Silicon–oxygen tetrahedron

Silicate tetrahedron.

A silicon–oxygen tetrahedron is the SiO₄ anionic group, or a silicon atom with four surrounding oxygen atoms arranged to define the corners of a tetrahedron. This is a fundamental component of most silicates in the Earth's crust. A variety of silicate minerals can be identified by the way that the tetrahedra links differ, and also by the cations present in the mineral.

Silicon–oxygen tetrahedra links

There are five common silicon–oxygen tetrahedra links:

1. Independent tetrahedra
2. Single chains
3. Double chains
4. Sheet silicates
5. Framework silicates

Independent tetrahedra

In this group, known as the nesosilicates or orthosilicates, the tetrahedra do not share any oxygen atoms and are independent. Attraction between the tetrahedra and positive ions holds minerals of this structure together. The positive ions could be metal cations such as iron or magnesium. A common mineral in this group is olivine.

Single chains

Single chain tetrahedra.

In single-chain silicates, which are a type of inosilicate, tetrahedra link to form a chain by sharing two oxygen atoms each. A common mineral in this group is pyroxene.

Double chains

Double chain tetrahedra.

Double-chain silicates, the other category of inosilicates, occur when tetrahedra form a double chain (not always but mostly) by sharing two or three oxygen atoms each. Common minerals for this group are amphiboles.

Sheet silicates

Sheet silicates.

In this group, known as phyllosilicates, tetrahedra all share three oxygen atoms each and in turn link to form two-dimensional sheets. This structure does lead to minerals in this group having one strong cleavage plane. Micas fall into this group. Both muscovite and biotite have very weak layers that can be peeled off in sheets.

Framework silicates

In a framework silicate, known as a tectosilicate, each tetrahedron shares all 4 oxygen atoms with its neighbours, forming a 3D structure. Quartz and feldspars are in this group.

See also

• Silicate minerals