroscopy may be used as a probe to obtain high spatial resolution (?～ 10 μm) information on Fe²⁺ ordering as well as on Fe³⁺ concentrations in minerals belonging to the spinel group.     

Mössbauer studies of synthetic and natural micas on the polylithionite-siderophyllite join

Mössbauer studies of micas on the polylithionite-side-rophyllite join show the existence of a relation between the quadrupole splitting (ΔE _Q) values of Fe²⁺ high spin doublets and both cationic and anionic composition of micas. This linear relation is positive as Li₂O content increases and negative as F content increases. In the lithium iron micas, the inner ferrous quadrupole doublet is assigned to the cis-site M(2), while the outer doublet is assigned to the trans-site M(1). A random distribution of Fe²⁺ is observed in fluorine-rich compositions, while slight enrichment of the M(1) site is noticed in hydroxyl compositions, perhaps due to a more sensitive oxidation in situ in M(2) than M(1) sites. The Mössbauer spectrum of siderophyllite K₂(Fe ₄ ²⁺ Al₂)(Si₄Al₄)O₂₀(OH)₄ shows the presence of only one ferrous doublet, which is assigned to M(2) sites. Hence from Mössbauer data we must consider a clintonite (“xanthophyllite”) structure for this mica. The ordered octahedral layer has two distorted ferrous cis-sites and one, more symmetrical, aluminum trans-site.     

Microanalysis of the iron oxidation state in (Mg,Fe)O and application to the study of microscale processes

We report application of the flank method using the electron microprobe to a suite of twelve (Mg,Fe)O samples with composition 2–47 wt% Fe and Fe³⁺/ΣFe = 1 to 11%, where Fe³⁺/ΣFe was determined independently using Mössbauer spectroscopy on the same grains used for the flank method measurements. A calibration curve of the form Fe²⁺ = A + B × (ΣFe)² + C × (Lβ/Lα) was fit to the data and gave excellent agreement between Fe³⁺/ΣFe calculated from the flank method and Fe³⁺/ΣFe determined using Mössbauer spectroscopy. We found the method to be sufficiently sensitive to determine meaningful variations in Fe³⁺/ΣFe for geophysically relevant compositions of (Mg,Fe)O (<25 wt% Fe), and calibration parameters remained constant within experimental uncertainty over the course of the entire study (20 months). Flank method measurements on an inhomogeneous sample of synthetic (Mg,Fe)O showed evidence of diffusion processes resulting from rupture of the capsule during the high-pressure experiment and the possibility to measure Lβ/Lα variations with a spatial resolution of a few microns. We detected the presence of exsolved magnesioferrite in a suite of (Mg,Fe)O single crystals using transmission electron microscopy and Mössbauer spectroscopy. Flank method measurements on the same suite of single crystals showed enhanced Fe³⁺/ΣFe values, consistent with the presence of magnesioferrite even though the grains were too small to be resolved by conventional electron microprobe measurements.     

Cordierite III: the site occupation and concentration of Fe3+

Cordierite has the ideal formula (Mg,Fe)₂Al₄Si₅O₁₈ ^.x(H₂O,CO₂), but it must contain some Fe³⁺ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe³⁺ in two Mg-rich natural cordierites have been investigated by EPR and ⁵⁷Fe Mössbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe³⁺ is located on T₁1 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mössbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe³⁺ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000?°C for 2?days causes an oxidation and/or loss of Fe²⁺ on T₁1, together with an expulsion of Na⁺ from the channels, whereas heating at the Fe–FeO buffer produces little Fe³⁺ in cordierite. Heating at 1,000?°C removes all class I H₂O, but small amounts of class II H₂O remain as shown by the IR measurements. No evidence for channel Fe²⁺ or Fe³⁺ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000?cm^?1 originating from charge-transfer between Fe²⁺ in the octahedron and Fe³⁺ in the edge-shared T₁1 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe³⁺. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.     

A comparison of iron redox ratios in silicate glasses determined by wet-chemical and 57Fe Mössbauer resonant absorption methods

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.     

