57Fe Mössbauer spectroscopy and magnetization of cation-deficient Fe2TiO4 and FeCr2O4. Part I:57Fe Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectra are presented for synthetic cation-deficient Fe₂TiO₄ and FeCr₂O₄ spinel particles (<1μm) at various temperatures. The spectra of ferrimagnetic cation-deficient Fe₂TiO₄ show characteristic features due to relaxation because of superparamagnetism and spin relaxation in the temperature range 5–294 K. At 5 K and 78 K, a superposition of at least two sextets is observed which appear to arise from Fe³⁺ onA-sites (Fe _A ³⁺ andB-sites (Fe _B ³⁺ ) of the spinal lattice with magnetic hyperfine fields at 5 K ofB _hf ((Fe _B ³⁺ )≈47.5 T andB _hf (Fe _B ³⁺ )≈51.0 T, respectively. Cation-deficient FeCr₂O₄ particles reveal at 78 K a fieldB _hf (Fe³⁺)≈46.9 T and exhibit relaxation spectra as a consequence of superparamagnetism in the temperature range 80 K - ～300 K. At 294 K, quadrupole splitting Δ(Fe _A ³⁺ )=0.92 mm/s and isomer shift δ(Fe _A ³⁺ )=0.29 mm/s (relative to metallic Fe) are measured. For both compounds the magnetic hyperfine fieldsB _hf are discussed in terms of supertransferred hyperfine fields involving vacancies and in the case of cation-deficient Fe₂TiO₄ also diamagnetic Ti⁴⁺ neighbours of the Fe ions.     

Electrical and magnetic properties of cronstedtite

Two samples of cronstedtite, a mixed valence serpentine with ideal formula {Fe ₂ ²⁺ ,Fe³⁺}[Si,Fe³⁺]O₅(OH)₄, have been examined by X-ray and neutron diffraction, thermopiezic analysis, magnetization and susceptibility measurements and Mössbauer spectroscopy. The conductivity is thermally activated, with activation energies of 0.25 eV in the basal plane and 0.37 eV in the perpendicular direction. The shape of paramagnetic Mössbauer spectra above 200 K is influenced by charge fluctuations in octahedral sites and fits of spectra at temperatures up to 410 K with a stochastic relaxation model give an activation energy of 0.19 eV. Static charge ordering sets in progressively below about 100 K. Cronstedtite orders antiferromagnetically below 12 K in a structure with antiferromagnetic octahedral sheets and moments perpendicular to the a-axis. Magnetic and charge-ordered structures are proposed for the ideal composition.     

Ilvaite: A study of temperature dependent electron delocalization by the mössbauer effect

The mixed valence iron silicate ilvaite, CaFe ₂ ²⁺ Fe³⁺Si₂O₈(OH), displays electron delocalization associated with Fe²⁺→Fe³⁺ charge transfer as observed by Mössbauer spectroscopy. Previous studies report the observation of an ‘electron hopping phenomenon’ with resolution of discrete valence states below 320 K. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded over a temperature range, 80 K to 575 K. Five quadrupole doublets were resolved by computer fitting and assigned to Fe²⁺(A), Fe²⁺(B), Fe³⁺(A), and Fe²⁺(A)→Fe³⁺(A)‖c and ⊥c. Contrary to prior work, doublets associated with electron delocalization are resolved at 80 K and preclude the use of a Verwey-type order-disorder model. We propose a thermal activation model and discuss its criteria from molecular orbital and mineralogical viewpoints.     

Laihunite—A new iron silicate mineral

Laihuite reported in the present paper is a new iron silicate mineral found in China with the following characteristics:

1. This mineral occurs in a metamorphic iron deposit, associated with fayalite, hypersthene, quartz, magnetitc, etc.
2. The mineral is opaque, black in colour, thickly tabular in shape with luster metallic to sub-metallic, two perfect cleavages and specific gravity of 3.92.
3. Its main chemical components are Fe and Si with Fe³⁺>Fe²⁺. The analysis gave the formula of Fe Fe _1.00 ³⁺ ·Fe _0.58 ²⁺ ·Mg _0.03 ²⁺ ·Si_0.96O₄.
4. Its DTA curve shows an exothermic peak at 713°C.
5. The mineral has its own infrared spectrum distinctive from that of other minerals.
6. This mineral is of orthorhombic system; space group:C _2h ^/5 ?P2₁/c; unit cell:α=5.813ű0.005,b=4.812ű0.005,c=10.211ű0.005,β=90.87°.
7. The Mössbauer spectrum of this mineral is given, too.

