Exploring the kinetics of monovalent thallium incorporation in hydropyrochlore and hydrokenopyrochlore

Pyrochlores, general formula A2–mB2X6–wY1–n and cubic Fd-3m(Z = 8) symmetry, are oxide and fluoridemineralswith a peculiar structure consisting of a framework of corner-linked BX6 octahedra. The framework is arranged such that it forms tunnels parallel to the [110] direction, which are occupied by the A and Y sites at the centre (or in slightly displaced neighbouring positions). Atoms hosted at the tunnel sites are particularly susceptible to ion-exchange processes, especially when these sites are occupied by highly mobile species (e.g. water) or are partially vacant; this is the case for hydropyrochlore and hydrokenopyrochlore from the Lueshe carbonatite, both extensively altered and hydrated due to intense weathering. Their ion-exchange properties are remarkable, making them suitable candidates for Tl+ sequestration from aqueous matrices. This study focuses on the kinetics of the ion-exchange reaction involving monovalent thallium, incorporated by single crystals through progressively longer imbibition experiments in Tl-rich solutions. Structural (single-crystal X-ray diffraction) and chemical (electron microprobe analysis) investigations carried out before and after each imbibition experiment showed that Tl+ is incorporated in the structure at both tunnel sites and tends to order preferentially at A with increasing A:B occupancy ratios. The entry of Tl+ seems to be accompanied by a progressive shift in colour of the crystals (from colourless to dark brown). The incorporation process is relatively quick, as it approaches a saturation limit (~70–75% of the A site occupancy) in, cumulatively, 450 minutes of treatment; longer imbibition experiments (930 minutes) only led to a slight redistribution of electrons among the tunnel sites.