Single-crystal electronic absorption spectroscopy of synthetic chromium-, cobalt-, and vanadium-bearing pyropes at different temperatures and pressures

 Single crystals of synthetic vanadium-, chromium- and cobalt-bearing garnets, Pyr:V0.06, Pyr:V0.13, Pyr:Cr0.04, Pyr:Co0.10, and Gt:Co3.00, and a natural vanadium-bearing grossular, Gross:V0.07 (Cr³⁺ < 0.005), were studied by electronic absorption spectroscopy in the wavenumber range 35 000–5000 cm⁻¹ under ambient conditions and at temperatures up to 600 K and pressures up to 8 GPa. The T and P behavior of the absorption band energies and intensities shows the following for the different transition metal-bearing garnets: Cr: The thermal expansion of chromium octahedra are similar to and the Racah parameter the same in synthetic Cr-doped pyrope, α_poly≅ 1.3 × 10⁻⁵ K⁻¹, and in natural pyrope, α_poly≅ 1.5 × 10⁻⁵ K⁻¹, and B=655 cm⁻¹, respectively. Ca^2+[8]-free garnets have a slightly stronger crystal field at the Y^[6] site and, therefore, the energies of the two spin-allowed Cr³⁺ dd bands are ca. 300 cm⁻¹ higher in Mg-pyrope than in natural Ca-bearing pyrope. Co: Increasing temperature causes only a small thermal expansion of the cobalt dodecahedra. Increasing pressure gives rise to appreciable compression, which is similar to that of the Fe²⁺-dodecahedra in almandine, where k=125 ± 25 GPa. T and P dependence of the Co band intensities may be caused by strong spin-orbit coupling. V: Occurs in at least two valence states and structural sites: (1) V³⁺ in octahedral sites gives rise to two spin-allowed bands, at 17 220 cm⁻¹ and 24 600 cm⁻¹, whose temperature dependence is typical for spin-allowed dd transitions in centrosymmetric sites. (2) V⁴⁺, which causes a set of dd absorption bands similar to those observed in the spectrum of V⁴⁺-doped Zr[SiO₄]. The P behavior of the V absorption bands indicates an interaction between V³⁺ and V⁴⁺ species. Received: 27 June 2001 / Accepted: 19 December 2001     

Polarized electronic absorption spectra of synthetic (Mg,Fe)-orthopyroxenes,ferrosilite and Fe3+-bearing ferrosilite

The assignment of spin-allowed Fe²⁺-bands in orthopyroxene electronic absorption spectra is revised by studying synthetic bronzite (Mg_0.8 Fe_0.2)₂Si₂O₆, hypersthene (Mg_0.5 Fe_0.5)₂Si₂O₆ and ferrosilite (Fe₂Si₂O₆). Reheating of bronzite and hypersthene single crystals causes a redistribution of the Fe²⁺-ions over the M1 and M2 octahedra, which was determined by Mössbauer spectroscopy and correlated to the intensity change of the spin-allowed Fe²⁺ d-d bands in the polarized absorption spectra. The 11000 cm^-1 band is caused by Fe²⁺ in M1 (⁵B_2g→⁵A_1g) and Fe²⁺ in M2 (⁵A₁→⁵A₁), the 8500 cm^-1 band by Fe²⁺ in M1 (⁵B_2g→⁵B_1g) and the 5000 cm^-1 band by Fe²⁺ in M2 octahedra (⁵A₁→⁵B₁). The Fe²⁺-Fe³⁺ charge transfer band is identified at 12500cm^-1 in the spectra of synthetic Fe³⁺ -Al bearing ferrosilite. This band shows a strong γ-polarization and therefore is caused by Fe²⁺ -Fe³⁺-ions in edge-sharing octahedra.     