     

Mössbauer,electrokinetic and refined lattice parameters study of synthetic manganoan lipscombite

Manganoan lipscombite (Fe _x ^/2+ , M _y ^/2+ ) Fe _3?(x ^+y)/3+ [OH)_3?(x+y)(PO₄)₂] was synthesized from pure chemicals. From the study of the Mn²⁺/Fe²⁺ atomic ratio by Mössbauer spectra, solubility, and electrokinetic properties, it was found that the crystal structure of lipscombite is not changed substantially by the manganese substitution. The unit cell parameters were determined from Guinier-Hägg X-ray diffraction patterns, which are identical for both synthetic ferrous-ferric and manganoan lipscombite. The two compounds crystallize in the tetragonal system with a=5.3020±0.0005 Å and c=12.8800±0.0005 Å.     

Crystallographic,Mössbauer and electrokinetic study of synthetic lipscombite

The transformation of vivianite and the direct synthesis starting from pure chemicals lead to the formation of lipscombite {Fe _x ²⁺ Fe _3?x ³⁺ [(OH)_3?x/(PO₄)₂]} with varying Fe²⁺/Fe³⁺ molar ratios. The influence of this ratio on the Mössbauer spectra, solubility, electrokinetic potential and infrared spectra has been studied. By means of Mössbauer spectroscopy, the distribution of the Fe²⁺ and Fe³⁺ ions between the octahedral sites I and II has been investigated. The unit cell dimensions have been determined from Guinier-Hägg X-ray diffraction patterns. The crystal system is tetragonal for synthetic lipscombite with a=5.3020±0.0005 Å and c=12.8800±0.0005 Å. Lipscombite has been found to show a negative and time-dependent zeta-potential which, moreover, is influenced by the pH of the suspension and the Fe²⁺/Fe³⁺ molar ratio. An explanation of the time-dependence of the zeta-potential on variations of solubility is proposed. Infrared absorption spectrum only is characterized by two absorption bands: v _OH(3,500 cm^?1) and v _P?O(1,100-960 cm^?1). The density at 25° C is determined in toluene as 3.36±0.01 g·cm^?3.     

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.     

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.     

Crystal chemistry of Fe3+-bearing (Mg,Fe)SiO3 perovskite: a single-crystal X-ray diffraction study

Magnesium silicate perovskite is the predominant phase in the Earth’s lower mantle, and it is well known that incorporation of iron has a strong effect on its crystal structure and physical properties. To constrain the crystal chemistry of (Mg, Fe)SiO₃ perovskite more accurately, we synthesized single crystals of Mg_0.946(17)Fe_0.056(12)Si_0.997(16)O₃ perovskite at 26 GPa and 2,073 K using a multianvil press and investigated its crystal structure, oxidation state and iron-site occupancy using single-crystal X-ray diffraction and energy-domain Synchrotron Mössbauer Source spectroscopy. Single-crystal refinements indicate that all iron (Fe²⁺ and Fe³⁺) substitutes on the A-site only, where \( {\text{Fe}}^{ 3+ } /\Upsigma {\text{Fe}}\sim 20\,\% \) based on Mössbauer spectroscopy. Charge balance likely occurs through a small number of cation vacancies on either the A- or the B-site. The octahedral tilt angle (Φ) calculated for our sample from the refined atomic coordinates is 20.3°, which is 2° higher than the value calculated from the unit-cell parameters (a = 4.7877 Å, b = 4.9480 Å, c = 6.915 Å) which assumes undistorted octahedra. A compilation of all available single-crystal data (atomic coordinates) for (Mg, Fe)(Si, Al)O₃ perovskite from the literature shows a smooth increase of Φ with composition that is independent of the nature of cation substitution (e.g., \( {\text{Mg}}^{ 2+ } - {\text{Fe}}^{ 2+ } \) or \( {\text{Mg}}^{ 2+ } {\text{Si}}^{ 4+ } - {\text{Fe}}^{ 3+ } {\text{Al}}^{ 3+ } \) substitution mechanism), contrary to previous observations based on unit-cell parameter calculations.     

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.     

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.     

