Abstract

The structure of vandendriesscheite, Z = 8, Pb (sub 1.571) [(UO 2 ) 10 O 6 (OH) 11 ](H 2 O) 11 , orthorhombic, a = 14.1165(6), b = 41.378(2), c = 14.5347(6) Aa, V = 8490 Aa 3 , space group Pbca, has been solved by direct methods and refined by full-matrix least-squares techniques to an agreement factor (R) of 12.1% and a goodness-of-fit (S) of 1.28 using 4918 unique observed reflections (F o > or =4sigma F) collected with MoKalpha X-radiation and a CCD (charge-coupled device) detector. The structure contains ten unique U (super 6+) positions, each of which is part of a nearly linear (UO 2 ) (super 2+) uranyl ion that is further coordinated by five equatorial (O (super 2+) , OH (super -) ) anions to form pentagonal bipyramidal polyhedra. There are two unique Pb positions; one is fully occupied, but site-scattering refinement gives an occupancy factor of 0.573(8) for the other. The Pb positions are coordinated by O atoms of the uranyl ions and by H 2 O groups. There are 11 unique H 2 O groups; five are bonded to Pb and the other six are held in the structure by hydrogen bonds only. The U polyhedra link by the sharing of equatorial edges to form sheets parallel to (001). The sheet of U polyhedra is not known from another structure and is the most complex yet observed in a uranyl oxide hydrate. The sheets are structurally intermediate to those in schoepite and becquerelite and are linked by bonds to the interlayer Pb cations and the H 2 O groups. The extensive network of hydrogen bonds that link adjacent sheets is derived on the basis of crystal-chemical constraints. The high mobility of U (super 6+) in oxidizing fluids, as opposed to Pb (super 2+) , causes the alteration products of Precambrian uraninite deposits to become progressively enriched in Pb relative to U. In the case of lead uranyl oxide hydrate minerals, there is a continuous sequence of crystal structures that involves a systematic modification of the sheets of U polyhedra and that corresponds to increasing sheet charge and increasing Pb content. Thus, a clear relationship exists between the crystal structures of lead uranyl oxide hydrates and their paragenesis, and this is relevant to the disposal of spent nuclear fuel.

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