An insight into crystal chemistry and cation order of columbite-(Fe) and columbite-(Mn) from worldwide occurrences

A set of nine columbite samples (Fe,Mn)(Nb,Ta)(2)O-6, preserved in the Mineralogical Museum, Naples University Federico II (Italy) and originating from several world-wide pegmatitic occurrences, was studied by electron-probe microanalysis and single crystal X-ray diffraction. All the samples are columbite-(Fe), except for two samples corresponding to columbite-(Mn). For seven samples Ta/(Nb+Ta) is <0.05 and for two samples <0.2. The (Ti+Sn)(4+) content of all samples is <0.12 pfu. Structure refinements showed that five samples displayed strong (Fe,Mn)(2+) vs. (Nb,Ta)(5+) disorder at the two octahedral sites A and B, whereas four samples were highly ordered. Two samples could be interpreted as "pseudo-ixiolite". The degree of cation order at A and B was calculated from cell dimensions and scattering behaviour at A and B. Both methods are in good agreement. The octahedral coordination of (Nb, Ta)5+ in columbite is characterized by out-of-center distortion, thus the average distortion at the B site was probed by calculating the difference between the longest and shortest B-O distance. For a Nb-rich and (Ti,Sn)-poor data set, the min-max distortional parameter shows very good linear correlation with the order parameter calculated from site occupancy.