ELECTRONIC ABSORPTION-SPECTRA OF CHROMIUM-BEARING SAPPHIRINE

Violet, non-pleochroic and greenish-blue, pleochroic chromium-substitu ted sapphirines were found in corundum-bearing spinel-websterite xenol ites from the Yakutian kimberlite pipes Noyabrskaya (N) and Sludyanka (S1), respectively. The crystallochemical formulae of sapphirine cryst als from such xenolites were determined by EMP to be (Mg3.40Fe0.23Al3. 25Cr0.16)[6] Al1.00[6][O2/Al4.53Si1.47O18] (N) and (Mg2.53Fe0.55 34+Al 3.55Cr0.083+)[6]Al1.00[6][O2/Al4.28Si1.73O18] (SI). Single crystal spe ctra in the range 35000-6000 cm1- showed a slightly polarization depen dent absorption. edge near 3200 cm1- (N) or 30000 cm1- (SI) and unpola rized bands at 25300 and 17300 cm1-, typical of spin-allowed transitio ns, derived from 4A2g --> 4T1g and 4A2g --> 4T2g, of Cr3+ in octahedra l sites, with point symmetry C1, of the structure. Another weak band a t 23000 cm-1 in the sapphirine-N spectra is attributed to low symmetry splitting of the excited 4T1 (F)-State of Cr2+. These assignments lea d to crystal field parameters Dq = 1730 cm-1 and B = 685 cm-1 of Cr3in sapphirine. Crystallochemical and spectroscopic arguments suggest t hat Cr3+ subsitutes for Al in the M(1) or M(8) sites of the sapphirine structure. In addition to Cr3+-transitions, spectra of S1 exhibit wea k dd-bands of Fe2+ at 10000 and 7700 cm1-, which are unpolarized in co nsistency with the C1 site symmetry of the octahedra in the structure. Spectra of S1 show also prominent, broad bands (DELTAupsilon1/2 congr uent-to 5000 cm1-) at 15000 and 11 000 cm1-, which occur in E//Y(//b) and E//Z(AND(c) = 12-degrees) only and exhibit an intensity ratio alph a(Y):alpha(Z) close to 1: 3. This result, the large half width, as wel l as band energy - MM-distance considerations suggest that these bands originate from Fe2+[6]-Fe3+[6] Charge-transfer transitions in wall oc tahedra M(1)M(2), M(6)M(7) etc., forming MM vectors of 30-degrees with the c-axis. The lack of Fe2+ - Fe3+ charge-transfer bands in sapphiri ne N might indicate a lower oxygen fugacity during the formation of th e websterite from the Noyabrskaya pipe compared to that from the Sludy anka pipe.