SYNTHESIS OF AND APPLICATIONS FOR THE CURIENITE-FRANCEVILLITE MINERAL SERIES

Curienite and Francevillite are isostructural uranyl vanadate minerals which most often occur in oxidizing zones containing reduced U and V. One notable occurrence of this mineral series is within a weathered alteration zone at the Mounana deposit north of the Oklo natural reactor zone in Gabon. Because their location within the proximity of the Oklo natural reactor and overall insolubility in aqueous environments, the thermodaynamic properties of Curienite and Francevillite are of interest and may provide information about the behavior of spent nuclear fuel over geologic time.

The crystal structures of these minerals are composed of sheets containing dimers of edge sharing V⁺⁵O₅ pyramids which share oxygen atoms with either one or two uranyl pentagonal bipyramids. Interlayer space in Curienite and Francevillite is populated by Pb and Ba, respectively, as well as water molecules. Although commonly found in nature with variable amounts of Pb and Ba, pure end member compositions have been obtained synthetically. Hydrothermal methods utilizing uranyl pyrovanadate as a starting reagent and the appropriate alkali hydroxide form pure powders of the desired mineral phase after 5 days at 170 ̊C with starting pH<3. Characterized using powder X-ray diffraction, thermogravimetric analysis, and inductively coupled plasma optical emission spectrophotometry, formation enthalpies for these mineral phases will be determined by high temperature oxide melt drop-solution calorimetry. Differences in thermodynamic properties resulting from variable composition will be investigated.