Reinvestigation of the crystal structure of kasolite, Pb[(UO 2)(SiO 4)](H 2O), an important alteration product of uraninite, UO 2+x

Abstract

The crystal structure of kasolite, Pb[(UO 2)(SiO 4)](H 2O), 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) Å 3, the space group P2 1/ c, has been solved by charge-flipping method and refined by the full-matrix least-squares techniques to an agreement factor ( R obs) of 2.2% and, a goodness-of-fit (GOF) of 1.26 using 1243 unique observed diffraction maxima ( I obs > 3 σ( I)) collected with MoKα X-radiation and a 4 K CCD area detector. The crystal structure of kasolite contains 1 unique U 6+ position that is part of a nearly linear uranyl ion (UO 2) 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 Pb 2+ in the interlayer. O ligands and 1 (H 2O) non-transformer group coordinate Pb 2+ 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.