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, 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 P212121, was solved by direct methods and refined to an agreement index (R) of 4.9% for 2309 unique reflections collected using MoKα X-radiation 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 U5+ The Uφ7 (φ: O, OH, H2O) polyhedra share edges and corners to form a unique sheet in which a CO3 group shares an edge with the U5+φ7 polyhedron. The structure contains one Ca site coordinated by seven anions. 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 Caφ7 polyhedron is linked to one adjacent sheet by sharing an edge with the CO3 group and an O atom with a U6+φ7 polyhedron. Structural units are linked together through hydrogen bonds only.