Silicate Minerals, Structure and Properties

Silicate Mineral Structure

The Silicate minerals group is of great importance because they constitute about 90% of the Earth’s crust. They are found in all the common rocks except limestone. To understand the differences between major silicate mineral groups, it is necessary to study their structure. Every silicate mineral contains oxygen and silicon, and all except quartz, contain one or more additional elements to complete their structure.

The basic unit in all silicate minerals is the “silicon-oxygen tetrahedron”. This structure is composed of four oxygen atoms with the silicon atom at its centre. These tetrahedra can occur in the silicate structures either as single units or joined into chains, sheets, and three-dimensional networks by sharing oxygen atoms. Depending upon the type of structure built by these tetrahedra, the slicates are classified into the following groups:

1. Nesosilicates. Nesosilicates include those minerals which are built up from isolated SiO4 tetrahedra. The atomic packing of the neosilicate structure is generally dense, which causes the minerals of this to have relatively high specific gravity and hardness. The crystal habit of these minerals is generally equidimensional, and they have poor cleavage. Olivine, Zircon, and Gernets are examples of this class.
2. Sorosilicate. Sorosilicates are characterised by a linked pair of SiO4 tetrahedra. In this structure, only one oxygen is shared, giving the ratio of Si : O =2:7. Hemimorphite [Zn4 (Si2O7) (OH)2 H2O] is an example of this class.
3. Cyclosilicate. Cyclosilicate is also known as the ring silicate. It contains rings of linked SiO4 tetrahedra having a ratio of Si:O = 1:3. These rings may consist of groups of three, four or six linked tetrahedra. Cyclosilicates from extremely strong minerals, such as beryl and tourmaline.
4. Inosilicate (Chain Silicate). In this group, SiO4 tetrahedra are linked by sharing oxygens to form straight chains of indefinite length. These chains may be single chains or a double-linked channel. In the single chain structure, two of the four oxygens in each SiO4 tetrahedron are shared, giving a ratio of Si:O = 1:3. In the double chain structure, half of the tetrahedra share three oxygens, while the other half share two oxygens yielding a ratio of Si:0 =4:11. Inosilicates split easily in one crystal direction because bonds within chains are strong but are weaker between them. These minerals commonly form needle-like crystals, such as asbestos. Pyroxenes are examples of single-chain minerals, and amphiboles are examples of double-chain minerals.
5. Phylosilicates (Sheet Silicates). The phylosilicates form sheet structures in which there is the continuous linking of hexagonal groups of silica tetrahedra. In this structure, three of the four oxygens in each SiO4 tetrahedron are shared with neighbouring tetrahedra, giving a ratio of Si:O =2:5. As the atomic bonding perpendicular to the sheet structure is generally weak, these minerals split easily into thin sheets. Flaky minerals, such as micas, chlorite and kaolinite, are examples of this class.
6. Tectosilicates. They are also known as the framework silicates. In tectosilicates, SiO4 tetrahedra are linked in a three-dimensional framework. All oxygens in each SiO4 tetrahedron are shared with neighbouring tetrahedra. This results in a strongly bonded structure in which the ratio Si:O is 1:2. The minerals belonging to the tectosilicate group possess uniform properties throughout. Quartz are felspars are the examples of this class.

Table showing Silicate Mineral Groups and Examples.

List of Silicate Minerals