Partial Ordering of Cations by the Wolframite Mechanism Using Fe2+- and Sc-Dominant Minerals of the Columbite Supergroup as Examples

In most cases, columbite-supergroup minerals are characterized by partial ordering of cations, which makes their identification based only on chemical composition and powder diffraction data difficult. Columbite-supergroup minerals with partially ordered cations were studied by means of electron microprobe analyses, powder and single-crystal X-ray diffraction, including crystal structure refinement in ixiolite-type and wolframine-type models. The samples originate from the Sakhanaiskiy granite massif, Eastern Siberia, Russia, (Sample 1) and from the Heftetjern pegmatite, Telemark, Norway (Sample 2). Their representative empirical formulae are (Fe2+0.81Mn2+0.53)Σ1.34Fe3+0.07(Ti0.23Zr0.11Sn0.02)Σ0.36(Nb1.45Ta0.26)Σ1.71W0.52O8 (Sample 1) and (Mn2+0.28Fe2+0.25)Σ0.53Sc1.08(Sn0.32Ti0.04)Σ0.36(Ta1.41Nb0.52)Σ1.93W0.10O8 (Sample 2). Based on these data and the results of the crystal structure refinement, the studied samples can be considered as columbite-supergroup minerals in which cations are partially ordered in accordance with the wolframite mechanism. An approach is suggested according to which the degree of cation ordering in such columbite-supergroup minerals can be estimated based on the electron contents refined for different sites in a monoclinic model. According to this criterion, the degree of cation ordering of Samples 1 and 2 is 91% and 26%, respectively. Despite a significant degree of cation disordering, transition from the wolframite to ixiolite model results in a significant enhancement of the R-factor of the structure refinement (from 0.0365 to 0.0764 and from 0.0207 to 0.0610, respectively).