Correlation of spectroscopic properties and substitutional sites of Cr3+ in aluminosilicates: II. Andalusite and sillimanite

This paper is an extension of the earlier one dealing with kyanite in which the best fitting value of the oxygen ligand distance for Cr³⁺ is adopted to study the spectroscopic properties of Cr³⁺ ions doped at the two possible Al sites in the other two polymorphs of the aluminosilicate group (Al₂O₃ · SiO₂), namely, andalusite and sillimanite. The superposition model and the crystal field analysis package recently developed for 3d ions doped at arbitrary low symmetry sites in crystals are used to predict energy levels and statevectors within the whole 3d ³ configuration. Then the values of the ground state zerofield splitting for Cr³⁺ ions at each Al sites in the two crystals are obtained. The splittings of the lower excited states ² E and ⁴ T ₂ as well as the admixture of ⁴ T ₂ into ² E have also been predicted. Comparison of our results with the available experimental data enable us to correlate the optical and EPR Spectroscopic properties with the substitutional Cr³⁺ sites. The conclusion is that in andalusite and sillimanite only the Al sites with nearly-octahedral six-fold coordination seem to be occupied by Cr³⁺ ions.     

Spectroscopic characterisation and crystal field calculations of varicoloured kyanites from Loliondo, Tanzania

Orange, ochre-coloured, light green and dark blue varieties of kyanite, ideally Al₂SiO₅, from Loliondo, Tanzania, have been characterised by electron microprobe analysis and polarised infrared and optical absorption spectroscopy. All colour varieties show elevated Fe contents of 0.39 to 1.31 wt.% FeO, but Ti contents only in the range of the EMP detection limit. Orange and ochre-coloured crystals have Mn contents of 0.23 and 0.06 wt.% MnO, respectively, the dark blue kyanite contains 0.28 wt.% Cr₂O₃, while the light green sample is nearly free from transition metal cations other than Fe. Polarised infrared spectra reveal OH defect concentrations of 3 to 17 wt.ppm H₂O with structural OH defects partially replacing the O_B (O2) oxygen atoms. Polarised optical absorption spectra show that the colour of all four varieties is governed by crystal field d-d transitions of trivalent cations, i.e. Fe³⁺ (all samples), Mn³⁺ (orange and ochre) and Cr³⁺ (blue kyanite), replacing Al in sixfold coordinated triclinic sites of the kyanite structure. Intervalence charge transfer, the prevalent colour-inducing mechanism in ‘usual’ (Cr-poor) blue kyanites, seems to play a very minor, if any, role in the present samples. Crystal field calculations in both a ‘classic’ tetragonal and in the semiempirical Superposition Model approach, accompanied by distance- and angle-least-squares refinements, indicate that Fe³⁺ preferably occupies the Al4 site, Cr³⁺ prefers the Al1 and Al2 sites, and Mn³⁺ predominantly enters the Al1 site. In each case specific local relaxation effects were observed according to the crystal chemical preferences of these transition metal cations. Furthermore, the high values obtained in the calculations for the interelectronic repulsion parameter Racah B correspond to a high ionic contribution to Me³⁺–O bonding in the kyanite structure. In the particular case of the blue sample, band positions specifically related to the high Racah B value enable this ‘unusual’ type of blue colouration of kyanite solely due to Cr³⁺ cations.     

