The structure of masuyite, Pb[(UO2)(3)O-3(OH)(2)](H2O)(3), and its relationship to protasite

The structure of masuyite,Pb[(UO2)(3)O-3(OH)(2)](H2O)(3), Z=2, monoclinic, a 12.241(3), b 7.008(2), c 6.983(2) Angstrom, beta 90.402(4)4 space group P n, has been solved by direct methods and refined by full-matrix least-squar es techniques to an agreement index (R) of 6.3% for 2473 unique observed re flections ( | F-o | greater than or equal to 4 sigma(F)) collected using Mo K alpha X-radiation and a CCD-based area detector. The structure contains t hree symmetrically distinct U6+ positions, each of which is occupied by nea rly linear (UO2)(2+) uranyl ions (Ur) that are coordinated by five addition al anions arranged at the equatorial corners of pentagonal bipyramids cappe d by the O-Ur anions. The uranyl pentagonal bipyramids share edges to form alpha-U3O8-type sheers that are parallel to (010). The interlayer contains two distinct Pb2+ sites as well as three H2O groups. The Pb(1) site is clos e to fully occupied and is coordinated by seven atoms of O that are contain ed in the sheets of uranyl polyhedra, and three H2O groups. The Pb(2) site is only similar to 12% occupied and is coordinated by six atoms of O from t he sheets of uranyl polyhedra and three H2O groups. The structure of masuyi te is closely related to that of protasite, but has an additional cation si te in the interlayer.