Uranocircite

A wider range of structural variation is exhibited by the autunite-type compounds that contain Ba. The barium uranyl phosphate decahydiate mineral uranocircite, Ba[(U02)(P04)]2(H20)io, is likely isostructural with its arsenate analogue, heinrichite, Ba[(U02)(As04)]2(H20)io, (Tables 12 and 13) [128, 131],... 

Crystal structure studies have only been made on members of the meta-autunite family. Abernathyite and metatorbernite have been studied by Ross and Evans, and Ross and coworkers,meta-uranocircite by Zolensky and Khosrawan-Sazedj, threadgoldite by Piret and co-workers, and hydrogen-meta-autunite by Morosin." These studies verify the general meta-autunite structure proposed by Beintema, but show that there are subtle structural differences among the species. The basic unit is an infinite sheet of 2-4 UOe polyhedra and (P,As)04 tetrahedra, which share corners. Ideally, this sheet has tetragonal symmetry. It is shown in Fig. 13, which also shows how the sheets are proposed to... 

Specimens of trogerite and meta-uranocircite are known to transform from tetragonal to lower symmetry at temperatures near room temperature. For trogerite the transformation is around 25°C for meta-uranocircite it is at 108°C. A slight distortion in the structure results and the crystals show an extensive cross-grid twinning. Similar transformations occur in other minerals also—for example, Ca and Pb meta-autunites, and may explain the reported meta-autunite II, which is orthorhombic. 

Zolensky M. E. The crystal structure and twinning of barium uranyl phosphate hexahydrate (meta-uranocircite), and comparison to the other members of meta-autunite group. Ph.D. thesis, Pennsylvania State University, 1983. 

Both detrital members interfinger and progressively merge. They can be interpreted as fluviatile deposits of meanders over a flood plain. The uranifefous mineralization is totally controlled by sandy lenses of the channel-shaped member, two-thirds of the metal occurring in the lower sequence and the remainder in the upper. Uranium-bearing species are mainly phosphates and vanadates (francevillite-tyuyamunite, autunite, uranocircite) uranium is associated with selenium, arsenic, zinc and lead. 

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