Description and classification of uranium oxide hydrate sheet anion topologies
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Description and classification of uranium oxide hydrate sheet anion topologies Mark L. Miller Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131
Robert J. Fincha) Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Peter C. Burnsb) and Rodney C. Ewing Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 (Received 12 January 1996; accepted 5 August 1996)
The sheets of uranyl ions (U6+ O2 )2+ in the structures of all uranyl oxide hydrates (UOH) (and the structurally related a- and b-forms of U3 O8 ) are based on only four structural unit chains. Each sheet type may be reduced to its underlying sheet anion topology to determine the chains present within each topology and to describe the structural relationships among these phases. Each sheet type is described by a chain stacking sequence. The four chain types required to construct the UOH sheet anion topologies are the H-chain, the R-chain, the P-chain, and the directional “arrowhead” chains denoted by U and D. The H-chain is found only in the sheet anion topology of a-UO2 (OH)2 and consists of hexagonal sites sharing opposing edges. In a –UO2 (OH)2 , all hexagons are populated with uranyl ions. The “arrowhead” chain is composed of pentagonal sites populated with uranyl ions and sharing edges and alternating with vacant triangular sites. Arrowhead chains are present in the sheet anion topologies of all other UOH sheets. Arrowhead chains are directional and can occur in both U and D “senses” within a single anion topology. The P-chain consists of edge-sharing pentagonal sites populated with uranyl ions forming a zigzag chain. The P-chain is flanked on both sides by arrowhead chains of the same “sense”. The remaining structural unit is a discontinuous “chain” of rhombic sites. This “R-chain” is produced when nested; adjacent U and D “arrowhead” chains are translated diagonally. The R-chain occurs in the sheet anion topologies of sheets which contain only 4-coordinate uranyl ions and those containing both 4- and 5-coordinate uranyl ions. The rhombic sites may be populated with a uranyl ion, a U4+ or other cation and two apical oxygens, or they may be vacant.
I. INTRODUCTION
The UO2 in spent nuclear fuel (SNF) will be unstable under the oxidizing conditions present at Yucca Mountain, the proposed nuclear waste repository in Nevada.1 The solubility-limiting phases will include the uranium oxide hydrates (UOH) which form as corrosion products of the spent nuclear fuel.1 The crystal-chemical systematics of this mineral group (Table I) are poorly understood, and the existing literature contains many contradictions and ambiguities.2 Smith et al.3 provide a systematic discussion of the chemistry of these phases; a)
Present address: Chemical Technology Division, Argonne National Laboratories, Argonne, Illinois 94304. b) Present address: Department of Geology, University of Illinois, Urbana, Il
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