Tetrahedral shape of SiO44– anion
The silicates containing SiO44– units are called orthosilicates.
Silicates are the metal derivatives of silicic acid (H4SiO4). The basic building unit of all silicates is the SiO44– anion.
In SiO44– ion , Si is sp3 hybridised, and it forms four covalent bonds with four negatively charged oxygen atoms. The SiO44– anion has a tetrahedral shape. Silicate structures are less close–packed than other oxide structures. Silicon atoms (Si) in SiO44– can be replaced by aluminium (Al), to obtain alumino–silicates.
Structure of Pyrosilicate
The Si2O76– unit, which exists in pyrosilicates.
Classification of Silicates Depending upon the number of corners (0, 1, 2, 3 or 4) of SiO44– tetrahedron shared with other tetrahedra, the silicates can be divided into the following classes.
Orthosilicates
In simple orthosilicates, the SiO44– tetrahedra do not share any corner with one another. These silicates therefore, contain discrete SiO44– tetrahedral anion. The O–atoms of each SiO44– anion are coordinated to the metal ion. Typical example is olivine, Mg2SiO4.
Pyrosilicates
Pyrosilicates are obtained when two SiO44– tetrahedra share one corner. These silicates contain Si2O76– anion, which is formed by the combination of two SiO44– units through one oxygen atom. This type of structure is known as island structure.
Structure of Cyclicsilicate
The SiO32– unit present in cyclic silicates.
Cyclic, or ring silicates
Such silicates contain the cyclic anions, Si3O96– or Si6O1812–, which are formed when two oxygen atoms of each SiO44– tetrahedron are shared with other SiO44– tetrahedra.
Beryl, Be3Al2Si6O18 is a typical mineral having ring anions. Example of cyclic silicate containing anion is Benitoite, BaTiSi3O9.
Chain silicates
Such silicates contain the linear chain of anions which is formed by sharing two oxygen atom of each SiO44– tetrahedra with others. It contains (SiO32−)n type of anion . The chains of the chain silicates lie parallel to each other, and the cations lie, in between these chains. Typical examples are, synthetic silicates, e.g. Li2SiO3 and Na2SiO3; pyroxene minerals, e.g. CaMg(SiO3)2. Asbestos also contains chain anions. If two chains are cross–linked, the resulting double–stranded silicates contain the anion [(Si4O11)6–]n, and are called amphiboles. Asbestos belongs to this class of silicates.
Si3O96–, or Si6O1812– anions, which exist in cyclic or ring silicates.
Structure of Three dimensional silicate
Sheet silicates
Two–dimensional silicates
Two–dimensional structure of sheet silicates contains Si2O52– units.
These silicates are formed when 3 corners of each SiO44– tetrahedron are shared with others. These two–dimensional sheet structures contain (Si2O52–)n anion. The minerals containing (Si2O52–)n anions are soft and can be cleaved easily. Such sheet–like anions are found in mica and different types of clay minerals. Examples include Talc, Mg2 (Si2O5)2, Mg(OH)2, Kaolin, Al2(OH)4(Si2O5).
Three–dimensional silicates
When all the four oxygen atoms of SiO44– tetrahedron are shared with others, then an infinite, three–dimensional structure is formed leading to different forms of silica. Such structures are found in quartz, tridymite, and crystobalite.