Pattern of minor element enrichment in columbites: A synchrotron radiation X-ray fluorescence (SRXRF) study
Introduction
Columbites are typical pegmatite minerals with general formula A2+B5+2O6 where A represents essentially manganese plus iron and B, niobium plus tantalum. These minerals figure out solid solutions involving the four oxide components taken two by two according to the charge and the dimension of metal ions – that is, Fe plus Mn and Nb plus Ta (Ercit et al., 1995). They are frequently used to interpret the internal evolution trends in granitic pegmatites, being recognized that an increased fractional crystallization induces higher Mn contents at the expense of Fe, combined with a similar situation for Ta vs. Nb (Černý et al., 1986, Mulja et al., 1996, Tindle and Breaks, 2000, Zhang et al., 2004).
Beyond economically important as niobium and tantalum ores (e.g., Černý and Ercit, 1989), columbites display physical properties that make them adequate prototypes for oxide materials with relevant technological applications (e.g., Lee et al., 1997).
It is well known that these minerals carry a vast span of minor elements, namely Ti, Sn, Zr, W, Sc, Y, Pb, and U (Černý et al., 1986, Ercit, 1994). However, the low level of incorporation is a difficulty when trying to differentiate the role of external geochemical conditions prevailing during mineral crystallization from intrinsic hindrance factors arising from crystal structure features.
With the aim of disclosing a pattern for minor element enrichment in columbite solid solutions, a crystal chemical study was undertaken on minerals from the Zambezia pegmatite fields in Mozambique, complemented by a theoretical topologic approach to columbite crystal structure based on bond valence calculations. A major goal was to enlighten the structural control of major chemical components simultaneously upon substitution schemes and bulk minor element contents. Taking advantage of the low detection limits attained for most elements when using synchrotron radiation to excite X-ray fluorescence (SRXRF), a photon microprobe was applied to figure out the pattern of minor element incorporation; conversely, major element analyses were performed with an electron microprobe.
Analytical data are reported and a discussion is presented on the main results, showing that minor and trace element contents display a general trend, pointing towards the preferential incorporation of those elements that minimize the stress induced in the crystal structure of the host mineral by the non-commensurability of metal ions with distinct valences and ionic radii.
Section snippets
Crystal chemistry of columbites revisited
Chemical composition of columbite group minerals – simplified formula AB2O6 – is currently plotted within a quadrilateral of pure double oxides FeNb2O6–MnNb2O6–MnTa2O6–FeTa2O6 (Fig. 1). A large area in the compositional field corresponds to phases with a triple stored α-PbO2 structural arrangement to which the general designation of columbite applies. Close to the FeTa2O6 corner – the mineral tapiolite (s.s.) – a trirutile crystal structure holds for a small compositional area and both
Materials and experimental
Studied minerals were collected from columbites sampled during an extensive fieldwork conducted in the sixties (Figueiredo de Barros and Martins Vicente, 1963) in the Pegmatite Province of Alto Ligonha, Zambezia/Mozambique (Claus and Hutchinson, 1956). Since then, these pegmatites have been exploited as niobium and tantalum ore deposits. Previous studies on the chemistry of Nb,Ta-oxides (columbites, ixiolites, microlites) from this metallogenetic province emphasize an extreme fractionation to
Results
Columbite compositions representative of a wide domain of Fe/Mn and Nb/Ta ratios are listed in Table 1. From the set of tentatively analyzed minor elements, only Ti, Zr and W have provided significant results.
When plotting iron vs. manganese (Fig. 3a) and niobium vs. tantalum (Fig. 3b) expressed as atoms per formula unit (apfu), a wide range of substitutions Fe–Mn and Ta–Nb became clearly apparent for the whole set of EPMA analyzed samples. These samples were further characterized with the
Discussion
As mentioned before, Bond Valence Model calculations applied to columbite crystal structure have shown that A-type cations are overbonded while B-cations are underbonded (Mirão, 2004), being then expected that the incorporation of trace elements will not worsen the basic lattice stress. Small amounts of tri- and tetravalent minor elements (Y3+, Ti4+, Zr4+, U4+) could then contribute to release stresses when entering the underbonded B-positions if metal – oxygen distances are not significantly
Conclusions
The plot of trace element abundances against the ratio Nb / (Nb + Ta) demonstrates that their uptake is conditioned by the dominant presence of niobium in columbite and that trace elements are mainly incorporated during early stages of crystallization. On the other hand, the scattering in the abundance of these elements in Nb-rich solid solutions is clear evidence that their incorporation is ultimately conditioned by elemental availability in the magmatic fluid.
It is worthwhile remarking that
Acknowledgements
The financial support from the EU through the Program Access to Research Infrastructures is acknowledged. Special thanks are due to Professor P. Chevallier, responsible for the photon-microprobe at the LURE. We also thank our colleague M.J. Basto for helping in spectra collection. [LM]
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