Electronic absorption by Ti3+ ions and electron delocalization in synthetic blue rutile

Polarized absorption spectra, σ and π, in the spectral range 30000–400 cm⁻¹ (3.71–0.05 eV) were obtained on crystal slabs // [001] of deep blue rutile at various temperatures from 88 to 773 K. The rutile crystals were grown in Pt-capsules from carefully dried 99.999% TiO₂ rutile powder at 50 kbar/1500 °C using graphite heating cells in a belt-type apparatus. Impurities were below the detection limits of the electron microprobe (about 0.005 wt% for elements with Z≥13). The spectra are characterized by an unpolarized absorption edge at 24300 cm⁻¹, two weak and relatively narrow (Δν_1/2≈3500–4000 cm⁻¹), slightly σ-polarized bands ν₁ at 23500 cm⁻¹ and ν₂ at 18500 cm⁻¹, and a complex, strong band system in the NIR (near infra red) with sharp weak peaks in the region of the OH stretching fundamentals superimposed on the NIR system in the σ-spectra. The NIR band system and the UV edge produce an absorption minimum in both spectra, σ and π, at 21000 cm⁻¹, i.e. in the blue, which explains the colour of the crystals. Bands ν₁ and ν₂ are assigned to dd transitions to the Jahn-Teller split upper E_g state of octahedral Ti³⁺. The NIR band system can be fitted as a sum of three components. Two of them are partly π-polarized, nearly Gaussian bands, both with large half widths 6000–7000 cm⁻¹, ν₃ at 12000 cm^–1 and the most intense ν₄ at 6500 cm⁻¹. The third NIR band ν₅ of a mixed Lorentz-Gaussian shape with a maximum at 3000 cm⁻¹ forms a shoulder on the low-energy wing of ν₄. Energy positions, half band widths and temperature behaviour of these bands are consistent with a small polaron type of Ti³⁺Ti⁴⁺ charge transfer (CT). Polarization dependence of CT bands can be explained on the basis of the structural model of defect rutile by Bursill and Blanchin (1983) involving interstitial titanium. Two OH bands at 3322 and 3279 cm⁻¹ in σ-spectra show different stability during annealing, indicating two different positions of proton in the rutile structure, one of them probably connected with Ti³⁺ impurity. Total water concentration in blue rutile determined by IR spectroscopy is 0.10 wt-% OH. The EPR spectra measured in the temperature interval 20–295 K show the presence of an electron centre at temperatures above 100 K and Ti³⁺ ions in more than one structural position, but predominantly in compressed interstitial octahedral sites, at lower temperatures. These results are in good agreement with the conclusions based on the electronic absorption data. Received: 24 March 1997 / Revised, accepted: 14 October 1997     

Electron paramagnetic resonance and polarized optical absorption spectra of Ni2+ in synthetic forsterite

The investigated Ni doped forsterite was grown with the floating zone technique. The EPR spectra were taken at room temperature using both 9.5 and 35 GHz. All specimens show EPR signals resulting from Mn²⁺ at M2 and Fe³⁺ at M1, M2, and Si positions. Ni²⁺ EPR signals are observed at 35 GHz but not at 9.5 GHz. The Ni²⁺ spectra are described by the spin Hamiltonian

Polarized optical absorption spectra of synthetic chromium doped Mg2SiO4 (forsterite)

Polarized optical absorption measurements were carried out on three single crystals of Mg₂SiO₄ (forsterite), differently doped with Cr. Two crystals containing average 0.013 and 0.027 weight% Cr, respectively, were pulled from the melt in air, whereas one crystal containing average 0.08 weight% Cr was pulled from the melt in an argon atmosphere. The absorption spectra of the three crystals agree with each other although the intensity of single absorption bands varies significantly. In all -polarized patterns a sharp absorption line around 18000cm^-1 (550 nm) appears. Conjectures are presented to assign this line to the lasing center in Cr doped forsterite which very likely exists as Cr⁴⁺ at the fourfold coordinated Si site.     

The role of Fe2+ and Fe3+ in synthetic Fe-substituted tetrahedrite

Summary Tetrahedrites with the composition between Cu₁₂Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ were synthesized at 457 °C and 500 °C from the elements and carefully studied by Mössbauer spectroscopy of⁵⁷Fe. Between Cu₁₂Sb₄S₁₃ and Cu₁₁Fe₁Sb₄S₁₃ iron is predominantly ferric. Between Cu₁₁Fe₁Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ iron is predominantly ferrous and occupies the tetrahedral M1-sites.

