GROUP 14 (CARBON FAMILY)
Silicates are the minerals containing silicon and oxygen in tetrahedral SiO44- units which are linked together in several patterns. Depending on the way the tetrahedral units are linked, the silicates are classified into the following types.
Ortho silicates (or Neso silicates) are the simplest silicates which contain discrete SiO44- tetrahedral units.
E.g. 1) Phenacite - Be2SiO4
2) Willemite - Zn2SiO4
Note: The Be and Zn ions are tetrahedrally surrounded by the oxygen atoms of silicate.
3) Olivine - (Fe/Mg)2SiO4 : The cations are octahedrally coordinated to the oxygen atoms of the silicate.
4) Zircone - ZrSiO4 : The coordination number of Zr4+ is 8.
Pyro silicates (or Soro silicates) contain Si2O76- ions which are formed by joining two tetrahedral SiO44- which share one oxygen atom at one corner (one oxygen is removed while joining).
E.g. 1) Thortveitite - Sc2Si2O7
Cyclic silicates contain (SiO3)n2n- ions which are formed by linking three or more tetrahedral SiO44- units cyclically. Each unit shares two oxygen atoms with other units.
E.g. 1) Benitoite - BaTi(SiO3)3 : contain three tetrahedra arranged cyclically.
2) Beryl - Be3Al2(SiO3)6 : contain six-silicate rings. It is an aluminosilicate. Each aluminium is surrounded by 6 oxygen atoms octahedrally.
Chain silicates or pyroxenes contain (SiO3)n2n- ions which are formed by linking ‘n’ number of tetrahedral SiO44- units linearly. Each unit shares two oxygen atoms with other units.
E.g. 1) Spodumene - LiAl(SiO3)2
2) Diopsite - CaMg(SiO3)2
3) Wollastonite - Ca3(SiO3)3
The general formula of double chain silicates (or Amphiboles) is (Si4O11)n6n- . There are two types of tetrahedra: those sharing 3 vertices and those sharing only 2 vertices.
E.g. 1) Asbestos - These are noncombustible fibrous silicates. They have been used for thermal insulation material, brake linings, construction material and filters. These are carcinogenic amphiboles affecting the lungs. Hence their applications are restricted nowadays.
2) Tremolite - Ca2Mg5(Si4O11)2(OH)2
Note: The single and double chain silicates are collectively known as Inosilicates.
The general formula of Sheet or Phyllo silicates is (Si2O5)n2n- . Each SiO4 tetrahedron shares three oxygen atoms with others and thus by forming two-dimensional sheets. These silicates can be cleaved easily just like graphite.
E.g. 1) Talc - Mg3Si4O10(OH)2 : It is the main ingredient of soap stone.
2) Micas : General formula is X2Y4–6Z8O20(OH,F)4
Where X = K, Na, or Ca
Y = Al, Mg, or Fe
Z = Si or Al
e.g. Muscovite mica - KAl2(AlSi3O10)(F,OH)2
Lepidolite - KLi2Al(Al,Si)3O10(F,OH)2
3) Clay : It is an aluminosilicate with sheet structure.
4) Kaolinite - AlSiO5(OH)5
The general formula of three dimensional or tecto silicates is (SiO2)n . All the oxygen atoms of SiO4 are shared with other tetrahedra and thus by forming three-dimensional network.
E.g. SiO2 - Quartz, Tridymite and Crystobalite - These are the crystalline forms of silica.
When SiO44- units are replaced by AlO45- units, three dimensional aluminosilicates are formed.
E.g. Feldspar, Zeolites, Ult etc.,