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.     

Ferric iron in tourmaline

Red Fe³⁺-rich and black Fe³⁺, Fe²⁺-rich tourmalines have been studied by optical and Mössbauer spectroscopies to determine the optical characteristics of Fe³⁺ in tourmaline. Prominent optical absorption features at 485 and 540 nm are assigned to transitions of multiple exchange-coupled Fe³⁺ pairs in several site combinations. These transitions are more intense than those of isolated Fe³⁺ and are polarized along the vector between the interacting ions, thus permitting site assignments. The 485 nm band occurs at an unusually low energy for Fe³⁺ in silicate minerals. Similar behavior has been observed in the spectrum of coalingite, Mg₁₀Fe ₂ ³⁺ (OH)₂₄CO₃·2H₂O, in which Fe³⁺ occurs in related pairs in edge-shared sheets. These lower energies are proposed to result from magnetic exchange in edge-shared geometries. Antiferromagnetic exchange has been confirmed by a variable temperature magnetic susceptibility study of a Kenyan dravite with 3.36 wt percent Fe. The Mössbauer spectrum of this sample is unusual in that it shows a pronounced decrease in width of component peaks from 298 K to 5 K.     

<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.     

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.     

Crystal chemistry of ferric iron in (Mg,Fe)(Si,Al)O<Subscript>3</Subscript> majorite with implications for the transition zone

Fifteen samples of (Mg,Fe)SiO₃ majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O₃ majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe³⁺ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe²⁺–Fe³⁺ charge transfer, where band intensity increases with increasing Fe³⁺ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe³⁺ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO₃ majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe³⁺ concentration of (Mg,Fe)(Si,Al)O₃ majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O₃ perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.     

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.     

Local environment and valence state of iron in microinclusions in fibrous diamonds: X-ray absorption and Mössbauer data

Iron valence state and local environment in a set of fibrous diamonds from Brazilian and Congolese placers were investigated using X-ray absorption and Mössbauer spectroscopies. It is shown that the diamonds could be divided into two main groups differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe²⁺ and Fe³⁺, most likely with Fe²⁺ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: The presence of metallic Fe and Fe₃O₄ is inferred from XAS measurements. Spatially resolved XANES and Mössbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of f_O2.     

<Superscript>57</Superscript>Fe Mössbauer spectroscopy,X-ray single-crystal diffractometry,and electronic structure calculations on natural sinhalites

Natural sinhalites, MgAlBO_4, from the Ratnapura District, Sri Lanka, and from Bodnar Quarry near Hamburg, Sussex Co., New Jersey, USA, have been characterized by ⁵⁷Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine the oxidation state and site occupancy of iron in the sinhalite structure. The samples contain about 3.35 and 1.46 wt% of total iron oxide, respectively. The structure refinement is successful and reproduces the total iron content provided that the substitution of Mg²⁺ by Fe²⁺ on the M2 position only is assumed. The ⁵⁷Fe Mössbauer spectra at 77, 293, 573 and 773 K can be resolved into two doublets with hyperfine parameters common for octahedrally coordinated high-spin Fe²⁺. There is no evidence for iron in the tetrahedral site. Electronic structure calculations in local spin density approximation yield hyperfine parameters for Fe²⁺ on the M2-site at 0, 293, 573 and 773 K in quantitative agreement with experiments. Calculated spectroscopic properties for Fe²⁺ on the M1-site are at variance with the experimental data and, thus, indicate that substitution of Al³⁺ by Fe²⁺, if occurring at all, must be accompanied by considerable local expansion and distortion of the M1-octahedron.     

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.     

<Superscript>57</Superscript>Fe Mössbauer spectroscopy,X-ray single-crystal diffractometry,and electronic structure calculations on natural alexandrite

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.     

