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.     

Polymorphism in sapphirine

Sapphirine-1Tc, a polymorph of sapphirine, was found in granulites near Wilson Lake, Labrador. It is triclinic with unit cell data: a=10.04, b=10.38, c=8.65 Å, =107°33, =95°07, =123°55. The X-ray diffraction patterns of several crystals of sapphirine from Wilson Lake were investigated and evidence was found of solid state reactions that lead from the ordered polymorph-1Tc to a disordered arrangement of domains with the structure of sapphirine-2M, that is normal sapphirine.     

The red-green change in chromium-bearing garnets

The colour of chromium-bearing pyrope-rich garnets may be red or green depending upon the Cr-O distance which varies with the chemical composition. Red garnets become green when the crystal field parameter ₀ for substitutional Cr³⁺ decreases below 17200 cm^–1 corresponding to about 1970 Å.     

Instability of sapphirine at high pressures

The stability field of Mg-sapphirines is limited at high pressures through the solid-solid breakdown reaction sapphirine?pyrope = corundum+spinel, the univariant curve originating from an invariant point located at 22 kb, 880°C to 30 kb, 1350°C. Under water pressures less than 22 kb sapphirines exhibit the same low-temperature breakdown into the assemblage chlorite+corundum+spinel as determined by Seifert (1974) between 1 kb and 7 kb thus resulting in one continuous univariant lower stability limit extending from 1 kb, about 650°C through 10 kb, 770°C to the invariant point at 22 kb, 880°C. If \(P_{{\text{H}}_{\text{2}} {\text{0}}} < P_{{\text{total}}} \) , the stability field of sapphirine will expand towards lower temperatures. The occurrence of sapphirine in mantle depths requires rather aluminous bulk compositions, high geothermal gradients and/or \(P_{{\text{H}}_{\text{2}} {\text{0}}} < P_{{\text{total}}} \) , with total pressures not exceeding 30 kb. Thus sapphirine is probably not a stable phase in the lower portions of lithospheric plates and the underlying asthenosphere.     

Electronic absorption spectra of Fe2+ ions in oxygen-based rock-forming minerals at temperatures between 297 and 600 K

 Polarized electronic single crystal spectra of natural Fe²⁺ ion-bearing oxygen-based minerals, in which ferrous ions enter octahedral sites of different symmetry and distortion (olivine, cordierite, ortho- and clinopyroxene, amphibole), eightfold sites in garnet (almandine) and clinopyroxene (M2), and tetrahedral sites in spinel, were studied at temperatures from 300 to ca. 600 K. In the minerals studied, the spin-allowed bands of Fe²⁺ display rather variable temperature behaviour. In most cases, due to the thermal expansion of the Fe²⁺-bearing polyhedra, bands shift to lower energies upon increasing temperature, though there are some exceptions to this rule: in cases of other than sixfold octahedral or close to octahedral coordination, in almandine and spinel the bands shift to higher energies, which can be explained by an increase in distortions of the Fe²⁺-bearing polyhedra. Splitting of the excited ⁵ E _g-level of Fe²⁺ ions usually, but not always, increases with temperature, reflecting thermally induced increase in distortion of the Fe²⁺-bearing sites in the minerals studied. Integral intensities of the bands in question do not always obey the general rule, according to which intensity should increase with temperature, when the 3d ^N-centred site is centrosymmetric, or should remain unchanged when the 3d ^N site lacks an inversion centre. The experimental results show that the response of the characteristics of absorption bands such as width, intensity and energy caused by dd transitions of Fe²⁺ in oxygen-based minerals to increasing temperature is not always uniform and is at variance with expectation. This temperature dependence cannot be used directly to solve band assignment problems, as earlier proposed in the literature. Received: 22 December 1999 / Accepted: 30 October 2000     

