Silicates - Sheetal Sequeira

Silicates - Sheetal Sequeira

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  SHEETAL SEQUEIRA 198227
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  INTRODUCTION  A Silicateis 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.
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  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.
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  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.
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  DIFFERENT TYPES OFSILICATES  ORTHO SILICATES.  PYRO SILICATES.  CHAIN SILICATES.  CYCLIC SILICATES.  SHEET SILICATES.  THREE-DIMENSIONAL SILICATES.
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  ORTHO SILICATES  Thesesimplest 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.
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  PYRO SILICATES  Thesecontains 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]
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  CHAIN SILICATES  Chainsilicates 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
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  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
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  CYCLIC SILICATES  Thesecontain (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.
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  SHEET SILICATES  Thegeneral 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
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  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.,