Determination of the structure of wyartite provides the first evidence for a pentavelent pentavalent-U in a mineral. The structure of wyartite, CaU5+(UO2)2(CO3)O4(OH)(H2O)7, Z = 4, orthorhombic, a = 11.2706(8), b = 7.1055(5), c = 20.807(1) Å, V = 1666.3(3) Å3, space group P2(1)2(1)2(1), was solved by direct methods and refined to an agreement index (R) of 4.9% for 2309 unique observed (| Fo| ³ 4s F) reflections collected using MoKa X-radiation and a CCD-based detector. The structure contains three unique U positions; two contain U6+ and involve uranyl ions with typical pentagonal pentagonal-bipyramidal coordination. Seven anions coordinate the other U position, but there is no uranyl ion present. The polyhedral geometry, the bond-valence sum incident upon at this U site, and electroneutrality requirements, all indicate that this site contains U5+. U The UO7 polyhedra share edges and corners to form a unique sheet, in which a CO3 group shares an edge with the U5+O7 polyhedron. The structure contains one Ca position site coordinated by seven O atomsanions. The Ca atom and its associated H2O groups occupy interlayer sites, along with two H2O groups that are held in the structure by H bonds only. The CaO 7 polyhedron is linked to one adjacent sheet by sharing an edge with the CO3 group and an O atom with a U6+O7 polyhedron. Structural units are linked together through H bonds only.