Pyrochlore nanomineralogy questions the immobility of niobium during tropical weathering

The origin of the mobility of low-solubility elements such as niobium during supergene weathering remains elusive, despite growing evidence, especially in strongly weathered regolith. In the critical zone, the low mobility of these high field strength elements (HFSEs) is assigned to the resistance of their mineral hosts. This property led to their use as geochemical invariants to quantify mass transfer during alteration. Pyrochlore is a quintessential weathering-resistant mineral accommodating most HFSEs. Here, we investigate the surface alteration layers of pyrochlore using a nanoscale approach within a well-characterized regolith subject to intense weathering over millions of years. In the profile, porous pyrochlore rims exhibit distinct chemistry with higher Th contents. Sharp chemical and textural nanometer-scale interfaces between Pb-rich and Th-rich pyrochlore provide compelling evidence for interface-coupled dissolution-reprecipitation. This demonstrates the alterability of the Nb octahedral framework in pyrochlore and the relative stability of its Th-rich counterpart during tropical weathering. Nanoscale analyses of Ce-rich pyrochlores in two additional lateritic horizons confirm the generality of this mechanism across the deposit. The presence of Nb in secondary nano-cerianite found in manganiferous veins establishes its transport during weathering. These nanomineralogical studies of secondary assemblages provide the first direct evidence of Nb mobility during weathering at the deposit scale, underscoring the need to reappraise HFSE geochemical cycles at Earth’s surface, especially in light of growing mining volumes. This paves the way for further investigations into the alteration of weathering-resistant minerals at the nanoscale, providing new models of mobilization and sequestration through previously undocumented processes.