Crystal structure of uranyl-oxide mineral wölsendorfite revisited

Abstract

The crystal structure of the rare supergene Pb2+-containing uranyl-oxide mineral wölsendorfite has been revisited employing the single-crystal X-ray diffraction. The new structure refinement provided deeper insight into the complex structure of this mineral, revealing additional H2O sites in the interlayer complex and confirming the entrance of the Ca2+ into the structure. Studied wölsendorfite is orthorhombic, space group Cmcm, with unit cell dimensions a = 14.1233(8) Å, b = 13.8196(9) Å, c = 55.7953(12) Å, V = 10890.0(10) Å3, and Z = 8. The structure has been refined to an agreement index (R) of 10.74% for 3815 reflections with I > 3σ(I) collected using a microfocus X-ray source from the microcrystal. In line with the previous structure determination, the refined structure contains U–O–OH sheets of the wölsendorfite topology and an interstitial complex comprising nine symmetrically unique Pb sites, occupied dominantly by Pb2+. Nevertheless, one of the sites seems to be plausible for hosting Ca2+. Its presence has been successfully modeled by the refinement and further supported by the crystal-chemical considerations. The structural formula of wölsendorfite crystal studied is Pb6.07Ca0.68[(UO2)14O18(OH)5]O0.5(H2O)12.6, with Z = 8, Dcalc. = 6.919 g·cm–3 (including theoretical 30.2 H atoms). The rather complex structure of wölsendorfite makes it the third most complex known uranyl-oxide hydroxy-hydrate mineral.