Reassignment of the absorption spectra of purple yoderite

Polarized spectra E∥X, E∥Y, and E∥Z of purple yoderite, taken at 295 and 100 K, result in a revised interpretation for the mineral. Major bands at around 16,900 (X>Y?Z), 18,600 (X?Y), and 20,600 cm^?1 (X≈Z>Y) may be attributed to spin-allowed transitions of Mn³⁺ in trigonal bipyramids (A2 or A3). Minor features may be assigned to single ion Fe³⁺. However, charge transfer possibilities for bands at 18,600 and 25,500 cm^?1 in yoderite cannot be ruled out and are discussed. The extremely high intensity of Mn³⁺ spin-allowed bands (?, 216 to 1,900 [1·g-atom^?1·cm^?1]) is attributed to fivefold coordination of the A2 and A3 position and to the covalency of the Mn³⁺-O bonds.     

Electron density and polarized absorption spectra of fayalite

The maxima of the electron difference densities of Fe²⁺ atM(1) andM(2) positions of fayalite, Fe₂SiO₄, determined by x-ray diffraction are considered to correspond to atomic dipoles. Provided the selection rules of dipole radiation are satisfied and the energy of the incident radiation lies within the appropriate range, the interaction of incident radiation with these atomic dipoles should lead to three absorption bands of which two originate from Fe²⁺ atM(1), one from Fe²⁺ atM(2). The relative intensities of the three bands, dependent on the polarization direction, are estimated. The result ist in excellent agreement with the interpretation of olivine spectra given by Burns (1970).     

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.     

Planar and line defects in the sapphirine polytypes

Transmission electron microscopy of various natural and synthetic sapphirines shows the existence of extensive polytypic variation in some samples. In addition to the previously described 1Tc and 2M polytypes, 3Tc, 4M and 5Tc structures are observed. The structural principles underlying the polytypism and the associated diffraction behaviour are reviewed, and the correlation between the nature of the observed stacking faults and their origin during crystal growth, in deformation or as a result of solid-state transformation is discussed. Pairs of c/2 partial dislocations observed in synthetic material provide a likely solid-state transformation mechanism, although the evidence suggests that most observed stacking variation arises during growth.A new type of non-conservative planar defect has been found in sapphirine from Finero, Italy, suggestive of unit-cell scale intergrowth with a surinamite- or pyroxene-like structure. A polysomatic model of the sapphirine and surinamite structures as stackings of pyroxene P and spinel S modules is proposed.     

Calculated optical absorption spectra of Ni2+-bearing compounds

The transition energies responsible for optical absorption spectra can be obtained by crystal-field analysis, but the transition intensities are notoriously difficult to calculate. This paper examines the basic ingredients of the calculation of optical spectrum intensities. Magnetic dipole and electric quadrupole transitions intensities are evaluated, as well as the direct d(Ni²⁺) to p(O²⁻) electric dipole transitions. All these contributions are shown to be small in the optical range, so that spectral intensities are due to the mixing of odd orbitals with the Ni²⁺ 3d ⁿ states. Received: 11 November 1997 / Revised, accepted: 6 September 1999     

Single crystal absorption spectra of synthetic Ti,Fe-substituted pyropes

Synthetic pyrope crystals up to 0.5 mm in diameter, substituted by titanium or by titanium plus iron, were grown under defined conditions of P, T, $f_{O_2 }$ in the presence of water using a piston-cylinder device. The crystals were characterized by X-ray and microprobe techniques. Their single-crystal optical absorption spectra were measured by means of a microscope-spectrometer. Two absorption bands at 16100 and 22300 cm{cm-1} in the spectra of pale-blue Fe-free Ti-bearing pyropes, grown under reduced conditions, were identified as originating from spin-allowed transitions, derived from ² T _2g → ² E _g of octahedral Ti³⁺ ions. The splitting value of the excited ²E _g state, 6200 cm^-1, and the crystal field parameter of Ti³⁺ in pyrope Δ ₀ = 19 200 cm^-1 are both in agreement with literature data. In spectra of brown Fe, Ti-bearing garnets, a broad band at 23000 cm^-1 was interpreted as a Fe^2+[8] → Ti^4+[6] charge-transfer band. The spectral position and width of this band agree with those observed for a FeTi charge transfer band in natural garnets. Fe, Ti-containing garnets synthesized at relatively high oxygen fugacity (10^-11,0 atm), which permits a fraction of Fe³⁺ to enter the garnet, show an additional Fe^2+[8] → Fe^3+[6] charge transfer band at 19800 cm^-1.     

