Mechanisms leading to exceptional niobium concentration during lateritic weathering: The key role of secondary oxides

Abstract

Niobium (Nb) is considered as one of the most immobile elements during supergene weathering due to its low solubility and the expected resistance of the pyrochlore mineral group (A2B2X6Y) in which Nb occupies the B site. Although the resistance of pyrochlore is challenged by direct evidence of alteration in lateritic profiles, the geochemical and mineralogical processes controlling its behavior in the critical zone remains elusive. This study uses a multiscale geochemical, mineralogical and spectroscopic approach to monitor Nb solid speciation in a thick weathering profile. The weathering of the pyrochlore-bearing core carbonatite facies from Morro dos Seis Lagos has resulted in a Fe-enriched laterite with exceptional Nb concentrations (2.91 wt% Nb2O5 on average). The preservation of the successive secondary phases (Ti-, Fe-, Ce-oxides) resulting from weathering processes along the horizons offers the opportunity to track Nb during pyrochlore alteration from the fresh parent rock to the final stages of weathering. Intense lateritization processes have led to an enrichment in Fe and Ti in the pyrochlore B-sites, resulting in the weakening of the structure and its ultimate destabilization due to weaker B-O bonds. After the release of Nb from pyrochlore, the fate of Nb is dependent on the secondary phases formed during weathering. In the lower horizons dominated by Fe oxides, goethite is the main host for Nb (1 wt% Nb2O5) due to its capacity to substitute Fe3+ for Nb5+, in contrast to hematite. In the upper horizons, where Ti oxides have formed during the reworking of the laterite, Nb5+ is preferentially incorporated into brookite and rutile (20 wt% Nb2O5), which may have formed owing to slightly acidic pH and high Nb activity during their growth. The presence of Nb in hollandite and cerianite in a vein from the manganiferous horizon involves the transport of Nb with mobile elements at the scale of the laterite. With this study, we describe a new model accounting for the formation of Nb-enriched laterites. The latter is based on the ability of Nb5+ to substitute for Fe3+, Ti4+, Ce4+ in minerals formed by lateritization processes after its release from pyrochlore. The nature of the secondary Nb-bearing oxides is dependent on the chemistry of the parent rock and the conditions of formation of the laterite. This work, evidencing the mobilization of Nb both at the scale of the mineralogical assemblage and of the entire profile questions the pervasive use of Nb as a geochemical invariant in the critical zone.