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THE DISTRIBUTION OF CHROMIUM IN CHROMIAN PUMPELLYITE FROM SARANI, URALS, RUSSIA: A TOF NEUTRON AND X-RAY RIETVELD STUDY

Mariko Nagashima and Masahide Akasaka

Department of Materials Creation and Circulation Technology, Graduate School of Science and Engineering, Shimane University, Matsue 690-8504, Japan

The crystal structure of chromian pumpellyite from Sarani in the Russian Urals was refined using complementary time-of-flight neutron and X-ray Rietveld methods, to investigate the crystal-chemical behavior of chromium in pumpellyite. The average chemical composition of the sample is CaO 22.06 ± 0.39, MgO 3.95 ± 0.37, Cr₂O₃ 14.39 ± 1.19, Al₂O₃ 16.71 ± 0.79, total Fe₂O₃ 0.15 ± 0.08, and SiO₂ 35.73 ± 0.63 wt.% (n = 38). The occupancy of Mg at the octahedral X site refined with the neutron-diffraction data is 0.48(3), the same as that obtained by chemical analysis. The occupancies of Cr and Al refined with the X-ray-diffraction data were 0.32(1)Cr and 0. 20Al at the X site, and 0.31Cr and 0.69Al at the Y site. Bond-valence considerations support the validity of the refined occupancies and the assumption that Cr is trivalent. Accordingly, the formula is established as Ca_8.00(Mg_1.92Cr_1.28Al_0.80)_4.00(Al_5.52Cr_{2. 48})_8.00Si_12.00O_42.12(OH)_13.88. Although Cr³⁺ ions occur at both the X and Y sites, the distribution coefficient [(Cr/Al)^X/(Cr/Al)^Y = 3.56] indicates a stronger preference of Cr for the X site than the Y site. The unit-cell parameters were refined as a 8.8193(5), b 5.9396(3), c 19.161(1) Å, β 97.603(2)° and V 994.86(9) ų (neutron data), and a 8.8132(2), b 5.9342(1), c 19.1466(3) Å, β 97.603(2)° and V 992.54(3) ų (X-ray data). Substitution of chromium for aluminum at the Y site increases the mean Y–O distance by expansion of the octahedra, which causes increase in the a, b, and c dimensions. However, the mean X–O distance is not correlated with mean ionic radius at the X site. Distortion parameters of the Y site tend to decrease with increasing mean Y–O distance and volume of the site. Expansion of octahedra by ionic substitutions of larger cations for Al at the Y site thus causes a gradual change of the octahedra to a highly symmetrical and regular form.

Keywords: pumpellyite, chromian pumpellyite, shuiskite, chromium, TOF, neutron, X-ray, Rietveld analysis, crystal structure, crystal chemistry, site distortion, site occupancy, Sarani, Urals, Russia.