Oursinite is a rare Co-bearing uranyl silicate of the uranophane group. The structure of oursinite, (Co0.8Mg0.2)[(UO2)(SiO3OH)]2(H2O)6, is orthorhombic, space group Cmca, a = 7.0494(5), b = 17.550(1), c = 12.734(1) Å, V = 1575.4(2) Å3, Z = 4. It was solved by direct methods and refined on the basis of F2 for all unique reflections using least-squares techniques to an agreement index (R1) of 2.66%. The structure contains an approximately linear (UO2)2+ uranyl ion that is present as a uranyl pentagonal bipyramid, one symmetrically distinct SiO3OH acid silicate group, and one M2+(OH,H2O)6 octahedron (M is dominated by Co). The uranyl pentagonal bipyramids and silicate tetrahedra are linked by the sharing of edges and vertices, giving a sheet based upon the uranophane anion topology. Adjacent sheets are linked by M2+(OH,H2O)6 octahedra located in the interlayer, and by hydrogen bonds. Each M2+(OH,H2O)6 octahedron contains two OH groups that are apical ligands of silicate tetrahedra in adjacent uranyl silicate sheets. Although several uranophane-group minerals contain sheets that are based upon the uranophane anion topology, the oursinite sheet involves novel orientations of silicate tetrahedra.