Electron spin resonance of trivalent chromium in forsterite,Mg2SiO4

The electron spin resonance (ESR) spectrum of Cr³⁺ in a synthetic single crystal of forsterite doped with Cr₂O₃ was studied at room temperature in the X-band frequency range. The dependence of the observed spectra on the crystal orientation with respect to the applied magnetic field was investigated. The ESR spectra are described by the spin Hamiltonian \(H = \beta HgS + D(S_Z^{\text{2}} - {\text{1/3}}S{\text{(}}S{\text{ + 1)) + }}E{\text{(}}S_x^{\text{2}} - S_y^{\text{2}} {\text{)}}\) with S=3/2. The spin resonance reveals that the chromium ions are located at both the M1 and M2 positions. Other possible substitutional or interstitial Cr³⁺ positions may be possible, but were not observed. The site occupancy numbers of Cr³⁺ at M1 and M2 are roughly 1.2×10^?4 and 0.8×10^?4, respectively, assuming that chromium is oxidized completely. The preference of the chromium ions for M1 was interpreted qualitatively in terms of crystal field criteria. The rhombic and axial spin Hamiltonian parameters, D and E, and the directions of the magnetic axes obtained for M1 and M2 are consistent with the respective oxygen coordination polyhedra.     

Optical absorption and electron spin resonance in blue and green natural beryl

A comparative study of blue and green beryl crystals (from the region of Governador Valadares, Minas Gerais, Brazil) using electron paramagnetic resonance (EPR) and optical absorption (OA) spectroscopy is reported. The EPR spectra show that Fe³⁺ in blue beryl occupies a substitutional Al³⁺ site and in green beryl is localized in the structural channels between two O₆ planes. On the other hand the infrared spectra show that the alkali content in the blue beryl is mostly at substitutional and/or interstitial sites and in green beryl is mostly in the structural channels. The OA spectra show two types of Fe²⁺. Thermal treatments above 200° C in green beryl cause the reduction of Fe³⁺ into Fe²⁺ accompanied by a change of color to blue. The blue beryl color does not change on heating. The kinetics of the thermal conversion of Fe³⁺ into Fe²⁺ is composed of two first order processes; the first one has an activation energy ΔE ₁=0.30 eV and the second one has an activation energy ΔE ₂=0.46 eV.     

Electronic absorption spectra of Cr3+ ions in natural clinopyroxenes

The polarized (E‖a′, E‖b and E‖c) electronic absorption spectra of five natural chromium-containing clinopyroxenes with compositions close to chromdiopside, omphacite, ureyite-jadeite (12.8% Cr₂O₃), jadeite, and spodumene (hiddenite) were studied. The polarization dependence of the intensities of the Cr³⁺ bands in the clinopyroxene spectra cannot be explained by the selection rules for the point groups C ₂ or C _2v but can be accounted for satisfactorily with the help of the higher order pseudosymmetry model, i.e. with selection rules for the point symmetry group C _3v. The trigonal axis of the pseudosymmetry crystal field forms an angle of 20.5° with the crystallographic direction c in the (010) plane. D _q increases from diopside (1542 cm^?1) through omphacite (1552 cm^?1), jadeite (1574 cm^?1) to spodumene (1592 cm^?1). The parameter B which is a measure of covalency for Cr³⁺-O bonds at M1 sites in clinopyroxene depends on the Cr³⁺ concentration and the cations at M2 sites.     

Hyperfine interactions and spin transfer between Cr3+ and Al3+ in a synthetic single crystal of Mg2SiO4. A theoretical approach to the interpretation of EPR data

The occurrence of Cr-Al pairs in Mg₂SiO₄ has been detected by EPR spectroscopy. In the case where Cr³⁺ replaces Mg at the M2 position three different neighboring Si sites may be substituted by Al³⁺, which should yield different superhyperfine interactions. A new spectrum is presented which shows the presence of two of these possible pair configurations. An assignment of the spectral features to a specific Cr-Al pair with Cr at M2 from the experimental data alone was not possible, therefore, MSX _α cluster calculations have been performed from which the differences in the superhyperfine interaction for the various pair configurations could be obtained. Best agreement with the data of the Cr³⁺(M2)-Al pair exhibiting the most intense group of lines in the EPR spectrum was obtained for the situation where Al³⁺ is at the Si position with the shortest distance to M2. The second observed Cr³⁺(M2)-Al pair, which is significantly weaker in intensity, could not yet be assigned.     

