Hexavalent uranium (uranyl) minerals are common in the oxidized portions of U deposits, and are important for understanding water-rock interactions in U deposits, the mobility of actinides in the environment, the weathering of U mine and mill tailings, and the alteration of spent nuclear fuel in a geological repository. This study will focus on Pb uranyl oxide hydrate minerals that occur in the weathered portions of Precambrian U deposits, as well as in weathered mine tailings that result from the utilization of such resources. These structurally complex minerals contain several different sheets of uranyl polyhedra, with Pb cations and water molecules located in interlayer positions. Preliminary work has demonstrated that these minerals show considerable compositional variability within some of the structure types, and led to the hypothesis that anionic substitution in the sheets of uranyl polyhedra may be the charge-balancing mechanism that permits variation of the interlayer content. The Pb uranyl oxide hydrate minerals vandendriesscheite, fourmarierite, curite, richetite, sayrite, and wlsendorfite will be investigated using extensive crystallographic studies, microprobe analysis, and the synthesis of mineral analogues. These studies will provide the details of the crystal-chemical variability of these complex minerals. The results of the investigations will be used to extend the structural hierarchy of uranyl minerals, and to develop an understanding of the relationship between the structures and paragenesis of these and other low-temperature minerals.
