Niobium and tantalum in minerals: Siderophile, chalcophile or lithophile, and polyvalent

Abstract

Most geoscientists are convinced that minerals of both niobium and tantalum contain those elements in the 5+ state only. In fact, there are several well-documented minerals containing Nb0, Nb2+, Nb4+ and Nb5+, and tantalum follows a similar pattern. Both elements can exhibit a siderophile, a chalcophile and a lithophile character, as well illustrated empirically, and as documented via XANES and XPS spectra. Of the XPS spectra that we report here on ferroniobium of metallurgical grade, pyrochlore, samarskite, niocalite, synthetic edgarite (FeNb3S6) and NbTe2, only the samarskite and niocalite contain only one valence state of Nb (5+). Vladimir Vernadsky understated the situation when he placed Nb and Ta in the “dispersed” category. Columbite-group minerals are most unusual in exhibiting variable mutual disorder involving (Nb,Ta) and (Fe,Mn). Oddly, samples must be heated to become ordered. We raise the possibility that at the stage of primary crystallization, the relevant magmas contained mixed valences of Nb, Ta, Fe and Mn, which made a disordered distribution along the octahedrally coordinated cations energetically more acceptable. Owing to the relative insolubility of members of this solid-solution series in the ambient aqueous fluid, they have remained metastably frozen in a partly or completely disordered state.