The crystal chemistry and mössbauer study of schorlomite

Six schorlomite samples with TiO₂ contents varying between 9.70 and 15.34 weight percent were studied by means of Mössbauer spectroscopy and chemical analysis. The measured Mössbauer spectra have complex shapes. The spectra of these samples were fitted with six doublets, which can be assigned to ^VIIIFe²⁺, ^VIFe²⁺, ^VIFe³⁺, ^IVFe³⁺ and two electron delocalizations, ^IVFe³⁺ ? ^VIIIFe²⁺ and ^IVFe³⁺ ? ^VIFe²⁺, respectively. The assignment of iron absorption doublets and their Mössbauer parameters are discussed in terms of the single crystal structure data of one of the samples studied in this work. Cation distributions are also given. The occupancies of cations at the tetrahedral (Z) site are Fe³⁺>Al³⁺, Ti⁴⁺, and the relative enrichments at Z site are always Fe³⁺>Ti⁴⁺. Most of the six samples contain Ti³⁺. Ti³⁺/ΣTi ratios range from 1.43 percent to 6.40 percent. Fe²⁺/ΣFe ratios vary from 8.84 percent to 11.31 percent. Four types of substitution must be considered for Ti entering the garnet structure.     

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.     

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 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.     

Determination of quantitative cation distribution in orthopyroxenes from electronic absorption spectra

Electronic and Mössbauer absorption spectra and electron microprobe data are correlated for iron-bearing orthopyroxenes. The correlation provides a means of quantitatively determining the distribution of Fe²⁺ between the M(1) and M(2) sites of orthopyroxene crystals from electronic spectra and electron microprobe analysis. The electronic spectra are used to analyze the changes in the Fe²⁺ distribution produced during heating experiments and confirm earlier results from Mössbauer spectra. Two components of the spin-allowed transition of Fe²⁺ in the M(1) site are identified at about 13,000 cm^?1 and 8,500 cm^?1 in γ. Molar absorptivity (?) values for all spin-allowed Fe²⁺ absorption bands in the near-infrared region are determined. The M(2) Fe²⁺ band at ～5,000 cm^?1 in β is the analytically most useful for site occupancy determinations. It remains linear with concentration (?=9.65) over the entire compositional range. The band at ～10,500 cm^?1 in α is the most sensitive to M(2) Fe²⁺ concentration (?=40.8), but deviates from linearity at high iron concentrations. The origins of spin-forbidden transitions in the visible region are examined.     

Channel constituents in beryl

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.     

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.     

Local heterogeneity in the distribution of iron ions in CR-spinel from Ural ultramafic massifs: Evidence from Mössbauer spectroscopy data

The crystal chemistry of Fe ions in Cr-spinel from the largest Ural ultramafic massifs has been studied by Mössbauer spectroscopy at room temperature and the boiling temperature of liquid nitrogen. The spectra substantially depend on the mineral composition (stoichiometry) and measurement temperature; Fe²⁺ and Fe³⁺ doublets significantly overlap; the Fe²⁺ doublet lines are markedly broadened. According to the Mössbauer data, the degree of iron oxidation is 7–35% and appreciably differs from that in the stoichiometric approximation. The disturbance of integral stoichiometry by di- and trivalent cations (deviation of the Me³⁺/Me²⁺ value from 2.0) may be caused not only by partial inversion of the mineral structure but also by local micro- and nanoscale heterogeneity of the mineral, clustering of Fe²⁺ and Al (Cr, Fe³⁺) cations, and the appearance of associates. Possible application of the QS-distribution method for analyzing nonequivalent nuclear iron states and the thermal dynamics of Mössbauer spectra for studying local clustering effects of iron cations is discussed. It is shown that these approaches give new information on local heterogeneity of structural sites occupied by iron ions.     

The crystal chemistry of ferric iron in Fe0.05Mg0.95SiO3 perovskite as determined by Mössbauer spectroscopy in the temperature range 80–293 K

Mössbauer spectra were recorded at multiple temperatures between 80 and 293 K to study the nature of Fe³⁺ in Fe_0.05Mg_0.95SiO₃ 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: Θ_M (Fe²⁺)=365±52 K and Θ_M (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.