---

Zusammenfassung Die Rolle von Fe²⁺ und Fe³⁺ in synthetischen Tetraedriten mit Fe-Substitution Tetraedrite mit einer Zusammensetzung zwischen Cu₁₂Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ wurden bei 457 °C und 500 °C aus den Elementen synthetisiert und sorgfdltig mit Mössbauer-Spektroskopie von⁵⁷Fe untersucht. Zwischen Cu₁₂Sb₄S₁₃ and Cu₁₁Fe₁Sb₄S₁₃ ist Eisen überwiegend dreiwertig. Zwischen Cu₁₁Fe₁Sb₄S₁₃ and Cu₁₁Fe₂Sb₄S₁₃ ist Eisen überwiegend zweiwertig und besetzt die tetraedrisch koordinierten M1-Plätze.

     

Polarized electronic absorption spectra of colourless chalcocyanite,CuSO4, with a survey on crystal fields in Cu2+ minerals

Polarized electronic absorption spectra of colourless chalcocyanite, CuSO₄, have been measured using microscope-spectrometric techniques. The spectra are characterized by a structured and clearly polarized band system in the near-infrared spectral range with components centred at 11,720, 10,545, 9,100, and 7,320 cm^?1, which have been assigned to crystal field d–d transitions of Cu²⁺ cations in pseudo-tetragonally elongated CuO₆ polyhedra with point symmetry C _i ( \(\bar{1}\) ). The polarization behaviour is interpreted based on a D ₂(C ₂″) pseudo-symmetry. Crystal field calculations were performed for the actual triclinic point symmetry by applying the Superposition Model of crystal fields, as well as in terms of a ‘classic’ pseudo-tetragonal crystal field approach yielding the parameters Dq _(eq) = 910, Dt = 395, and Ds = 1,336 cm^?1, corresponding to a cubically averaged Dq _cub = 679 cm^?1. A comparative survey on crystal fields in Cu²⁺ minerals shows that the low overall crystal field strength in chalcocyanite, combined with a comparatively weak pseudo-tetragonal splitting of energy levels, is responsible for its unique colourless appearance among oxygen-based Cu²⁺ minerals. The weak crystal field in CuSO₄ can be related to the lower position of the SO₄ ^2? anion compared to, e.g. the H₂O molecule in the spectrochemical series of ligands.     

Optical absorption spectroscopy of synthetic tourmalines

Tourmaline solid solutions containing Fe, Fe+ Ti, Cr, Ni, Cu, Co, Mn chromophoric centers have been grown hydrothermally at 650° C and 1,5 kbar on natural seeding plates close to the elbaite composition. The newly grown tourmalines were characterized by chemical analyses and optical absorption spectroscopy in the range 26316-5000 cm^-1 at 297 K and in the range 26316-9090 cm^-1 at 77 K. Most characteristic of Fe²⁺, Fe³⁺-bearing specimens is the presence of intensive σpolarized absorption bands caused by exchange-coupled Fe²⁺-Fe³⁺ pairs in Y- and Z-sites of the tourmaline structure. An additional intensive absorption band 12500 cm^-1 (σ-polarisation) appears in some specimens but is not yet found in spectra of natural tourmalines. The colour and spectroscopic properties of the Fe³⁺, Mn³⁺ and Cu²⁺ containing tourmalines are significantly affected by the presence of even the smallest Li-contents. The results suggest that Fe²⁺, Cu²⁺, Co²⁺, Ni²⁺-ions occupy, predominantly, Y-sites of the tourmaline structure, whereas the Cr³⁺-ions seem to enter the smaller Z-octachedra.     

