The structure of richetite, approximate formula M x Pb (sub 8.57) [UO 2 ) 18 O 18 (OH) 12 ] 2 (H 2 O) 41 , Z = 1, triclinic, a 20.9391(3), b 12.1000(2), c 16.3450(3) Aa, alpha 103.87(1), beta 115.37(1), gamma 90.27(1) degrees , V 3605.2 Aa 3 , space group P1, has been solved by direct methods and refined by full-matrix least-squares techniques to an agreement factor (R) of 8.9% and a goodness-of-fit (S) of 1.79 using 12,383 unique observed reflections (F o >4sigma F ) collected with MoKalpha X-radiation and a CCD (charge-coupled device) area detector. The structure contains 36 unique U (super 6+) positions, each of which is part of a near-linear (U (super 6+) O 2 ) (super 2+) uranyl ion that is further coordinated by five (O, OH-) anions, forming pentagonal bipyramids. The uranyl polyhedra share edges to form symmetrically distinct but topologically identical alpha -U 3 O 8 -type sheets at z nearly equal 0.25 and z nearly equal 0.75. Although alpha -U 3 O 8 -type sheets of uranyl polyhedra occur in several structures, the richetite sheets are unique in their arrangement of OH (super -) anions. There are 13 partially occupied unique Pb (super 2+) sites, two octahedrally coordinated M sites that may contain Fe (super 3+) or other cations, and 41 unique H 2 O groups in two distinct interlayers at z nearly equal 0 and z nearly equal 0.5. Both the Pb (super 2+) and M cations link to uranyl-ion O-atoms from adjacent sheets, and thus provide linkage of the sheets to the interlayer constituents. An extensive network of H bonds provides additional linkage.