Vertex-sharing Tetrahedra. Silicates

The linking of tetrahedra takes place predominantly by sharing vertices. Edge-sharing and especially face-sharing is considerably less frequent than among octahedra. 

Tetrahedra linked via three vertices correspond to a composition MX1 1X3 2 or MX2 5 = M2X5. Small units consisting of four tetrahedra are known in P4O10, but most important are the layer structures in the numerous sheet silicates and aluminosilicates with anions of the compositions and [AlSiOj-]. Because the terminal vertices of the single 

Some forms of rings and chains of vertex-sharing tetrahedra in silicates. How the chain conformations adapt to the size of the cation octahedra is shown for two chains (the octahedron chain is a section of a layer) 

Some arrangements of tetrahedra in sheet silicates. The lower image in each case represents a side view 

Dots mark the polyhedron vertices shared when the octahedron and the tetrahedron layers are linked. 

A chain of vertex-sharing tetrahedra results when every tetrahedron has two terminal and two bridging atoms the composition is MX274X2A2 or MX3. The chain can be closed to form a ring as in [SOj], [PO3 Ij, ISiO I3 or [SiOf jg. Endless chains have different shapes depending on the mutual conformation of the tetrahedra (Fig. 16.19). They occur especially among silicates, where the chain shape is also determined by the interactions 

From the German zwei = two, drei = three, zweier = two-membered, dreier = three-membered, Kette = chain. 

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Phosphate tetrahedra, P04 , can link up via oxygen-sharing to give polyphosphates in the form of chains and rings. The phosphates, unlike the silicates, contain a central atom with a valency of five. Thus, the maximum number of oxygen atoms that each tetrahedron can share is three, since there must always be one vertex of the tetrahedron that is a terminal oxygen, bound as P=0. 

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