Electronic absorption spectra of chromium-bearing sapphirine

Violet, non-pleochroic and greenish-blue, pleochroic chromium-substituted sapphirines were found in corundum-bearing spinel-websterite xenolites from the Yakutian kimberlite pipes Noyabrskaya (N) and Sludyanka (Sl), respectively. The crystallochemical formulae of sapphirine crystals from such xenolites were determined by EMP to be (Mg_3.40Fe_0.23Al_3.25Cr_0.16)^[6] Al _1.00 ^[6] [O₂/Al_4.53Si_1.47O₁₈] (N) and (Mg_2.53Fe_0.55 Mn_0.04Ti _0.03 ⁴⁺ Al_3.55Cr _0.08 ³⁺ )^[6]Al _1.00 ^[16] [O₂/Al_4.28Si_1.73O₁₈] (Sl). Single crystal spectra in the range 35000–6000 cm^1- showed a slightly polarization dependent absorption edge near 3200 cm^1- (N) or 30000 cm^1- (Sl) and unpolarized bands at 25300 and 17300 cm^1-, typical of spin-allowed transitions, derived from ⁴A_2g→⁴T_1g and ⁴A_2g→⁴T_2g, of Cr³⁺ in octahedral sites, with point symmetry C₁, of the structure. Another weak band at 23000 cm^?1 in the sapphirine-N spectra is attributed to low symmetry splitting of the excited ⁴T₁ (F)-State of Cr³⁺. These assignments lead to crystal field parameters Dq=1730cm^?1 and B= 685cm^?1 of Cr³⁺ in sapphirine. Crystallochemical and spectroscopic arguments suggest that Cr³⁺ subsitutes for Al in the M(1) or M(8) sites of the sapphirine structure. In addition to Cr³⁺-transitions, spectra of Sl exhibit weak dd-bands of Fe²⁺ at 10000 and 7700 cm^1-, which are unpolarized in consistency with the C₁ site symmetry of the octahedra in the structure. Spectra of Sl show also prominent, broad bands (Δv_1/2～-5000 cm^1-) at 15000 and 11000 cm^1-, which occur in E//Y(//b) and E//Z(//_c=12°) only and exhibit an intensity ratio α_Y∶α_z close to 1∶3. This result, the large half width, as well as band energy — MM distance considerations suggest that these bands originate from Fe^2+[6]-Fe^3+[6] charge-transfer transitions in wall octahedra M(1)M(2), M(6)M(7) etc., forming MM vectors of 30° with the c-axis. The lack of Fe²⁺-Fe³⁺ charge-transfer bands in sapphirine N might indicate a lower oxygen fugacity during the formation of the websterite from the Noyabrskaya pipe compared to that from the Sludyanka pipe.     

Single crystal infrared spectra in the range of OH fundamentals of paragenetic garnet,omphacite and kyanite in an eklogitic mantle xenolith

Summary Single crystals of paragenetic garnet (gt), omphacite (cpx) and kyanite (ky) were isolated from an eclogite xenolith from the Zagadochnaya kimberlite, Yakutia (grospydite Z13) and studied by polarized FTIR-microspectrometry in the OH valence vibrational region and by microprobe analyses. The coexisting minerals are homogeneous with respect to major and minor elements and have compositions near gross₄₉pyr₂₆alm₂₀uvar₄ (gt), jad₄₅dio₄₇hed₆kos₂ (cpx) and ky_>97. Single crystal spectra show one _OH-band for gt at 3630 cm^–1 (halfwidth ca. 100 cm^–1) which is very likely caused by vibrations of tetrahedral (OH)₄-^4– clusters replacing SiO₄ ^4– tetrahedra. Cpx shows one strong, but weakly pleochroic band at 3464 cm^–1 (halfwidth ca. 160 cm^–1) and a weak satellite band centered at 3620 cm^–1) with a distinct pleochroism. Ky OH spectra exhibit two sets of weak sharp pleochroic bands, a triplet, characteristic for high pressure ky, at 3439, 3410, 3387 cm^–1) and a doublet at 3279, 3264 cm^–1) (halfwidths ca. 10 cm^–1) From integral and linear absorbances in the unpolarized spectra defect-hydroxyl contents in the three coexisting minerals were estimated and found near O.OX wt% H₂O. The distribution scheme of hydrogen in the paragenetic minerals was evaluated to be c^ky < c^cpx < c^gt.

---

Einkristall Infrarotspektren im Bereich der OH Grundschwingungen einer Granat-Omphacit-Kyanit Paragenese in einem eklogitischen Mantelxenolith

