Silicates

The silicate minerals

In the silicates, the mineral is built in the form of tetrahedra, each composed of one silicon atom and four oxygen atoms, as in the left of the diagram below. This is the simplest arrangement that can exist in a stable form, and is the one found in what we know as nesosilicates (e.g. olivine).

From this simple arrangement, however, a range of different structures can be built,. through the simple process of sharing an oxygen ion between two tetraheda. By sharing a single oxygen, for example, a double tetrahedra can be created to produce what are known as sorsilicates, of which epidote is an example. Joining the sorosilicate structure, again via shared oxygens, can produce inosilicates, comprising chains of tetrahedra: either a single chain (an example of which pyroxene) or a double chain (e..g. amphibole). Double up on these and we can produce sheets of tetrahedra - a phyllosilicate, exemplified by the mica minerals. Thus six groups of silicates are formed (Figure below).

Beyond these, however there are more silicate structures that respond to a different construction logic. Cycloslicates are built of rings of tetrahedra, in a variety of structures. Tourmaline is one example. Even more complex structures, which are too complex to illustrate clearly like the other forms. These are displayed by the tectosilicates, which are built of a 3-dimensional framework of linked tetrahedra, in which each silica is linked to four shared oxygens (Figure below). This important group includes quartz and the feldspars.

Nesosilicate (isolated tetrahedra) and sorosilicate (double tetrahedraO

Inosilicate (single chain silicate), tectosilicate (double chain silicate) and phyllosilicate (sheet silicate)

Nesosilicates
These comprise the structurally simplest silicate minerals and include olivine, garnet, zircon and the aluminium silicates such as andalusite and kyanite. The minerals tend to form as crystals of equidimensional habit, without clear cleavage, and have high specific gravity (i.e. density) and hardness due to the dense packing of the atoms within the structure. Substitution of the silica in the minerals (e.g. by aluminium) is rare.

Sorosilicates
These represent a more complex structure than the nesosilicates, with a framework of double tetrahedra, comprising seven oxygens for every silica atom (one oxygen is shared). They display varied crystal habits and moderate cleavage an typically form in situations of low to medium-grade metamorphism. There are few common minerals in this group, except for epidote and zoisite.

Inosilicates
Inosilicates are chain silicates, comprising either a single or a double chain of tetrahedra. Single chain inosilicates include the pyroxenes (e.g. augite, diopside, enstatite), while double chains are characterised by the amphibole minerals (e.g. hornblendes in various forms) and. Both have relatively high melting points so form in hot (and deep) igneous environments, and early in the cooling process - the single chains first and then the double chains). Many are therefore prone to weathering when exposed at the surface.

Phyllosilicates
The phyllosilicates are so called because they comprise sheets of silicate tetrahedra within which oxygens are multiply shared to give a fairly stable structure, but between which the bods are relatively weak. Because of this they produce minerals with a flaky or tabular structure, and with properties that differ substantially between the faces and the edges of the sheets. Micas are the most common constituent minerals, but they also include chlorite and serpentine.

Tectosilicates
Tectosilicates are frame silicates, which comprise a three-dimensional structure of tetrahedra in which each tetrahedron is bonded to four others, with an oxygen shared at every corner. They thus contain two oxygens for every one silicon. They are especially important geologically because they account for about 75% of the material in the Earth's crust. They have a low melting point which means that they tend to crystallise late in the cooling process, and they characteristically of low-moderate density, hard and resistant to weathering. As well as quartz and feldspar, they include the zeolites.