Advances in Understanding of the Crystal Chemistry of Hexavalent Uranium
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Advances in Understanding of the Crystal Chemistry of Hexavalent Uranium Peter C. Burns Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556, U.S.A. [email protected] ABSTRACT Research concerning the crystal chemistry of hexavalent U by the Environmental Mineralogy and Crystal Structures research group at Notre Dame has resulted in the description of more than 110 new structures of uranyl compounds (including 36 minerals). New insights into the crystal chemistry of U6+ are presented, with emphasis on recently discovered novel structural connectivities. The structural hierarchy of uranyl minerals and compounds, which was first established for 180 structures in 1996, has been extended to include 145 new structures. The hierarchy is based upon polymerization of polyhedra containing higher-valence cations, and consists of five distinct classes: structures containing isolated polyhedra (7), finite clusters of polyhedra (41), chains of polyhedra (52), sheets of polyhedra (184), and frameworks of polyhedra (41). The dominance of sheets in uranyl compounds (57% of known structures) arises from the unequal distribution of bond-valences within the uranyl polyhedra. Topological relations of the sheets in uranyl compounds are best understood by analysis of the topological distribution of anions within sheets in which sharing of polyhedral edges dominates, and by graphical representation of the connectivity of polyhedra in cases where sharing of vertices of polyhedra dominates the sheet. INTRODUCTION Phases containing U6+, which is usually present as the UO22+ uranyl, have been the focus of extensive research over two centuries. Efforts of the Environmental Mineralogy and Crystal Structures group at University of Notre Dame have resulted in the publication of more than 110 new uranyl structures, of which 36 correspond to minerals. These structures exhibit fascinating and complex atomic arrangements, owing to the myriad of possible types of connections of uranyl polyhedra with other uranyl polyhedra and polyhedra involving other high-valence cations. Research concerning uranyl compounds is driven by the search for novel solids with important materials properties [1-3], as well as their importance in the environment [4-6]. Uranyl phases in nature (uranyl minerals) exhibit considerable structural and chemical diversity, and reflect geochemical conditions dominant during their formation. Uranyl minerals are of considerable environmental importance. They are significant for understanding the genesis of U deposits, as well as water-rock interactions in U-rich rocks. Uranyl minerals impact the mobility of actinides in contaminated soils [7,8] and in vadose zone sediments polluted with actinides, such as the Hanford and Savanna River sites in the U.S. [9]. Precipitation of uranyl phosphate minerals in the vadose zone of contaminated sites, by the addition of phosphate, has been proposed as a means to mitigate uranium plumes in groundwater [10]. Uranyl minerals
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