Metamict Fergusonite from Kolonna and Masimbula, Sri Lanka

Two radioactive mineral specimens (with compositions ThO₂ 2.9-4.6wt% and UO₂ 5.8-7.8wt%, respectively), collected from a placer deposit near Kolonna and a quarry near Masimbula, Sri Lank.a, were identified as metamict fergusonite. Identification was based on samples' chemical compositions and the observation that both specimens were transformed to crystalline fergusonite upon dry annealing at 550-600°C, including the spectroscopic observation of a high-temperature α---- β transition (≥750°C) that is characteristic of this mineral. Strong negative Eu anomalies in chondrite-normalised REE patterns indicate reducing formation conditions. Both samples have experienced (perhaps even multiple) chemical alteration processes that led to the formation of decidedly heterogeneous internal textures. Fluid-driven alteration, especially along fractures and domain boundaries, has resulted in vast chemical changes.