Optical absorption spectra of iron ions in vivianite

Absorption bands are determined in polarized optical spectra of vivianite Fe₃(PO₄)₂·8H₂O, recorded at room and low temperatures. These bands are caused by spin-allowed d-d transitions in structurally nonequivalent Fe _A ²⁺ (～11000 cm^-1 (γ-polarization) (and) ～12000 cm^-1 (β-polarization)) (and) Fe _B ²⁺ (～8400 cm^-1 (γ, α-polarization) and ～11200 cm^-1 (α-polarization)) ions. A charge transfer band (CTB) Fe _B ²⁺ +Fe _B ³⁺ →Fe _B ²⁺ +Fe _B ²⁺ (～15000 cm^-1) also determined, has polarizing features giving evidence of a change in the Fe _B ²⁺ -Fe _B ³⁺ bond direction, when compared with Fe _B ²⁺ -Fe _B ²⁺ . Bands of exchange-coupled Fe³⁺-Fe³⁺ pairs (～19400, ～20400, ～21300 and ～21700 cm^-1) which appear on oxidation of Fe²⁺ in paired Fe _B octahedra are also characterized.     

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.     

Local distortion of the spodumene structure around isolated cations using E.P.R. and the superposition model

The superposition model is used to attempt to explain the EPR spectrum of Fe³⁺ in spodumene (LiAlSi₂O₆). Two different models for the distortion of the local environment of Fe³⁺ are considered. If the local structure around Fe³⁺ is distorted toward that of its isomorphic compound LiFeSi₂O₆, it is possible to obtain an agreement between experimental data and calculated constants, only if it is considered that the values of \(\bar B_2\) are different for the two non equivalent oxygens O₁ and O₂. A second model of local distortion of the structure, based on crystal chemical considerations, is proposed. With this model, it is possible to explain the EPR spectrum with a single value of \(\bar B_2\) . It is seen that the local arrangement around isolated Fe³⁺ in spodumene may be different from that in LiAlSi₂O₆.     

The structural state of iron in oxidized vs. reduced glasses at 1 atm: A57Fe Mössbauer study

A general model for the structural state of iron in a variety of silicate and aluminosilicate glass compositions in the systems Na₂O-Al₂O₃-SiO₂-Fe-O, CaO-Al₂O₃-SiO₂-Fe-O, and MgO-Al₂O₃-SiO₂-Fe-O is proposed. Quenched melts with variable Al/Si and NBO/T (average number of nonbridging oxygens per tetrahedrally coordinated cation), synthesized over a range of temperatures and values of oxygen fugacity, are analyzed with⁵⁷Fe Mössbauer spectroscopy. For oxidized glasses with Fe³⁺/∑Fe>0.50, the isomer shift for Fe³⁺ is in the range ～0.22–0.33 mm/s and ～0.36 mm/s at 298 K and 77 K, respectively. These values are indicative of tetrahedrally coordinated Fe^3?. This assignment is in agreement with the interpretation of Raman, luminescence, and X-ray,K-edge absorption spectra. The values of the quadrupole splitting are ～0.90 mm/s (298 K and 77 K) in the Na-aluminosilicate glasses and compare with the values of 1.3 mm/s and 1.5 mm/s for the analogous Ca- and Mg-aluminosilicate compositions. The variations in quadrupole splittings for Fe³⁺ are due to differences in the degree of distortion of the tetrahedrally coordinated site in each of the systems. The values of the isomer shifts for Fe²⁺ ions in glasses irrespective of Fe³⁺/∑Fe are in the range 0.90–1.06 mm/s at 298 K and 1.0–1.15 mm/s at 77 K. The corresponding range of values of the quadrupole splitting is 1.75–2.10 mm/s at 298 K and 2.00–2.35 mm/s at 77 K. The temperature dependence of the hyperfine parameters for Fe²⁺ is indicative of noninteracting ions, but the values of the isomer shift are intermediate between those values normally attributable to tetrahedrally and octahedrally coordinated Fe²⁺. The assignment of the isomer-shift values of Fe²⁺ to octahedral coordination is in agreement with the results of other spectral studies. For reduced glasses (Fe³⁺/∑Fe≈<0.50), the value of the isomer shift for Fe³⁺ at both 298 K and 77 K increases and is linearly correlated with decreasing Fe³⁺/∑Fe in the range of \(f_{O_2 } \) between 10^?3 and 10^?6 atm when a single quadrupole-split doublet is assumed to represent the absorption due to ferric iron. The increase in value of the isomer shift with decreasing \(f_{O_2 } \) is consistent with an increase in the proportion of Fe³⁺ ions that are octahedrally coordinated. The concentration of octahedral Fe³⁺ is dependent on the \(T - f_{O_2 } \) conditions, and in the range of log \(f_{O_2 } \) between 10^?2.0 and 10^?5 a significant proportion of the iron may occur as iron-rich structural units with stoichiometry similar to that of inverse spinels such as Fe₃O₄, in addition to isolated Fe²⁺ and Fe³⁺ ions.     

