Tetrahedra sharing edges only

Each octahedron shares six edges with dodecahedra and only corners with tetrahedra. Each tetrahedron shares two edges with dodecahedra octahedra and dodecahedra share only corners. Each dodecahedron... 

The telluride halides crystallize in monoclinic lattices, but only In-TeBr and InTel are isotypic 162). InTeCl forms a layer type of structure, as do InSCl and its analogs, but, owing to the size of the Te atom and the enhanced covalency of the In-Te bond, only a coordination number of 4 for indium is realized. The structure is built up of strongly distorted, InTesraCli/j tetrahedra that share the corners and edges occupied by Te atoms. The Cl atoms are coordinated to one tetrahedron each, and do not take part in the layer formation 324, 325). 

The most numerous and most important tetrahedral structures are those in which only vertices are shared. The sharing of faces of tetrahedral groups AX4 would result in very close approach of the A atoms (to 0-67 AX or OAl XX, where XX is the length of the tetrahedron edge) and a very small A-X-A angle (38° 56 for regular tetrahedra) and for these reasons need not be considered. 

Figure 5. Schematic representation in two dimensions of a dehydrated but fully hydroxylated colloidal silica particle. The fourth oxygen coordinated with Si is above or below the plane of the paper. The figure is only a diagram, not a model. In an amorphous silica model the Si-O-Si bond angle may vary, but the Si-O distances are constant each oxygen ion is linked to not more than two cations the coordination number of oxygen ions about the control cation is 4 or less oxygen tetrahedra share comers, not edges or faces and at least two comers of each tetrahedron are shared.

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