Chapter 4
B.sc 1st year Book
(Page 11)
SILICATES
Silicates can be prepared by fusing an alkali or alkaline earth metal carbonate and others with silica.
Soluble silicates:
Soluble silicates Na2SiO3,Na2Si2O5,Na4SiO4,Na6Si2O7,etc. are soluble silicates. They are used in the preparation of liquid detergents to keep the pH high, so that fat grease can be dissolved by forming a soap. Sodium silicates are also used as adhesives.
Insoluble silicates:
They are in general naturally occurring. Zeolite is one of the synthetic insoluble silicates which is used as an ion exchanger.
Both soluble and insoluble silicates are structurally similar.
Structure of Silicates:
In the crystal lattice, silicon exists in +4 and oxygen exists in −2 oxidation states. The radius ratio in Si IN :O∥ is 0.29 which suggests that the structure of silicates is based on four co-ordination silicon, i.e. tetrahedral [SiO4]4 in which four oxygen atoms are arranged at the corners of the tetrahedron with the silicon atom at the center. Thus, in silicates, the basic structural unit is [SiO4]4 in which the Si−O bond distance is 1.62 A∘ and O−O bond distance is 2.70 A0. All silicates comprise [SiO4]4− units which are formed by sp 3 hybridization of Si orbitals. [SiO4]4 units condense together by sharing one or more oxygen atoms, thus giving different types of silicates. Silicon in [SiO4]4 units can be replaced by A(III) to give alumino-silicates due to their similar size.
The electronegativity of oxygen is 3.5 and that of silicon is 1.8. The electronegativity difference of 1.7 suggests that Si−O bonds are covalent in nature and it would appreciable degree of ionic character.
Classification of Silicates:
Depending upon the linkage of [SiO4]4 tetrahedral units, silicates may be classified into the following types :
1. Orthosilicates:
Discrete [SOO4] units occur in the orthosilicates M2H2SiO4 (where M=Be, Mg, Mn, Fe, and Zn)and in zircon, ZrSiO. In which [SiO4]4 tetrahedral units do not share comers with one another. Some minerals containing discrete orthosilicate ions are – phenacite Be2SiO4, Willemite Zn2SiO4, Zircon ZrSiO4, and olivine (Mg1Fe2SiO4. Both Na4SiO4 and K4 SO4 have also been shown to be orthosilicates, Another important group of orthosilicates is games. The large class of garnets is composed of minerals of the general formula
2. Pyrosilicates or Disilicate anions:
Pyrosilicates contain the discrete [Si2O7]0− units. The [Si2O7]6 – units are formed by joining two tetrahedral units through one oxygen atom. It may be noted that any oxygen atom which does not form a bridge has to pick up an electron from some metal to complete its octet and thus acquire a negative charge, In the disilicate ion. [Si2O7]6− – there are six oxygen atoms that do not form bridges carrying 6 negative charges: as shown in figure 2. The metal atoms from which the electrons are picked up by the oxyg= atoms are present in the silicate structure as cations. The number of cations is such that they balance the negative charge on the silicate anion.
Thortveitite, Sc2Si2O7, and hemimorphite Zn4(OH)2Si2O7H2O are two examples of phyllosilicates. In Sc2Si2O7, the six negative charges on Si2O7G−anion are balanced by the same number of positive charges on two Sc3+. It has octahedral Still and a linear Si-0-Si bond between staggered tetrahedra. There is also a series of lanthanoid disilicates LnSi2O7 in which the Si−O−Si angle decreases progressively from 1800 to 133∘ and the coordination of Ln increases from 6 through 7 to 8 as the size of Ln increases from 6- coordinated Lull to 8-coordinated Nd III. Barysilito, MnPb9(Si2O7)3 as well as vesuvianite and epidote which contain both SiO44− and Si2O76-ions.
3. Cyclic silicates:
They have cyclic structures of general formula (SiO3)2n−, occurring in several metasilicates such as benitoite, BaTiSi3O9, cataplexy, Na2ZrSi3O9⋅H2O, dioptase, Cu6Si6O18⋅6H2O and beryl, Be3Al2 S6O18. These are formed by sharing two oxygens per silicon atom resulting in the formation of a cyclic structure as shown in figure 4 .10 shown below. The primary unit, in this case, is (SiO3)2−. The cyclic ion, (Si3O9)6− shown in figure 4.10(a) occurs in wollastonite, Ca3Si3O2, and bentonite, Bali Si3O9 while the cyclic ion, (Si1O16)12 – shown in figure 4.10( b) occurs in beryl, Be3Al2Si6O18. (Silicates)
4. Single and double Chain metasilicate Ions (SiO3Power2)infinity or Pyroxenes:
Infinite chains of formula, (SiO3)n 2n-are found in minerals called ‘pyroxenes’.
