Tetrahedra sharing vertices only

Because Si04 tetrahedra can share any number of vertices from none, in the orthosilicate ion (Si04) to four, in Si02, the extensive oxygen chemistry of silicon provides examples of all types of tetrahedral structures. Their variety is 

(a) and (b) Porlions of infinite chains in which some tetrahedra share tw o and others three vertices, (c) Portion of A3X7 (melilite) layer in which some tetrahedra share three and 

In the structures we have considered above a shared vertex is common to two tetrahedra only. One structure in which each shared vertex is common to three tetrahedra is that of AlOCl (and the isostructural GaOCl) the layer is illustrated in Fig. 10.16 (p. 408). 

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