Zusammenfassung Aus einer Granat (Gt)-Omphacit (Cpx)-Kyanit (Ky) Paragenese eines Eklogit Xenoliths aus dem Zagadochnaya Kimberlit, Jakutien (Grospydit Z13), wurden Einkristalle isoliert und mit polarisierter FTIR-Mikrospektrometrie im OH Valenzschwingungsbereich und mit der Elektronenstrahl-Mikrosonde untersucht. Die koexistierenden Minerale sind hinsichtlich ihrer Haupt- und Nebenelemente homogen und haben Zusammen setzungen von etwa Gross⁴⁹Pyr²⁶Alm²⁰Uvar₄ (Gt), Jad₄₅Dio₄₇Hed₆Kos₂ (Cpx) and Ky_>97 Einkristallspektren von Gt zeigen eine _OH-Bande bei 3630 cm^–1) (Halbwerts breite ca. 100 cm^–1) die wahrscheinlich von Schwingungen tetraedrischer (OH)₄ ^4– Gruppen, die SiO₄ ^4– Tetraeder ersetzen, herrührt. Cpx zeigt eine starke, aber schwach pleochroitische Bande bei 3464 cm^–1 (Halbwertsbreite ca. 160 cm^–1) and eine schwache, deutlich pleochroitische Satellitenbande bei 3620 cm^–1) Ky OH Spektren zeigen zwei Gruppen von schwachen, scharfen pleochroitischen Banden, ein fur Hochdruck Ky charakteristisches Bandentriplett bei 3439, 3410, 3387 cm^–1) and ein Bandendublett bei 3279, 3264 cm^–1) (Halbwertsbreiten ca. 10 cm^–1) Aus den integralen and linearen Extinktionen der nicht-polarisierten Spektren wurde der Defekt-Hydroxyl Gehalt der drei koexistierenden Minerale abgeleitet and mit O.OX Gew% H₂O festgelegt. Das Verteilungsschema des Wasserstoffs kann in der Mineralparagenese mit c^Ky < c^Cpx c^Gt angegeben werden.

---

---

Dedicated to Prof. Dr. J. Zemann on the occasion of his 70th birthday

---

With 3 Figures     

Polarized IR spectra of synthetic smoky quartz

Polarized IR spectra of planeparallel (0001) plates of synthetic smoky quartz, with E rotating around [0001], show that the absorption figures of OH^– related absorption bands at 3380 (room temperature), 3365 and 3305 cm^–1 (liquid nitrogen temperature, -196° C) are strongly anisotropic and violate the trigonal symmetry of low quartz. This effect is correlated with a non-uniform substitution of Si by Al on the three symmetrically equivalent Si sites, as revealed by EPR measurements. Random distribution of Al over the three Si sites, obtained by dry annealing of the samples in air, yields isotropic absorption figures in the (0001) plates. It is thus experimentally evident that the absorption bands at 3380, 3365 and 3305cm^–1 are caused by the OH^– stretching vibrations coupled with Al substituting for Si. For each experimentally determined integral absorption coefficient of the three absorption bands a theoretical absorption coefficient was calculated, based on the symmetry of low quartz and the given Al distribution. This was done for various orientations of the OH^– dipoles with respect to the a axes of low quartz. By comparing the experimentally determined and calculated absorption coefficients, the orientation of the corresponding OH^– dipoles with respect to the a axes could be determined.     

Infrared reflectivity spectra of single crystal cassiterites

The infrared reflectivity spectra of two natural cassiterite single crystals from Portugal and Vietnam have been studied and analysed in the frequency range of 15–4000 cm^–1 at room temperature. The optical mode parameters are deduced by simulation of the experimental spectra using the factorised form of the dielectric function. The difference with synthetic SnO₂ spectra is a dip near 500 cm^–1 understood as the activation of an infrared forbidden E _g mode due to the amount of Fe³⁺ and Ti⁴⁺ impurities. Another result of this work is the derivation of the correct values of the static dielectric constant of cassiterite.     

Inclusions of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe,Yakutia

The results of study of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe are reported. Most of the studied samples are characterized by high concentrations of Sr, Ca, Na, and LREEs in comparison with minerals of the LIMA series from kimberlites of South Africa, whereas the average concentrations of Ba and K are significantly lower. Crichtonite group minerals in pyropes are characterized by predomination of Na over K in most samples and by a high concentration of Al₂O₃ (up to 4.5 wt %). Findings of inclusions of crichtonite group minerals with high concentrations of incompatible elements provide evidence for the metasomatic origin of host chromium-rich pyropes.     

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.     

