Abstract

Florencite-(La) (La/Ce = 1.09) with fissiogenic REEs and florencite-(Ce) (La/Ce = 0.62) have been identified in illite from the clay mantle surrounding a natural, 2 Ga fission reactor at Bangombé and in sandstone beneath the reactor zone, respectively. Florencite-(Ce) is apparently unrelated to nuclear processes and occurs with monazite-(Ce), apatite, TiO2 (probably anatase), zircon, and illite. Grains of florencite-(Ce) contain inclusions of thorite, chalcopyrite, and galena. Florencite-(La) was found 5 cm from the “core” of the reactor and contains inclusions of galena and U-Ti-bearing phases. Secondary uraninite and coffinite have precipitated on some of the florencite grains. The chemical composition of florencite-(La) as determined by electron microprobe analysis is (La0.38Ceo.35Nd0.06Sm0.01Ca0.03Sr0.17)(Al2.98Fe3+0.02)(PO4)[PO3.80(OH)0.20](OH)6. Secondary ion mass spectrometry revealed that between 27 and 30% of Nd and 67 and 71% of Sm in florencite-(La) is fissiogenic. The presence of fissiogenic REEs in “florencite” from the reactor zone in Bangombe and their preferential concentration in florencite relative to the bulk sample of clay demonstrate that aluminous phosphates may have played a more significant role in the fixation of fissiogenic REEs released from uraninite after the sustained fission reactions than sorption onto clays.

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