Crystal chemistry and structure of the orthorhombic (Fe,Mn)(Ta,Nb)2O6 family of compounds

A new approach for the determination of multiple cation locations and ordering, using the example of natural and heat-treated columbites

“…The site assignment has been done following earlier publications (dos Santos et al, 1999Santos et al, , 2001Bancroft, 1973) based on the correlation between the quadrupole splitting with the octahedral distortions. Indeed, the distortion of the octahedron has been investigated on partially ordered singlecrystal (Fe 0.65 (Tarantino et al, 2003).…”

“…wodginite (dos Santos et al, 1999Santos et al, , 2001Ercit et al, 1992). According to our Mö ssbauer spectroscopy discussed above, all the Fe atoms of this sample are located on the same type of cation sites.…”

“…Order-disorder phenomena connecting these AB 2 O 6 minerals have been extensively investigated (dos Santos et al, 2001). In the context of the present discussion, a 100% ordered columbite means all the Fe and Mn atoms occupying the A site, and all the Ta and Nb occupying the B site.…”

“…The tantalitecolumbites are sometimes referred to simply as columbite when the Nb content is larger than that of Ta, and tantalite when the Ta concentration is larger than that of Nb. The coexistence of phases in a natural tapiolite sample has been investigated by different techniques including Mö ssbauer spectroscopy, X-ray diffraction and microprobe analysis (dos Santos et al, 2001;Kinast et al, 2002;Tarantino et al, 2003). These minerals are economically important since they are the major source of tantalum.…”

“…Consequently, Wenger et al (1991) have considered that the single-crystal X-ray diffraction data should be used to differentiate between ixiolite and columbite. Other criteria have been tentatively established (dos Santos et al, 2001) to estimate the degree of cation order in the (Fe,Mn)(Ta,Nb) 2 O 6 system, but none have been found to be sufficient, leading to some mistakes being reported in the literature. This motivated us to develop our work on the analysis of selected natural compounds to combine different techniques aimed at probing, unambiguously, the degree of cation order.…”