Abstract

Titanium-enriched members of the columbite family (orthorhombic) commonly contain exsolved niobian-tantalian rutile, or are aggregated with a coprecipitated rutile phase. This assemblage is the Fe, Mn, Nb, Ta-rich counterpart of the Ti-dominant niobian rutile+exsolved titanian columbite pair. The rutile phase is enriched in Fe (super 2+) , Ta, Fe (super 3+) and Sn, whereas the orthorhombic phase favors Mn, Nb, Sc, W and Zr. Most samples of the orthorhombic phase are defined as titanian columbite - tantalite; they have low to moderate Ti content, moderately to considerably disordered structure, and convert to highly ordered columbite-tantalite on heating. A minor number of samples of the orthorhombic phase develops the highly ordered structure of wodginite on heating, which identifies them as titanian ixiolite, the disordered counterpart of titanowodginite. These samples are highly disordered in the natural state. They are Ti-rich, with Mn>Fe and Ta>Nb; these features correspond to the composition of natural titanowodginite and of the only titanowodginite synthesized to date. However, subordinate but significant Sn seems to be present in all samples of titanian ixiolite as well as natural titanowodginite, and it may affect the path of ordering. Excess of Ta over the (Fe, Mn)(Nb, Ta) 2 stoichiometry of columbite, characteristic of most compositions of wodginite, may play a significant role. Ordered nuclei potentially present in the bulk of natural disordered phases also may be important.

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