The Sharing of Polyhedron Corners, Edges, and Faces

The decrease in stability arises from the cation-cation Coulomb terms. The sharing of an edge between two regular tetrahedra brings the cations at their centers to a distance from each other only 0.58 times that obtaining when the tetrahedra share a corner only and the sharing of a face decreases this distance to 0.33 times that for a shared comer (Fig. 13-21). The corresponding 

In agreement with this rule, it is observed that silicon tetrahedra tend to share only corners with other silicon tetrahedra or other poly-hedra. No crystal is known in which two silicon tetrahedra Bhare an edge or a face, and in most of the silicate structures only corners are shared between silicate tetrahedra and other polyhedra also. This rule 

—The sharing of a comer, an edge, and a face by a pair of tetrahedra and by a pair of octahedra. 

This rule, like the others mentioned in this chapter, oan be used as a criterion for the largely ionic character of the bonds in a substance. The rule is obeyed by all of the forms of silica, in which the bonds have as much ionic character as covalent character. In SiSs, on the 

TS These values are for undistorted polyhedra. Some compensating distortion always occurs, as discussed in a later paragraph. 

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