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

Mariko NAGASHIMA, Masahide AKASAKA, Ko IKEDA, Atsushi KYONO, Kuniaki MAKINO
2010 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
more » ... 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.
doi:10.2465/jmps.090717b fatcat:76d6duqwyrfk5dpitdi57jo6ye