Silicates

Every silicate mineral — however chemically complex it may appear — is built from just one repeating structural unit: the $\text{SiO}_4^{4-}$ tetrahedron, in which a central silicon atom is $sp^3$ hybridised and covalently bonded to four oxygen atoms occupying the corners of a regular tetrahedron. The enormous structural diversity of natural and synthetic silicates arises entirely from the different ways in which neighbouring $\text{SiO}_4$ tetrahedra can share their corner oxygen atoms with one another.

If the tetrahedra share no corners at all — each $\text{SiO}_4^{4-}$ unit standing isolated, balanced by surrounding metal cations — the result is an orthosilicate (e.g. the mineral olivine). If exactly one corner oxygen is shared between two tetrahedra, a discrete two-tetrahedron unit, $\text{Si}_2\text{O}_7^{6-}$, results — a pyrosilicate. If each tetrahedron shares two of its corners with its neighbours, the tetrahedra link up into an infinite one-dimensional chain, and this is the structural basis of the chain (pyroxene-type) silicates. Starting from the basic unit $\text{SiO}_4^{4-}$ and removing one shared, bridging oxygen for every silicon (since each bridging oxygen is now counted as belonging to two silicons rather than one), the repeat unit of this infinite chain works out to $\text{SiO}_3^{2-}$ per silicon — so an $n$-silicon chain segment has the overall formula $(\text{SiO}_3)_n^{2n-}$, exactly the general chain-silicate formula. If each tetrahedron instead shares three of its four corners, the linkage extends in two dimensions into a sheet silicate (e.g. the minerals mica and talc, whose characteristic easy cleavage into thin flakes reflects the weaker bonding between sheets compared with the strong covalent bonding within each sheet). Finally, if every one of the four corners is shared, the tetrahedra build up into a fully three-dimensional framework silicate; because every oxygen atom is now shared between two silicons, such a framework carries no net charge at all (quartz, $\text{SiO}_2$, is the simplest example), and no additional metal cations are required to balance charge — although, as the next section shows, replacing some of the framework silicon atoms with aluminium reintroduces a net negative c …