Wyartite; crystallographic evidence for the first pentavalent-uranium mineral

Determination of the structure of wyartite provides the first evidence for a pentavalent-U mineral. The structure of wyartite, CaU 5+ (UO 2 ) 2 (CO 3 )O 4 (OH)(H 2 O) 7 , Z = 4, orthorhombic, a = 11.2706(8), b = 7.1055(5), c = 20.807(1) Å, V = 1666.3(3) Å 3 , 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 reflections collected using MoKα Xradiation and a CCD-based detector. The structure contains three unique U positions; two contain U6+ and involve uranyl ions with typical 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 at this U site, and electroneutrality requirements, all indicate that this site contains U 5+ . The Uφ 7 (f: O, OH, H 2 O) polyhedra share edges and corners to form a unique sheet in which a CO 3 group shares an edge with the U 5+ φ 7 polyhedron. The structure contains one Ca site coordinated by seven anions. The Ca atom and its associated H 2 O groups occupy interlayer sites, along with two H 2 O groups that are held in the structure by H bonds only. The Caφ 7 polyhedron is linked to one adjacent sheet by sharing an edge with the CO 3 group and an O atom with a U 6+ φ 7 polyhedron. Structural units are linked together through hydrogen bonds only.