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)O-4(O H)(H2O)(7), Z = 4, orthorhombic, a = 11.2706(8), b = 7.1055(5), c = 20.807( 1) Angstrom, V = 1666.3(3) Angstrom(3), space group P2(1)2(1)2(1), was solv ed by direct methods and refined to an agreement index (R) of 4.9% for 2309 unique reflections collected using MoK alpha X-radiation and a CCD-based d etector. The structure contains three unique U positions: two contain U6+ a nd involve uranyl ions with typical pentagonal-bipyramidal coordination. Se ven anions coordinate the other U position, but there is no uranyl ion pres ent. The polyhedral geometry, the bond-valence sum incident at this U site, and electroneutrality requirements, all indicate that this site contains U 5+. The U phi(7) (phi: O, OH, H2O) polyhedra share edges and corners to for m a unique sheet in which a CO3 group shares an edge with the U(5+)phi(7) p olyhedron. The structure contains one Ca site coordinated by seven anions. The Ca atom and its associated H2O groups occupy interlayer sites, along wi th two H2O groups that are held in the structure by H bonds only. The Ca ph i(7) polyhedron is linked to one adjacent sheet by sharing an edge with the CO3 group and an O atom with a U(6+)phi(7) polyhedron. Structural units ar e linked together through hydrogen bonds only.