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Description and Classification of Uranium Oxide Hydrate Sheet Topologies

Published online by Cambridge University Press:  15 February 2011

M. L. Miller ,
R. J. Finch ,
P. C. Burns  and
R. C. Ewing
M. L. Miller
Affiliation:
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque NM 87131, USA, [email protected]
R. J. Finch
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
P. C. Burns
Affiliation:
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque NM 87131, USA, [email protected]
R. C. Ewing
Affiliation:
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque NM 87131, USA, [email protected]
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Abstract

The uranyl oxide hydrates (UOH) are important corrosion products of uraninite and UO 2 in spent nuclear fuel under oxidizing conditions. However, the systematics of the crystal chemistry, thermodynamic parameters, and solubilities of this mineral group are poorly understood. With the exception of the synthetic UO2 (OH)2 polymorphs, all UOH crystal structures are based on sheets of edge-sharing 5- and 4-coordinated uranyl dipyramids. This structural similarity suggests that it is possible to develop a model by which to estimate the thermodynamic behavior of UOHs from data on structural endmember phases. Toward this end, a method of quantitatively describing all known UOH sheets has been developed. Only four structural unit chains are required to construct the uranyl oxide hydrate sheets (as well as the structurally similar U3O8 sheets). The H-chain is restricted to α-UO2 (OH)2 and is made up of hexagonally coordinated uranyl ions sharing opposing edges. The “arrowhead” chain composed of pentagonal dipyramids sharing edges and alternating with trigonal vacancies is present in all other UOH sheets. These arrowhead chains are directed and can occur in both an ↑ and ⇓ “sense” within a single sheet. The P-chain consists of edge-sharing pentagonal dipyramids 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 dipyramids. This “R-chain” is produced when nested adjacent ⇑ and ⇓arrowhead chains are translated by a diagonal shift. This “chain” occurs in sheets which contain only 4-coordinate uranyl ion and those containing both 4- and 5-coordinate uranyl ions.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 412: Symposium V – Scientific Basis for Nuclear Waste Management XIX , 1995 , 369
Copyright
Copyright © Materials Research Society 1996

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