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Schoepite and Dehydrated Schoepite

Published online by Cambridge University Press:  15 February 2011

F. C. Hawthorne,
R. J. Finch and
R. C. Ewing
F. C. Hawthorne
Affiliation:
University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada
R. J. Finch
Affiliation:
University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada
R. C. Ewing
Affiliation:
University of New Mexico, Albuquerque, New Mexico, 87131, U.S.A
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Abstract

Schoepite, [(UO2)8O2 (OH)12]·12H2O, dehydrates irreversibly to a defect structure-derivative of α-UO2 (OH)2 with composition UO3·0.75H2O (dehydrated schoepite) according to the reaction.[(UO2)8O2 (OH) 12]+12H2O ↠ 8[(UO2 )O0.25 (OH)1 5.] + 12H2O.Natural crystals change from translucent yellow schoepite to opaque yellow, polycrystalline dehydrated schoepite. The complete transformation occurs in three steps: (I) loss of interlayer H20 in schoepite causing collapse of the layers; (2) atomic rearrangement within the sheets from a schoepite-type arrangement to a configuration which may be similar to that of rnetaschoepite, UO3.2H2O; (3) a second rearrangement to the defect α-UO2 (OH)2-type sheet. Although dehydrated schoepite may transform to stoichiometric α-UO 2(OH)2 in water above approximately 290°C, it does not hydrate at any temperature. Schoepite transforms slowly in air at ambient temperature to metaschoepite, although this may take geologically long periods. The transformation of schoepite to dehydrated schoepite can occur rapidly at room temperature if crystals, which have only partially converted to metaschoepite. are subjected to external stress.

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