Reinvestigation of the crystal structure of kasolite, Pb[(UO2)(SiO4)](H2O), an important alteration product of uraninite, UO2+x

The crystal structure of kasolite, Pb[(UO2)(SiO4)](H2O), Z = 4, monoclinic, with a = 6.7050(3), b = 6.9257(2), c = 13.2857(5) Å, β = 105.064(4)°, V = 595.74(3) Å3, the space group P21/c, has been solved by charge-flipping method and refined by the full-matrix least-squares techniques to an agreement factor (Robs) of 2.2% and, a goodness-of-fit (GOF) of 1.26 using 1243 unique observed diffraction maxima (Iobs > 3σ(I)) collected with MoKα X-radiation and a 4 K CCD area detector. The crystal structure of kasolite contains 1 unique U6+ position that is part of a nearly linear uranyl ion (UO2)2+, coordinated in the equatorial plane by five O ligands, forming pentagonal bipyramid. The uranyl pentagonal bipyramids share edges to form chains parallel to [0 1 0]. The additional edge of uranyl polyhedra is shared by silicate tetrahedra to form sheets parallel to (1 0 0). There is one unique position of Pb2+ in the interlayer. O ligands and 1 (H2O) non-transformer group coordinate Pb2+ exhibiting [2 + 6] coordination. A network of H-bonds provides an additional linkage of an interlayer to the sheets besides Pb-O bonds. Chemical composition of the studied crystals, obtained by the electron microprobe, is reported and is in agreement with the crystal structure refinement.