Meta-uranocircite

Figure 33. Variation in Ba compounds with the autunite-type sheet, after [128,131]. At left heinrichite, Ba[(U02)(As04)]2(H20)io, projected along [100], with Ba atoms shown as black spheres and O atoms as larger gray spheres. At center, the structure of meta-uranocircite 1, Ba[(U02)(P04)]2(1120)7, projected along [001]. At right, the structure of meta-uranocircite II, Ba[(U02)(P04)]2(H20)g, projected along [101]. For clarity, hydrogen bonding is not shown.

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. 

---