Measurement of the polarized absorption spectra of synthetic transition metal-bearing silicate microcrystals in the spectral range 44,000-4,000 cm−1

A new single beam microtechnique has been developed for measuring the polarized absorption spectra in the region 44,000-4,000 cm^?1. Spectra of a natural garnet (Spess₇₀Alm₃₀), measured by the microtechnique and by conventional macrotechniques, are consistent and thus prove the applicability of the microtechnique described. It is possible to obtain well resolved spectra down to about 13,000 cm^?1 with crystals as small as about 10 μm. Thus spectra of crystals obtained in routine high-pressure high-temperature silicate syntheses can be measured. The polarized spectra of Mn³⁺, Fe³⁺, Fe²⁺, and Cr³⁺ in the following synthetic silicate minerals are presented: piemontite (I), acmite (II), orthoferrosilite (III), and kyanite (IV) or uvarovite (V), respectively. O-Cr³⁺, O-Mn³⁺, and O-Fe²⁺ charge transfer band maxima in the UV region are identified at 38,700 cm^?1, in V; at 33,200, 35,300, and 39,000 cm^?1, in I; and at 32,800, 35,200, and 37,300 cm^?1, in III, respectively. Bands in the region ≦25,000 cm^?1 are assigned to spin-allowed and spin-forbidden dd transitions as predicted from crystal field theoretical considerations for the foregoing ions in the respective structures.     

Synthetic and natural chromium-bearing spinels: an optical spectroscopy study

Four samples of synthetic chromium-bearing spinels of (Mg, Fe²⁺)(Cr, Fe³⁺)₂O₄ composition and four samples of natural spinels of predominantly (Mg, Fe²⁺)(Al, Cr)₂O₄ composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr₂O₄ spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr³⁺ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed ⁴ A _2g  → ⁴ T _2g and ⁴ A _2g  → ⁴ T _1g transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions ⁴ A _2g  → ² E _g and ⁴ A _2g  → ² T _1g of Cr³⁺, intensified by exchange-coupled interaction between Cr³⁺ and Fe³⁺ at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm^?1. A vague broad band in the range from ca. 15,000 to 12,000 cm^?1 in synthetic spinels is tentatively attributed to ^IVCr²⁺ + ^VICr³⁺ → ^IVCr³⁺ + ^VICr²⁺ intervalence charge-transfer transition. Iron, mainly as octahedral Fe³⁺, causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O^2? → Fe³⁺, Fe²⁺ transitions). As tetrahedral Fe²⁺, it appears as a strong infrared absorption band at around 4,850 cm^?1 caused by electronic spin-allowed ⁵ E → ⁵ T ₂ transitions of ^IVFe²⁺. From the composition shift of the U-band in natural and synthetic MgCr₂O₄ spinels, the coefficient of local structural relaxation around Cr³⁺ in spinel MgAl₂O₄–MgCr₂O₄ system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O₆ polyhedra known so far. The octahedral modulus of Cr³⁺ in MgCr₂O₄, derived from pressure-induced shift of the U-band of Cr³⁺, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr³⁺ in natural and synthetic MgCr₂O₄ spinels indicates that Cr–O-bonding in octahedral sites of MgCr₂O₄ has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl₂O₄ spinel, B does not much depend on pressure.     

Local relaxations around Fe3+ and Cr3+ in Al sites in minerals

Second-order zero-field splitting (ZFS) parameters from the literature for Fe³⁺ in twelve and for Cr³⁺ in seven minerals substituting for Al were evaluated by application of the superposition model. For Fe³⁺ in monoclinic site symmetries a fair agreement of the observed splitting patterns with those calculated from the crystal structure data was observed in most cases, but the distortions for Fe³⁺ appear to be usually larger than those of the unrelaxed Al sites. In cases of not too large local relaxation the unknown sign of the axialZFS parameterb ₀ ² could be predicted, in two cases a different sign than that reported was postulated. In cordierite and scolecite the reportedEPR spectra could thus be assigned to the sites with larger average bond distances. For Fe³⁺ in beryl the relaxation of the axial site can be deduced within narrow limits. For Cr³⁺ significantly larger differences between observed and calculatedZFS patterns are found suggesting additional relaxations due to the non-spherical electron distribution in the ground state of this ion.     

