X-ray single-crystal and optical spectroscopic study of chromian pumpellyite from Sarany, Urals, Russia
Journal of Mineralogical and Petrological Sciences
The crystal structure of a chromian pumpellyite K 0.03 Na 0.07 Ca 7.82 Mn 2+ 0.01 Mg 1.95 Fe 3+ 0.05 V 3+ 0.01 Cr 3+ 3.77 Al 6.52 Ti 0.02 Si 11.82 O 42.06 (OH) 13.94 (Z = 1) from Sarany in the Russian Urals was studied using optical spectroscopy and singlecrystal Xray diffraction methods. Five discrete peaks were observed in an optical spectrum. Of these, three at 392(υ 2 ), 574(υ 1 ) and 726 nm were attributed to Cr 3+ ; one at 456 nm to Fe 3+ ; and a peak at 291 nm is related to M -O charge
... ed 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 VI M -O distance of 1.954 Å. The crystal structure of chromian pumpellyite with a = 8.809(3), b = 5.9330(9), c = 19.137(1) Å, β = 97.606(2)º at 90 K, and a = 8.807(6), b = 5.943(4), c = 19.18(1) Å, β = 97.44(2)º 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 Cr 3+ 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, K D = (Cr/Al) X /(Cr/Al) Y , of 4.39. The substitution of Cr 3+ for Al in the Y site directly results in structural changes, whereas that in the X site is not effective for structural change.