X-ray single-crystal and optical spectroscopic study of chromian pumpellyite from Sarany, Urals, Russia

The crystal structure of a chromian pumpellyite K0.03Na0.07Ca7.82Mn2+0.01Mg1.95Fe3+0.05V3+0.01Cr3+3.77Al6.52Ti0.02Si11.82O42.06(OH)13.94 (Z = 1) from Sarany in the Russian Urals was studied using optical spectroscopy and single-crystal X-ray diffraction methods. Five discrete peaks were observed in an optical spectrum. Of these, three at 392(υ2), 574(υ1) and 726 nm were attributed to Cr3+; one at 456 nm to Fe3+; and a peak at 291 nm is related to M-O charge transfer. The crystal field parameters determined here (B, C and 10Dq) give 94% ionic character of Cr-O bonds and a mean VIM-O distance of 1.954 A. The crystal structure of chromian pumpellyite with a = 8.809(3), b = 5.9330(9), c = 19.137(1) A, β = 97.606(2)° at 90 K, and a = 8.807(6), b = 5.943(4), c = 19.18(1) A, β = 97.44(2)° at 293 K was refined using 1297 unique reflections measured at 90 K, converging at R = 2.3%. Site occupancies of the main constituent elements at the two independent octahedral X and Y sites are X = 0.49Mg + 0.308Cr + 0.202Al and Y = 0.742Al + 0.258Cr. Although Cr3+ ions occupy both the X and Y sites, the site preference of chromium for the X site is stronger than that for the Y site, as indicated by the distribution coefficient of chromium between the X and Y sites, KD = (Cr/Al)X/(Cr/Al)Y, of 4.39. The substitution of Cr3+ for Al in the Y site directly results in structural changes, whereas that in the X site is not effective for structural change.

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