Niobium is usually considered to be one of the least mobile elements during alteration of rocks in geological processes and is thought to only be mobile at elevated temperatures and/or pressures. Since niobium is assumed to be unaffected by weathering it can be used in geochemical-evolution calculations to estimate the loss of material from the weathering of rocks. There are roughly a hundred niobium-bearing minerals, including those in the pyrochlore group, which comprises the main ores for Nb and Ta. Furthermore, pyrochlores are identified as potential hosts for nuclear waste because of their resistance to weathering. Here we show that niobium can be highly mobile even at near-surface geochemical conditions as a soluble and unreactive polyoxometalate ion. Discovery of the first natural hexaniobate minerals, all formed at near-surface conditions, provides evidence that niobium is being actively leached out of nearby primary minerals and easily transported as polyoxometalate ions. In addition, we show how Nb and Ta can be fractionated during low-temperature processes.