Vibrational structure of luminiscence spectrum of Cr3+ in MgAl2O4

The optical absorption and luminescence spectra of MgAl₂O₄:Cr³⁺ natural spinel (from Ural) have been measured at 77 K and 293 K. The luminescent emission from ⁴ T _2g , ² E _g covers wide region of 600–750 nm. The emission spectrum at 77 K shows a very rich vibrational structure which can be mainly explained through the vibrational modes of the oxygen octahedron.     

Beryl-II,a high-pressure phase of beryl: Raman and luminescence spectroscopy to 16.4 GPa

The Raman and Cr³⁺ and V²⁺ luminescence spectra of beryl and emerald have been characterized up to 15.0 and 16.4 GPa, respectively. The Raman spectra show that an E _1g symmetry mode at 138 cm^?1 shifts negatively by ?4.57 (±0.55) cm^?1/GPa, and an extrapolation of the pressure dependence of this mode indicates that a soft-mode transition should occur near 12 GPa. Such a transition is in accord with prior theoretical results. Dramatic changes in Raman mode intensities and positions occur between 11.2 and 15.0 GPa. These changes are indicative of a phase transition that primarily involves tilting and mild distortion of the Si₆O₁₈ rings. New Raman modes are not observed in the high-pressure phase, which indicates that the local bonding environment is not altered dramatically across the transition (e.g., changes in coordination do not occur). Both sharp line and broadband luminescence are observed for both Cr³⁺ and V²⁺ in emerald under compression to 16.4 GPa. The R-lines of both Cr³⁺ and V²⁺ shift to lower energy (longer wavelength) under compression. Both R-lines of Cr³⁺ split at ~13.7 GPa, and the V²⁺ R₁ slope changes at this pressure and shifts more rapidly up to ~16.4 GPa. The Cr³⁺ R-line splitting and FWHM show more complex behavior, but also shift in behavior at ~13.7 GPa. These changes in the pressure dependency of the Cr³⁺ and V²⁺ R-lines and the changes in R-line splitting and FWHM at ~13.7 GPa further demonstrate that a phase transition occurs at this pressure, in good agreement with our Raman results. The high-pressure phase of beryl appears to have two Al sites that become more regular under compression. Hysteresis is not observed in our Raman or luminescence spectra on decompression, suggesting that this transition is second order in nature: The occurrence of a second-order transition near this pressure is also in accord with prior theoretical results. We speculate that the high-pressure phase (beryl-II) might be a mildly modulated structure, and/or that extensive twinning occurs across this transition.     

Electron delocalization in ilvaite,a reinterpretation of its 57Fe Mössbauer spectrum

The Mössbauer spectra of ilvaite CaFe ₂ ²⁺ Fe³⁺[Si₂O₇/O/OH] and their temperature dependence between 298 K and 455 K can be satisfactorily least-squares fitted by a superposition of the resonances for Fe²⁺(8d), Fe³⁺(8d) and Fe²⁺(4c). The relative areas under the three resonances are nearly equal and vary only weakly with temperature. No additional resonances or line broadenings have to be introduced, if we assume that the hyperfine interactions of Fe²⁺(8d) and Fe³⁺(8d) fluctuate between their values due to electron hopping between the iron ions at the 8d sites. Hopping can be assumed to occur homogeneously among nearly equivalent sites. The fluctuation rate is described by an Arrhenius law with a pre-exponent of about 9 × 10⁸ s^?1 and an activation energy of 0.11 eV indicating non-adiabatic hopping. In addition to the intersite hopping process, the strong decrease of the quadrupole splitting and the isomer shift of Fe²⁺(8d) between 298 K and 360 K suggests the occurrence of intrinsic charge delocalization from Fe²⁺(8d) which does not involve the neighbouring Fe³⁺(8d) ions.     