The crystal structure of natural and synthetic holtedahlite

Summary The crystal structure of synthetic holtedahlite, Mg₁₂(PO₃OH, PO4)(PO₄)₅(OH,O)₆,P31m,a = 11.186(3),c = 4.977(1) Å,Z = 1, has been refined toR = 0.033 for 718 observed reflections. Natural holtedahlite, Mg₁₂(PO₃OH, CO₃)(PO₄)₅(OH, O)₆,a = 11.203(3),c = 4.977(1) Å, was refined toR = 0.031 for 1202 observed reflections. The structure contains pairs of face-sharing Mg-octahedra linked by edge-sharing to form double chains alongc. Hydrogen phosphate groups on three-fold axes are partially replaced by carbonate groups in natural holtedahlite. Structural similarities with ellenbergerite are pointed out.

---

Die Kristallstruktur von natürlichem und synthetischem Holtedahlit

Zusammenfassung Die Kristallstruktur von synthetischem Holtedahlit, Mg₁₂(PO₃OH,PO₄)(PO₄)₅(OH,O)₆,P31m,a = 11,186(3),c = 4,977(1) Å,Z = 1, wurde für 718 beobachtete Reflexe aufR = 0,033 verfeinert. Natürlicher Holtedahlit, Mg₁₂(PO₃OH, CO₃)(PO₄)₅(OH, O)6,a = 11,203(3),c = 4,977(1) A wurde für 1202 beobachtete Reflexe aufR = 0,031 verfeinert. Die Struktur enthält Paare von über eine Fläche verknüpften Mg-Oktaedern, die weiter durch Kantenverknüpfung Doppelketten nachc bilden. Saure Phosphatgruppen auf dreizähligen Achsen sind im natürlichen Holtedahlit teilweise durch Karbonatgruppen ersetzt. Strukturelle Ähnlichkeiten zu Ellenbergerit werden aufgezeigt.

---

---

With 4 Figures     

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.     

Mie scattering and charge transfer phenomena as causes of the UV edge in the absorption spectra of natural and synthetic almandine garnets

 The UV edge in the electronic absorption spectra of minerals, in many cases influencing their colour, is generally interpreted as the low-energy wing of very strong UV bands caused by ligand–metal charge transfer (CT) transitions (e.g. Burns 1993). However, Mie scattering theory shows that the presence of randomly distributed submicroscopic inclusions with narrow size distribution and a refractive index n _i in a matrix with different refractive index n _m may give rise to a λ-dependent, band-like scattering (e.g. Kortüm 1969). Such scattering bands have so far not been considered as contributing to the UV edge. Single-crystal electronic absorption spectra of eight natural almandine-rich garnets (Alm₆₀–Alm₈₈), two synthetic almandine samples (Alm₁₀₀), all of different colours, and synthetic spessartine were studied by means of a Zeiss microscope-spectrometer in the range 40 000–20 000 cm⁻¹. Special techniques of spectral measurements with crossed analyzer and polarizer, which enable the registration of the scattering effect directly, were used as well. Four of the above garnets were also investigated using transmission electron microscopy. Different types of inclusions, from 10 to several 100 nm in size, were observed in the garnet matrices. They are abundant in cores of synthetic garnets, but very rare in most natural almandines studied. Electronic absorption spectra of the natural almandine garnets show largely varying UV edge position and, hence, intensity at a given wavenumber which correlates with the intensities of spin-forbidden dd bands of Fe³⁺ ions at 27 000 and 28 000 cm⁻¹, superimposed on the long energy slope of the UV absorption. There are also positive correlations between Ti⁴⁺ and Fe³⁺ content, the latter recalculated on the basis of garnet stoichiometry, and UV edge intensity. Thus, the presence of Ti⁴⁺ and Fe³⁺ ions in octahedra, even in very low concentrations (0.0n at. pfu), leads to CT phenomena, that probably involve Fe²⁺ ions in edge-shared dodecahedral position and intensifies ligand- to-metal CT. The different colours of natural almandine garnets with similar Fe²⁺ contents studied here are caused by this effect. Consistent with the absence of inclusions in most natural garnets studied, λ-dependent scattering plays no role in their UV absorption. In contrast, in synthetic almandine and spessartine crystals, a different intensity of UV absorption was observed in inclusion-free rims and inclusion-enriched cores. Some of the latter demonstrate typical scattering patterns when measured at crossed polarizers. Received: 10 April 2001 / Accepted: 27 September 2001