Siamese dodecahedron

The stmcture of UP04C1(H20)2 is also adopted by its Br analog [33]. From the Wyckoff positions of the atoms, the actinide site in the stmcture appears to contain UO6X2 (X = Cl, Br) polyhedra that approximate a Johnson snub disphenoid, also known as a Siamese dodecahedron, but as the halogen site is half-occupied, the actual coordination number is seven [35]. These polyhedra share vertices with phosphate tetrahedra to form an open framework with voids that extend parallel to the [100] direction. The halogen atoms that coordinate uranium line these voids (Fig. 5). 

In contrast to the relative paucity of structures with the actinides in sixfold or sevenfold coordination, eightfold coordination is shown by at least six different structure-types (Table 3). The actinide-centered polyhedra in these structure generally correspond to a Johnson snub disphenoid (also known as a Siamese dodecahedron, or a twelve-faced convex deltahedron). These six structure types are presented in order of increasing pol5mierization and decreasing crystallographic symmetry. 

Xenotime-(REE), (REE)(P04), and pretulite, Sc(P04) (Table 11), belong to the zircon, Zr(Si04), group. The larger trivalent cation is coordinated by eight 0-atoms in an arrangement that is known as a Siamese dodecahedron (Hawthorne and Ferguson 1975). 

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