The crystal structure of the lead uranyl-carbonate mineral widenmannite has been solved from precession electron-diffraction data and refined using both electron-diffraction data and synchrotron powder-diffraction data. Widenmannite is orthorhombic, Pmmn, with a = 4.9744(9), b = 9.3816(16), c = 8.9539(15) Å, and V = 417.86(12) Å3. The structure was solved by charge-flipping and refined to an R1 = 0.1911 on the basis of 301 unique, observed reflections from electron diffraction data, and to Rp of 0.0253 and RF of 0.0164 from X-ray powder data. The idealized structure formula of widenmannite is Pb2(OH)2[(UO2)(CO3)2], Z = 2. However, both data sets suggest that the widenmmanite structure is not that simple. There are two symmetrically independent, partly occupied U sites. The substitution mechanism can be written as U(1)O2 + Pb(OH)2 ↔ U(2)O2. When the U(2) site is occupied, the U(1)O2 group is absent, the two OH groups are substituted by O2− and one Pb2+-vacancy. The chemical formula of the real structure should be written as Pb2−x(OH)2−2x[(UO2)(CO3)2], where x is the probability of the substitution U(2) → U(1). The probability of occurrence of U(2) refines to x = 0.074(15) from the powder-diffraction data and to x = 0.176(4) from the electron-diffraction data. There is one Pb site (nearly fully occupied), which is coordinated by 11 anions (up to the distance of 3.5 Å), including O and OH. The shorter Pb-O bonds form a sheet structure, which is linked by the weaker bonds to the uranyl-carbonate chains to form a three-dimensional framework structure.

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