2. INTRODUCTION
A Silicate is an anions consisting of silicon and oxygen.
General formula [SiO4-x
(4-2x)- ]n .
Silicates occur in earth’s crust in abundantly in the form of silicate minerals
and aluminosilicate clay.
Silicate anions are often large polymeric molecules with an extense variety
of structures, including chains and rings, double chains and sheets.
Silicates are extremely important materials, both natural and artificial, for all
sorts of technological and artistic activities.
3. STRUCTURE OF SILICATE
Silicate structure consists of (SiO4)4- units which are formed by sp3 hybridisation of Si
orbitals.
The electron configuration of Silicon atom in the excited state is 3s1 p1
x p1
y p1
z thus it forms
four bonds with oxygen atoms by utilizing sp3 hybrid orbits and yielding (SiO4)4- units.
The oxygen atoms pickup one electron each from some metal in order to complete their
octets.
4. PROPERTIES OF SILICATES
Most of the silicates are insoluble in water.
Sodium silicate, Na2SiO3, is the only common silicate which is soluble in
water.
The electronegativity of oxygen is 3.5 and that of silicon is 1.9 . The
electronegativity difference of 1.6 suggests that Si-O bond, though covalent
,has an appreciable degree of ionic character.
6. ORTHO SILICATES
These simplest silicates which contain discrete (SiO4)4- units.
Example; 1) Phenacite - Be2SiO4
2) Willemite - Zn2SiO4
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.
7. PYRO SILICATES
These contains disilicate anions , (Si2O7)6- units.
These (Si2O7)6- units are formed by joining two tetrahedral (SiO4)4- units through
one oxygen atom.
Example; Thortveitite - Sc2Si2O7
Hemimorphite- [Zn4(OH)2Si2O7]
8. CHAIN SILICATES
Chain silicates or pyroxenes contain (SiO3)n
2- ions which are formed by linking ‘n’ number of tetrahedral
SiO4
4- units linearly. Each unit shares two oxygen atoms with other units.
Examples;
1) Spodumene - LiAl(SiO3)2
2) Diopside - CaMg(SiO3)2
3) Enstatite - MgSiO3
9. DOUBLE CHAIN SILICATES
The general formula of double chain silicates (or Amphiboles) is (Si4O11)n
6- . There are two types of
tetrahedra: those sharing 3 vertices and those sharing only 2 vertices.
EXAMPLES;
1) Tremolite - Ca2Mg5(Si4O11)2(OH)2
2)Amosite – ( MgFe)7Si8O22(OH)2.
3)Crocidolite
10. CYCLIC SILICATES
These contain (SiO3)n
2n- ions which are formed by linking three or more tetrahedral SiO4
4- units cyclically.
Each unit shares two oxygen atoms with other units.
Examples; 1) Bentonite - 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.
11. SHEET SILICATES
The general formula of Sheet or Phyllo
silicates is (Si2O5)n
2n- . 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.
Examples; 1)Kaolinite - AlSiO5(OH)4
2) Talc – Mg3Si4O16(OH)3
12. THREE DIMENSIONAL SILICATES
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 SiO4
4- units are replaced by AlO4
5- units, three
dimensional aluminosilicates are formed.
E.g. Feldspar, Zeolites, Ult etc.,
![INTRODUCTION
A Silicate is an anions consisting of silicon and oxygen.
General formula [SiO4-x
(4-2x)- ]n .
Silicates occur in earth’s crust in abundantly in the form of silicate minerals
and aluminosilicate clay.
Silicate anions are often large polymeric molecules with an extense variety
of structures, including chains and rings, double chains and sheets.
Silicates are extremely important materials, both natural and artificial, for all
sorts of technological and artistic activities.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)