Structure and physical property relations of Mn ilvaite

Structural and compositional data as well as ⁵⁷Fe Mössbauer parameters were determined on a natural Mn-rich monoclinic ilvaite crystal (ideal composition CaFe ₂ ²⁺ Fe³⁺Si₂O₈(OH)) which was used for electrical conductivity and thermopower measurements (part 2 of this paper). A zonar structure was found by electron microprobe analysis with a strong decrease in Mn concentration from the rim to the centre of the crystal in a plane perpendicular to the [001] direction. X-ray powder diffraction analysis of the most Mn-rich composition was performed. Mn²⁺ cations populate preferentially M2 sites of the ilvaite unit cell (space group P21/a), to a lower extent they reside on M1 and a reduced part is on Ca sites. The monoclinic angle was determined to β=90.178(4)°. The structural results are compared to literature data for other natural Mn-rich as well as low-impurity ilvaites; this concerns in particular the lattice b parameter and the undecided issue of the varying β angle. In the literature, the order parameter σ, which describes the varying degree of ordering of Fe²⁺–Fe³⁺ pairs on M11 and M12 sites in chains running parallel to the [001] direction, and structural defects are thought to be related to β. The interrelationship between β and σ with respect to a possible twin domain structure is discussed. Various ⁵⁷Fe Mössbauer spectra were recorded between 151 K and 327 K. Mössbauer parameters and Fe²⁺/Fe³⁺ concentration ratios were determined from the fits to the spectra. Fitting of subspectra was accomplished with the idea to find assignments of Fe²⁺ and Fe³⁺ doublets in agreement with X-ray results. The fraction of Mn²⁺ substituting Fe²⁺ on M1 sites could be estimated.     

Spectroscopy of red dravite from northern Tanzania

Low-Fe dravite with a formula of Na_0.66Ca_0.16Mg_2.62Fe_0.33Mn_0.02Ti_0.02Al_5.95B₃Si_6.04O₂₇(OH)₄ is described from Engusero Sambu, northern Tanzania (On maps, Engusero Sambu may be found to be marked as belonging to Kenya, but in reality, it is located near the border in northern Tanzania). The sample has an unusual red color that is distinctly different from the red dravite from the Osarara, Narok district, in Kenya that was formerly studied by Mattson and Rossman (Phys Chem Miner 14:225–234, 1984) and Taran and Rossman (Am Mineral 87:1148–1153, 2002). This unique sample has been characterized by optical and Mössbauer spectral measurements to investigate underlying cause of the intense bands in absorption spectra that give rise to the red color. These features are shown to be caused by exchange-coupled Fe³⁺–Fe³⁺ interactions. Thermal annealing of the samples causes an increase in Fe³⁺ contents due to oxidation of ^[Y]Fe²⁺. However, heat treatment does not change the high-energy absorption edge, which is probably caused by intense ligand-to-Fe³⁺ charge-transfer UV bands. In fact, Mössbauer results show that high-temperature annealing initiates breakdown of the tourmaline into an Fe oxide and causes accompanying redistribution of Fe³⁺ within the structure. Because of the popularity of tourmaline as a gemstone, this work has implications for understanding the causes of color in tourmaline, facilitating recognition of the distinctions between naturally occurring and treated tourmalines in the gem industry and enabling heat treatments for color enhancement.     

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.     

Characterization and origin of two Fe3+ EPR spectra in kaolinite

The electron paramagnetic resonance (EPR) spectra of Fe³⁺ in a well cristallized kaolinite from Decazeville in France are well resolved. It is shown that in this sample there are mainly two slightly different spectra, well separated at low temperature and characterized at -150° C by the constants B ₂ ⁰ = 0.112 cm^?1, B ₂ ² = 0.0688 cm^?1 for one and B ₂ ⁰ = 0.116 cm^?1, B ₂ ² = 0.0766 cm^?1 for the second. These two spectra arise from Fe³⁺ substituted for Al³⁺ at the two octahedral positions in equal amounts. The temperature dependence of EPR spectra was studied and was explained by a modification of the octahedral sites.