Examples of pyroxenes include enstatite, MgSiO3, diopside, CaMg(SiO3)2,Na2SiO3; Ca3Si3O2,Ba2Si2O5 (Mn.Ca) Si5O15,(Fe,Ca)7Si7O21, Pb12Si12O36, etc. They have chain or ribbon structures as shown in the figure.. formed by corner-sharing of SiO4 tetrahedra units. In these chains, the silicon atom shares two of the four tetrahedrally coordinated oxygen atoms with adjacent atoms. If further sharing of oxygen atoms occurs by half of the silicon atoms, a double chain or band structure is formed. This type of structure is found in amphiboles. Amphiboles contain the Si4O116 – repeating unit as well as metal and hydroxide ions. For example, crocidolite, Na2Fe5(OH)2[Si4O11]2 also known as blue asbestos, and amosite, (MgFe)7(OH)2[Si4O1l2, grey-brown asbestos. A linkage by the complete sharing of three oxygen atoms per silicon results in the layer or sheet structure. The general formula of such type of metasilicate is [Si2O5]2−.
5. Silicates with the 3-dimensional network:
When all the oxygen atoms of [SiO4]4− tetrahedron are shared with the neighboring [SlO4]4 – tetrahedron, a network is obtained in which every oxygen atom is common to two tetrahedral SiO4 groups. Thus, the whole network has the composition SiO2. This type of composition is found in:
(a) Quartz:
In this type of silica, the regular arrangement is somewhat distorted so that spirals of – O−Si−O−Si−O – chains lie around the trigonal screen axes of symmetry.
(b) Feldspar, Zeolite and Ultramarine:
If in the framework of [SinO2n]0 one or more silicon atoms are replaced by Al atoms, the framework of feldspar, zeolite, and ultramarine is obtained. By this replacement of Si atoms with Al atoms negative charge is developed in the new framework which is balanced by introducing a cation of appropriate size. (Silicates)
(i) Feldspars:
The feldspar, of the general formula MAl2−xSi2+OB, are the most important rock-forming minerals, comprising some 2/3rd of igneous rocks, such as granite, which is a mixture of quartz, feldspar, and micas. The feldspars are divided into two groups according to the symmetry of their structures and the difference between the ionic radii of their cations.
Ortho-clase feldspars. These are monoclinic in symmetry, examples are – orthoclase, KAlSi3O8, and celsian, CaA2Si2O8
Paige-cease feldspars: These are triclinic in symmetry, examples are – Albite, NaAlSi 3O8, and Anorthite, Ba-Al Si2O8 –
(ii) Zeolite:
The zeolites are aluminosilicate framework minerals of the general formula. In fibrous zeolites, the chain is made up of regular r of a unit of five tetrahedra so that the formula of all these fibrous zeolites contains the unit.
(SiAl) O10 or it multiple. Some examples of fibrous zeolites are heulandite and NaCa2Al5Si5O206H2O. They have a much more open structure than the feldspars and contain in them the cavities into which the gases for example CO2.NH3 and liquids for example water, ethyl alcohol etc., can enter and can be absorbed there. The cations can also diffuse into the cavities and this property of zeolites enables them to undergo cation exchange reaction. For example, sodium zeolite such as analcite on treatment with s solution of AgNO3, the sodium is replaced by silver.
Na[AlSi2O3]⋅H2O+AgNO3=Ag[AlSl2O6]⋅H2O+NaNO3
These reactions are reversible and have been used for the separation of ions and in the softening of water. The zeolite framework behaves in some ways like a clathrate cage about a guest molecule. They are characterized by open structures that permit the exchange of cations and water molecules. Both natural and synthetic zeolites find wide applications as cation exchangers since the ions can migrate rather freely through the open structure. Zeolites may also behave as acidic catalysts. The acidity may be of the Bronsted type if hydrogen ions are exchanged for mobile cations(such as Na+) by washing with acids. If zeolite is heated, water may then be eliminated from the Bronsted sites leaving aluminum atoms coordinated to only three oxygen atoms. These will act as Lewis acids. The catalytic sites occur at high density and are uniform in their activity because of the microcrystalline nature of the zeolites.(Silicates)
(iii) Ultramarines:
These are synthetic colored or colorless sodium alumina silicates. In addition to the Framework [(SIA) )nOin ] and balancing cations, these also contain the anion such as S2, Cl−, SO4 2-. For example ultramarine, Nag[Al6Si6O24]S, sodalite, Nag[Al8Si6O24Cl2 and noselite, Nag[Al6Sl6O24)SO4. The ultramarine is used as a blue pigment. It is also used to discharge yellowish Inge of cotton, linen, sugar, etc.