[en] The nucleation of a mineral crystal and its growth in groundwater carrying fractures 300 m above the Palmottu uranium deposit provide an impressive example of geochemical selectivity of uranium. Fracture-filling material was collected from a 3 mm thick fracture at depth 74.8-75 m (drillcore R348). SEM and EDS analyses on a thin section of the original fracture-filling show that the fracture filling is heterogeneous, composing mineral crystal particles and very porous clay-rich aggregates. The results of INAA on millimetre-sized single mineral crystals and aggregates selected from grinded fracture-filling show that porous aggregates (composed of clays and micrometer sized mineral particles) contain up to 1000 ppm U, which is higher than the average of the whole fracture-filling (400 ppm) and host rock related millimetre sized mineral particles (18-100 ppm). ²³³U/²³⁸U isotope exchange proves that a large fraction of the uranium in the fracture-filling is not easily exchanged with uranium in the solution. The amount of ²³⁸U released in the isotope exchange experiment is too high to be explained by reversible U(VI) sorption. Oxidation state analyses show that 30% of the uranium exists as U(IV). Laboratory batch experiment at anoxic conditions proved that pyrite can immobilise U(VI). (orig.)