Electron paramagnetic resonance and ENDOR studies of Cr3+ - Al3+ pairs in forsterite

The electron paramagnetic resonance (EPR) spectrum of Cr³⁺ in synthetic crystals of forsterite consists primarily of lines of Cr³⁺ “isolated” at the M1 and M2 positions in a “perfect” crystal environment without local charge compensation. In addition it shows two nonequivalent superhyperfine-split sextets with different intensities which are due to strong interaction of the Cr³⁺ electron spin S (S=3/2) with an adjacent nuclear spin I(I=5/2). Electron nuclear double resonance (ENDOR) experiments revealed that the sextets result from Cr³⁺ (M1) - Al³⁺ and Cr³⁺ (M2) - Al³⁺ pairs, Al being located at adjacent tetrahedral Si sites. The g, D, A, and A′ tensor components of the Cr³⁺ - Al³⁺ pairs have been determined at room temperature. The values of the pairs are distinct although they are not very different from the corresponding data of “isolated” Cr³⁺. From the intensities of the EPR spectra the relative concentration of the Cr³⁺ - Al³⁺ pairs with respect to “isolated” Cr³⁺ has been estimated.     

Crystal-field spectra for blue and green diopsides synthesized in the join CaMgSi2O6-CaCrAlSiO6

Synthetic diopsides in the join CaMgSi₂O₆ CaCrAlSiO₆ have been studied by means of crystal-field theory. These diopsides are either blue or pale green in colour. The former forms at lower temperatures and the latter at higher temperatures. The optical spectra and colours can be unequivocally explained based on the assumption that Cr³⁺ions occupy both tetrahedral and octahedral sites in the diopsides. In the blue diopsides Cr³⁺ions are present in two kinds of spin state, i.e., tetrahedrally coordinated low spin and octahedrally coordinated high spin. On heating the blue diopsides, tetrahedral occupancy of chromium decreases sharply due to spin transformation from tetrahedral low spin to octahedral high spin. Above 1,160° C nearly all chromium ions are present in octahedral sites with high spin state and the diopsides become pale green in colour. Some petrogenetic applications of the resultes are discussed.     

Evaluation,characterization and analytical application of a new composite material for removing metal ions from wastewater

A new organic/inorganic composite based on polyacrylonitrile and stannic molybdophosphate (PAN–SMP) as an adsorbent was synthesized under various conditions. The physicochemical properties of this material were specified by elemental analysis, scanning electron microscopy, infrared spectroscopy and thermogravimetry studies. The synthesized material was found to be stable in demineralized water, in dilute acids, under gamma radiation up to the total radiated of 100 kGy doses and in high temperature up to 500 °C. Ion exchange capacity of the synthesized composite and its distribution coefficient (K _d) for several metal ions were determined. The results showed that PAN–SMP has a great affinity toward some metal ions such as T^l+, Sr²⁺, Ba²⁺, UO₂ ²⁺ and La⁴⁺. Based on the determined K _d values, two binary quantitative separations of metal ions (Cr⁶⁺ from Cu²⁺ and Pb²⁺ from Cu²⁺) have been achieved on columns of this ion exchanger. The ability of PAN–SMP to decontaminate low-level liquid waste was also investigated.     

Use of ultrasonically modulated electron resonance to study S-state ions in mineral crystals: Mn2+, Fe3+, in tremolite

The use of ultrasonically modulated electron resonance (UMER) to study S-state ions in substitutional sites of mineral single crystals is discussed. Mn²⁺ and Fe³⁺ in natural single crystals of tremolite are used as examples. Combined electron paramagnetic resonance (EPR) and UMER measurements establish almost certainly that Mn²⁺ enters predominantly into the distorted M4 sites occupied by Ca²⁺ in the ideal tremolite structure and only to a minor extent into the M1, M2 and M3 sites normally occupied by Mg²⁺. Fe³⁺ in tremolite gives rise to the well known high spin resonance with g _eff?4.3 but there is considerable uncertainty as to the site of the impurity ion.     

