Wolsendorfite: A Masterpiece of Structural Complexity

Peter C. Burns

 

Uranyl minerals are currently receiving considerable attention because of their significance to the environment; they are present in the mill tailings that result from the mining of uranium, are precipitated in soils that are contaminated with actinide elements, and are expected to form in abundance in geological repositories for spent nuclear fuel. Owing to the primitive c repeat of 56 Å, it was impossible to determine the structure of wölsendorfite prior to the introduction of CCD-based detectors of X-rays. The structure of wölsendorfite, Pb6.16Ba0.36[(UO2)14O19(OH)4](H2O)12, Z = 8, orthorhombic, a = 14.131(1), b = 13.885(1), c = 55.969(4) Å, V = 10,982 Å3, space group Cmcm, has been solved by direct methods and refined by full-matrix least-squares techniques to an agreement factor (R) of 6.4% and a goodness-of-fit (S) of 1.13 using 6215 unique observed reflections collected with MoKa X-radiation and a CCD detector. The structure contains eight unique U6+ positions, each of which is part of a nearly linear (UO2)2+ uranyl ion. The uranyl ions (Ur) are further coordinated by four or five anions (f ) arranged at the equatorial corners of square and pentagonal bipyramids, respectively. The structure contains two unique Urf 4 square bipyramids and six unique Urf 5 pentagonal bipyramids that link by the sharing of equatorial corners and edges to form infinite sheets that are parallel to (100). The sheets are by far the most complex sheet of uranyl polyhedra yet observed in a mineral or synthetic compound. The interlayer between the uranyl sheets contains Pb2+ and Ba cations, as well as H2O groups that are either bonded to the interlayer cations or are held in the structure by H bonding only. There are eight unique cation positions in the interlayer that are coordinated by six to ten anions.
The structure of wölsendorfite is remarkable both in the complexity of the sheets of uranyl polyhedra and the connectivity of the interlayer. By using the sheet anion-topology approach, it is shown that the wölsendorfite sheet is composed of slabs of the simpler a -U3O8 and b -U3O8-type sheets. It is possible that many as yet unknown complex sheets of uranyl polyhedra exist that are based upon anion topologies that are combinations of slabs of simpler topologies.

 

The wolsendorfite sheet and corresponding sheet anion-topology.

 

 

Development of the wolsendorfite sheet anion-topology as a chain-stacking sequence. This demonstrates that the complex wolsendorfite sheet is composed of slabs of the simpler alpha-U3O8 and beta-U3O8 sheets.