CHARACTERIZATION OF A LA-CE-SR-CA ALUMINOUS HYDROXY PHOSPHATE IN NUCLEAR ZONE 13 IN THE OKLO URANIUM DEPOSIT (GABON)

A La-Ce-Sr-Ca aluminous hydroxy phosphate of the goyazite-florencite-c randallite series was observed in the nuclear reactor zone 13 in the O klo uranium deposit (Gabon). It has a trigonal elementary cell with a= 6.98 +/- 0.03 Angstrom and c = 16.35 +/- 0.04 Angstrom (space group P3 m). It occurs in a U-rich sample depleted in U-235 (U-235/U-238 = 0.00 47). Its chemical composition, measured by ion microprobe analyses. sh ows that it contains fission products (Zr, LREE) whereas the isotopic composition of Ba and Sr is natural. Ion microprobe imaging indicates that the nearby uraninite crystals are depleted in fissiogenic LREE. T his study shows that the migration of some fission products, probably related to a dolerite dike intrusion, is limited by the formation of a luminous hydroxy phosphates.