Schoepite, [(UO 2 ) 8 O 2 (OH) 12 ](H 2 O) 12 , is orthorhombic, a 14.337(3), b 16.813(5), c 14.731(4) Aa, V 3551(2) Aa 3 , space group P2 1 ca, Z = 4. The structure has been solved by direct methods and refined on F 2o to a weighted R index of 5.8% based on 4534 unique reflections measured with MoKalpha X-radiation on a single-crystal diffractometer (equivalent to an R index of 2.7% for F o >4sigma F o ). The refinement indicates that the formula contains eight more H 2 O groups per unit cell than previously assumed. The structure consists of neutral [(UO 2 ) 8 O 2 (OH) 12 ] sheets of edge-and corner-sharing Uphi 7 pentagonal dipyramids (phi : O, OH), hydrogen-bonded to each other through interstitial H 2 O groups. These sheets are topologically identical to those found in fourmarierite. The [(UO 2 ) 8 O 2 (OH) 12 ] sheets are interleaved with almost planar sheets of interlayer H 2 O groups. There are twelve symmetrically distinct H 2 O groups in the interlayer sheet; these are arranged in two pentagonal rings with two linking H 2 O groups. H-atom positions were not resolved, but an H-bonding scheme is suggested on the basis of stereochemical and bond-valence arguments. The structure displays strong Pbca pseudosymmetry, especially among the U atoms. The lower symmetry is primarily due to H-bond interactions between interlayer H 2 O groups and O(uranyl) atoms of the structural sheet.