ALBERT

Notes: Abstract The assignment of spin-allowed Fe2+-bands in orthopyroxene electronic absorption spectra is revised by studying synthetic bronzite (Mg0.8 Fe0.2)2Si2O6, hypersthene (Mg0.5 Fe0.5)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 (5B2g→5A1g) and Fe2+ in M2 (5A1→5A1), the 8500 cm-1 band by Fe2+ in M1 (5B2g→5B1g) 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.

Notes: Abstract 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 (Mg3.40Fe0.23Al3.25Cr0.16)[6] Al 1.00 [6] [O2/Al4.53Si1.47O18] (N) and (Mg2.53Fe0.55 Mn0.04Ti 0.03 4+ Al3.55Cr 0.08 3+ )[6]Al 1.00 [16] [O2/Al4.28Si1.73O18] (Sl). Single crystal spectra in the range 35000–6000 cm1- showed a slightly polarization dependent absorption edge near 3200 cm1- (N) or 30000 cm1- (Sl) and unpolarized bands at 25300 and 17300 cm1-, typical of spin-allowed transitions, derived from 4A2g→4T1g and 4A2g→4T2g, 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 cm1-, which are unpolarized in consistency with the C1 site symmetry of the octahedra in the structure. Spectra of Sl show also prominent, broad bands (Δv1/2∼-5000 cm1-) at 15000 and 11000 cm1-, which occur in E//Y(//b) and E//Z(//c=12°) only and exhibit an intensity ratio αY∶αz close to 1∶3. This result, the large half width, as well as band energy — MM distance considerations suggest that these bands originate from Fe2+[6]-Fe3+[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.

Notes: Abstract Polarized optical absorption spectra of Mn(IV) in octahedral crystal fields of Mn(SeO3)2 have been studied by means of microscope-spectrometry in the range 40000-4000 cm−1 and at temperatures between 113 K and 293 K. Intense charge-transfer absorptions (linear absorption coefficient α ≫ 30000 cm−1) completely mask the d-d transitions in the UV and VIS region above ≈23000 cm−1. The optical electronegativity χ opt of Mn(IV) in Mn(SeO3)2 is estimated to be 2.7. In accordance with the d 3 configuration of tetravalent manganese three d-d bands observed at ambient temperatures at 13250, 14137 (α≈50 cm−1) and ≈18500 cm−1 (α≈500–800 cm−1) are assigned to the spin forbidden 4 A 2g →2 E g and 4 A 2g →2 T 1g transitions as well as to the first spin allowed 4 A 2g →4 T 2g transition, respectively. These assignments allow the calculation of the following ligand field parameters: Dq ≈ 1850 cm−1, B 55 = 869 cm−1 (β 55 = 0.82), and C = 2346 cm−1 (293 K).

Notes: Abstract  High-pressure electronic absorption spectra at room temperature and at pressures 10−4〈P[GPa]〈8 were measured in the spectral range 380〈λ[nm] 〈780(26218〉ν˜[cm−1]〉12820) on analysed single crystal slabs, about 20 μm thick, of Cr3+-bearing spinel (I), kyanite (II), corundum (III), pyrope (IV) and uvarovite (V) using DAC-cell techniques in combination with single-beam microscopespectrometry. Ligand field theoretical evaluation of the spectra yielded following results: (i) the octahedral crystal field parameter, 10DqCr3+[6], linearly shifts on increasing pressure to higher energies with slopes, (δ10DqCr3+[6]/δP), of 103.1 (I), 99.5 (II), 104.0 (III), 111.7 (IV) and 110.3 [cm−1/GPa] (V) (reliability parameters r≥0.92), (ii) The Racah-parameter BCr3+[6], reflecting the covalency of the Cr–O bonds, does not significantly change with pressure up to 8 GPa, in those cases where it could be evaluated from the spectra (III, IV, V). This result is contrary to the behaviour of BCr3+[6] with increasing temperature (Taran et al. 1994) and shows that P and T are not inversely correlated parameters with respect to BCr3+[6], a decrease of which reflects an increase in covalency. (iii) This result enabled to extract octahedral compression moduli, kCr3+[6], from the pressure slopes of 10DqCr3+[6]: 312+48 (I), 297+70 (II), 298+44 (III), 275+35 (IV), 257+32 GPa (V). Quotients kcr3+[6]/kbulk,phase are nearly the same (ca. 1.6) for I, II, IV and V but significantly lower (ca. 1.1) for III. Deviations between spectroscopically determined kCr3+[6] and published kAl[6], obtained by HP-XRD on ruby and pyrope, are interpreted by lattice strain induced by [Cr3+, Al3+−1][6] substitution.

Notes: Abstract The effect of raising temperature on spin-allowed dd-transitions of octahedral Cr3+ was studied for various point symmetries of the Cr3+-bearing structural sites, i.e. 3 m and 3 with inversion center in spinel and garnets, respectively, or 32, 3, 2 and 1, lacking the inversion centre, in beryl, corundum, diopside and topaz, respectively. For this purpose, crystals of Cr3+-bearing spinel, pyrope, andradite, grossular, uvarovite, emerald, ruby, diopside and topaz were analyzed by microprobe, oriented, and measured in polarized radiation (except for the cubic minerals) in the spectral range 30 000 to 11 100 cm-1 and at temperatures between 77 and 797 K. The evaluation of the intensities, half widths, and energy positions of bands due to Cr3+-transitions derived from 4 A 2g → 4 T 2g (F) and → 4 T 1g (F) as well as of Dq- and B-values derived, had the following results: In all cases, red shift of the above bands and, hence, independent on the site symmetry of Cr3+, decreases in the Dq-values were obtained. The dependcies of Dq on T are nearly linear above room temperature and amount between -1.6% in topaz and -5.1% in pyrope in the temperature range studied. From this, values for the local thermal expansion of the Cr3+-centered octahedra, α loc, were derived on the basis of the R M-0 -5 -proportionality of 10Dq. Such values are consistently higher than those obtained from X-ray refinements, a method averaging rm-o for all the respective octahedral positions. On increasing temperature, the half band widths increase. The evaluation for spinel, pyrope and uvarovite showed $$\Delta \tilde v_{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}$$ to increase by about 60%, for both types of Cr3+ transitions, in the temperature range studied. The temperature dependence of band intensities is complex, because it depends on the site symmetry and related symmetry selection rules: symmetry forbidden transitions increase strongly on heating. This was observed for Cr3+ in centosymmetric sites $$\bar 3$$ m of spinel and $$\bar 3$$ of garnets as well as for the symmetry-forbidden Cr3+-transitions in the acentric sites 32 in beryl and 3 in ruby. Where no symmetry-related selection rules exist, as in point symmetry 1 in topaz, almost no temperature dependence of band intensities was observed. This holds also for diopside M 1-octahedra with point symmetry 2, although here the U-band should be forbidden at least in one direction. As for the intensity of the U-bands in spinel, pyrope and uvarovite, the observed intensity increase with T, agreed with that calculated on the basis of vibronic coupling with an odd vibration near ω= 450 cm-1. In order to differentiate between the influences of static and dynamic relaxation of the Laporte rule on the intensities, the relative band intensities, subject to static effects only, were calculated for Cr3+ in ruby and emerald. This proved the temperature dependence of the U π and Y π -band, respectively, to be caused by vibronic coupling only.