Silicate and zeolite structures

Silicon has a very high affinity for oxygen which explains the existence of the large number of silicate minerals. The silicate structures are best discussed in terms of the simple [SiO₄]^4- unit. The [SiO₄]^4- unit has tetrahedral coordination of silicon by oxygen and is nicely represented by a small tetrahedron (Si atom at the centre and O atoms at the vertices).

In most silicates the [SiO₄]^4- tetrahedra link by sharing vertices, but occasionally share edges or faces. Each terminal oxygen atom adds –1 to the charge of the unit, but each shared oxygen contributes 0.

Through sharing one or more oxygen atoms the tetrahedra are able to link and form chains, rings and layers.The negative charges on the silicate framework are balanced out by metal cations within the lattice.

The next page shows a structural classification of some mineral silicates. (fig. 19)