Structure of Cr2SiO4 and possible metal-metal interactions in crystal and melt

Cr₂SiO₄ has been prepared both as quench crystals and as an apparently stable subsolidus phase, at T=1600 and 1650 °C, P=37 kbar on Cr₂SiO₄ and Cr-SiO₃ bulk compositions. Crystal structure determination by Rietveld analysis of X-ray powder data from 3 samples show the structure, in space group Fddd, to be related to that of Cd₂SiO₄ (thenardite Na₂SO₄-V structure) but with approximately “square planar” coordination of the Cr²⁺ (d ⁴) atom. In one of the axial ligand positions, normal to the strongly bonded CrO₄ equatorial plane, is found a Cr atom (Cr-Cr=2.75 Å). Stereochemistry permits this contact to be a weak metal-metal bond, though it cannot be ruled out to be a non-bonded result of polyhedral connectivity.     

Chromium solubility in anhydrous Phase B

The crystal structure and chemical composition of a crystal of (Mg_14?x Cr _x )(Si_5?x Cr _x )O₂₄ (x ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr₂O₄–Mg₂SiO₄ at 12 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pmcb, with lattice parameters a = 5.900(1), b = 14.218(2), c = 10.029(2) Å, V = 841.3(2) Å³ and Z = 2. The structure was refined to R ₁ = 0.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145 Å) and Si at the octahedral Si1 site (mean bond distance: 1.856 Å), according to the reaction Mg²⁺ + Si⁴⁺ = 2Cr³⁺. Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe²⁺-bearing Anh-B. Taking into account that Cr³⁺ is not expected to be Jahn–Teller active, it appears that both the Cr³⁺–for–Mg and Cr³⁺–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg_13.66(8)Si_4.70(6)Cr_0.62(4)O₂₄ stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345 km depth in several subcontinental and subduction zone environments.     

Structure analysis on synthetic emerald crystals

Single crystals of emerald synthesized by means of the flux method were adopted for crystallographic analyses. Emerald crystals with a wide range of Cr³⁺-doping content up to 3.16 wt% Cr₂O₃ were examined by X-ray single crystal diffraction refinement method. The crystal structures of the emerald crystals were refined to R ₁ (all data) of 0.019–0.024 and wR ₂ (all data) of 0.061–0.073. When Cr³⁺ substitutes for Al³⁺, the main adjustment takes place in the Al-octahedron and Be-tetrahedron. The effect of substitution of Cr³⁺ for Al³⁺ in the beryl structure results in progressively lengthening of the Al–O distance, while the length of the other bonds remains nearly unchanged. The substitution of Cr³⁺ for Al³⁺ may have caused the expansion of a axis, while keeping the c axis unchanged in the emerald lattice. As a consequence, the Al–O–Si and Al–O–Be bonding angles are found to decrease, while the angle of Si–O–Be increases as the Al–O distance increases during the Cr replacement.     

Thermal annealing-induced electric dipole relaxation in natural alexandrite

Electrical properties of natural alexandrite (BeAl₂O₄:Cr³⁺) are investigated by the thermally stimulated depolarization current (TSDC) technique. Samples are submitted to consecutive annealing processes and TSDC is carried out after each annealing, yielding bands with different parameters. These bands are fitted by a continuous distribution of relaxation parameters: activation energy and pre-exponential factor of the Arrhenius equation. It has been observed that annealing influences the dipole relaxation behavior, since it promotes a modification of Fe³⁺ and Cr³⁺ impurity distributions on sites of distinct symmetry: Al₁ and Al₂. In order to have a reference for comparison, TSDC is also carried out on a synthetic alexandrite sample, where the only impurity present is Cr³⁺ ion.