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   

Structure and Mössbauer spectroscopy of barbosalite Fe2+Fe3+ 2 (PO4)2(OH)2 between 80 K and 300 K

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 V_zz, 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.     

A micro-Mössbauer study of chromites included in diamond and other mantle-related rocks

Oxygen fugacity ( $ f_{{{\text{O}}_{ 2} }} $ f O 2 ) is a fundamental but little known intensive variable in mantle processes. It influences the P/T position of a mantle solidus and the composition of mantle-derived melts and fluids and constrains mantle-core equilibria and a number of geophysical properties of the mantle. An important source of information on oxidation states is the ferric–ferrous iron ratio in mantle spinels. Since the magnetite component is low in mantle spinels, normal analytical errors translate into considerable $ f_{{{\text{O}}_{ 2} }} $ f O 2 uncertainties. In this study, we analyzed the Fe³⁺–Fe_tot ratio of chromites present as inclusions in diamond and other mantle-related occurrences by point-source Mössbauer spectroscopy using single-crystal absorbers as well as conventional Mössbauer spectroscopy using powder absorbers. The studied spinels have been previously analyzed by single-crystal X-ray diffraction and electron microprobe. The ferric–ferrous ratios found are normally similar to the different techniques apart from some samples where a large number of grains have been used for the analyses (powder absorbers). The general agreement between the different techniques allows us to conclude that the studied chromites are stoichiometric. However, conventional Mössbauer spectroscopy on powder absorbers should be conducted with great care, since the method requires a relatively large amount of sample material. Spinel frequently occurs as small grains, and the large number of crystals required may possess different degrees of oxidation/alteration and, consequently, different ferric–ferrous ratio leading to possible errors in the interpretation of the results.     

<Superscript>57</Superscript>Fe Mössbauer measurements and electronic structure calculations on natural lawsonites

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.     

Pressure effect on the electronic structure of iron in (Mg,Fe)(Si,Al)O3 perovskite: a combined synchrotron Mössbauer and X-ray emission spectroscopy study up to 100 GPa

We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg_0.88Fe_0.09)(Si_0.94Al_0.10)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.     

Determination of [Fe2+]/[Fe3+]-ratios in arfvedsonite by Mössbauer spectroscopy

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.     

Mössbauer effect study of anapaite, Ca2Fe2+(PO4)2 · 4H2O, and of its oxidation products

Mössbauer spectra (MS) of anapaite (Ca₂ Fe²⁺(PO₄)₂?·?4H₂O) and of a sample after being immersed in a 4% H₂O₂ solution at room temperature (RT) over 12 days (hereafter an4ox) were collected at temperatures in the range 4.2 to 420?K and 11 to 300?K respectively. All MS consist of symmetrical doublets, hence magnetic ordering was not observed. The temperature dependencies of the Fe²⁺ centre shifts of anapaite and an4ox were analysed with the Debye model for the lattice vibrations. The characteristic Mössbauer temperatures were found as 370?K?±?25?K and 340?K?±?25?K, and the intrinsic isomer shifts as 1.427?±?0.005?mm/s and 1.418?±?0.005?mm/s respectively. From the external-field (60?kOe) MS recorded at 4.2 and 189?K for the non-treated sample, the principal component V _zz of the electric field gradient (EFG) is determined to be positive and the asymmetry parameter η?≈?0.2 and 0.4 respectively. The temperature variations of the quadrupole splittings, ΔE _Q(T), cannot be interpreted on the basis of the thermal population of the ⁵ D electronic levels resulting from the tetragonal compression of the O₆ co-ordination. The low-temperature linear behaviour of ΔE _Q(T) is attributed to a strong orbit-lattice coupling. A field of 60 kOe applied to anapaite at 4.2?K produces magnetic hyperfine splitting with effective hyperfine fields of ?136, ?254 and ?171?kOe along the principal axes Ox, Oy and Oz of the EFG tensor respectively. Additional oxidation treatments in solutions with various H₂O₂ concentrations up to 20% and subsequent Mössbauer experiments at room temperature, have revealed that the anapaite structure is not sensitive to oxidation since eventually only a small amount of Fe²⁺ (～6.5%) is converted into Fe³⁺.