| Abstrakt: |
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 (MgFeAlCr) Al[O/AlSiO] (N) and (MgFe MnTiAlCr)Al[O/AlSiO] (Sl). Single crystal spectra in the range 35000-6000 cm showed a slightly polarization dependent absorption edge near 3200 cm (N) or 30000 cm (Sl) and unpolarized bands at 25300 and 17300 cm, typical of spin-allowed transitions, derived from A→T and A→T, of Cr in octahedral sites, with point symmetry C, of the structure. Another weak band at 23000 cm in the sapphirine-N spectra is attributed to low symmetry splitting of the excited T (F)-State of Cr. These assignments lead to crystal field parameters Dq=1730cm and B= 685cm of Cr in sapphirine. Crystallochemical and spectroscopic arguments suggest that Cr subsitutes for Al in the M(1) or M(8) sites of the sapphirine structure. In addition to Cr-transitions, spectra of Sl exhibit weak dd-bands of Fe at 10000 and 7700 cm, which are unpolarized in consistency with the C site symmetry of the octahedra in the structure. Spectra of Sl show also prominent, broad bands (Δv∼-5000 cm) at 15000 and 11000 cm, which occur in E//Y(//b) and E//Z(//=12°) only and exhibit an intensity ratio α∶α 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-Fe 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 Fe-Fe 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. [ABSTRACT FROM AUTHOR] |
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