Synthesis and optical absorption spectra of Cr2+-containing orthosilicates

By reacting chromium metal with appropriate oxides in a neutral atmosphere at high temperature it is possible to prepare Mg₂SiO₄ with the olivine structure containing Cr⁺⁺ and also Cr₂SiO₄. X-ray powder diffraction data for chromous orthosilicate may be indexed on an orthorhombic cell with a=5.690, b=11.262, and c=9.584 Å. The compound is not isotructural with olivine. Optical absorption spectra of Cr²⁺ in both hosts have been obtained and also indicate large differences between the two structures.     

Polarized electronic absorption spectra of Cr2SiO4 single crystals

Polarized electronic absorption spectra, E∥a(∥X), E∥b(∥Y) and E∥c(∥Z), in the energy range 3000–5000?cm^–1 were obtained for the orthorhombic thenardite-type phase Cr₂SiO₄, unique in its Cr²⁺-allocation suggesting some metal-metal bonding in Cr²⁺Cr²⁺ pairs with Cr-Cr distance 2.75?Å along [001]. The spectra were scanned at 273 and 120?K on single crystal platelets ∥(100), containing optical Y and Z, and ∥(010), containing optical X and Z, with thicknesses 12.3 and 15.6?μm, respectively. Microscope-spectrometric techniques with a spatial resolution of 20?μm and 1?nm spectral resolution were used. The orientations were obtained by means of X-ray precession photographs. The xenomorphic, strongly pleochroic crystal fragments (X deeply greenish-blue, Y faint blue almost colourless, Z deeply purple almost opaque) were extracted from polycrystalline Cr₂SiO₄, synthesized at 35?kbar, above 1440?°C from high purity Cr₂O₃, Cr (10% excess) and SiO₂ in chromium capsules. The Cr₂SiO₄-phase was identified by X-ray diffraction (XRD). Four strongly polarized bands, at about 13500 (I), 15700 (II), 18700 (III) and 19700 (IV) cm^–1, in the absorption spectra of Cr₂SiO₄ single crystals show properties (temperature behaviour of linear and integral absorption coefficients, polarization behaviour, molar absorptivities) which are compatible with an assignment to localized spin-allowed transitions of Cr²⁺ in a distorted square planar coordination of point symmetry C₂. The crystal field parameter of Cr²⁺ is estimated to be 10?Dq?10700?cm^–1. A relatively intense, sharp band at 18400?cm^–1 and three other minor features can, from their small half widths, be assigned to spin-forbidden dd-transitions of Cr²⁺. The intensity of such bands strongly decreases on decreasing temperature. The large half widths, near 5000?cm^–1 of band III are indicative of some Cr-Cr interactions, i.e. δ-δ* transitions of Cr₂ ⁴⁺, whereas the latter alone would be in conflict with the strong polarization of bands I and II parallel [100]. Therefore, it is concluded that the spectra obtained can best be interpreted assuming both dd-transitions of localized d-electrons at Cr²⁺ as well as δ-δ* transitions of Cr₂ ⁴⁺ pairs with metal-metal interaction. To explain this, a dynamic exchange process 2 Cr_loc ²⁺?Cr_{2, cpl} ⁴⁺ is suggested wherein the half life times of the ground states of both exchanging species are significantly longer than those of the respective optically excited states, such that the spectra show both dd- and δ-δ*-transitions.     

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.     

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.