Silicates

Silicates may be regarded as the metal derivatives of the silicic acid, H4SiO4. Silicates can be prepared by fusing metal oxides or metal carbonates with sand (SiO2). For example,

Na2CO3 Na2SiO4.(Na2SiO3)n etc.

Only some silicates of alkali metals are soluble in water while others are insoluble. The insolubility is due to the great strength of Si − O bond. When silicates are treated with HF, Si − O bond is broken.

Classification and structure of silicates: X−ray diffraction studies of various types of silicates have shown that all the silicates have units which are formed by sp3 hybridisation of Si atom. Si atom in its excited state contains 4 unpaired electrons and hence forms 4 covalent bonds with four negatively charged O−atoms, resulting in the formation of anion. Each O−atom becomes negatively charged by picking up an electron from some metal.

Since Si atom undergoes sp3 hybridisation, anion has tetrahedral shape as shown in fig (a), (b) and (c).

Various ways of showing the tetrahedral shape of anion

It is evident from the above discussion that all silicates contain the tetrahedral anions. These anions differ from one another in the manner in which these anions are linked together.

Silicates are classified according to the nature of linking between the tetrahedral anions. This basis of classification gives the following types of silicates.

(i) Orthosilicates: These silicates contain discrete tetrahedral anions. The O−atoms of each anion is also coordinated to the metal ion to impart electrical neutrality of the structure. Examples of such silicates are phenacite Be2SiO4 willemite Zn2SiO4, olivine Mg2SiO4, zircon ZrSiO4, garnets (where M2+ = Mg2+ Ca2+ Fe2+ etc and M3+ = Al3+, Cr3+, Fe3+ etc). In garnets, SiO4 tetrahedra are arranged about M2+ and M3+ ions so they become 8 and 6 coordianted respectively. Garnets are very hard and find extensive uses as industrial abrasives.

(ii) Pyrosilicates: These silicates contain the discrete anion which is formed by joining two SiO4 tetrahedral units through one oxygen atom as shown in figure.

Structure of ion found in pyrosilicates

Examples of silicates containing anion are thorrteveitite, Sc2(Si2O7) and hemimorphite, Zn3(Si2O7).Zn(OH)2.H2O.

(iii) Cyclic or ring silicates: Such silicates contain the cyclic or ring anions like or . The structure of these anions is given in figure.  These anions are obtained when each SiO4 tetrahedral unit shares an O−atom with each of the two neighbouring SiO4 units.

Structure of (a) (b) anions found in cyclic silicates

Examples of cyclic silicates containinganion are benitoite BaTiSi3O9, wollastonite Ca3Si3O9, catapleite Na2ZrSi3O9.2H2O while the example containing anion is beryl Be3Al2Si6O18.

(iv) Chain silicates: Such silicates contain the anions which are formed by the sharing of two oxygen atoms by each tetrahedron. The anions may be one of the types

(a) (b)

The chains in the silicates containing anions lie parallel to each other and the cations lie between the chains and bind them together. Such silicates are presented by the pyroxene minerals and several synthetic silicates. Examples are

Structure of (a) (b) anions found in chain silicates

(a) synthetic silicates: e.g. Li2SiO3, Na2SiO3

(b) pyroxene minerals: e.g. spodimene LiAl(SiO3)2, Jadeite NaAl(SiO3)2, enstatite MgSiO3, diopside CaMg(SiO3)2.

The silicates containing (Si4O11)6n− anions have double chains in which the simple cations are held together by shared oxygens. Such silicates are represented by the amphibole minerals which include most asbestos minerals. Structurally amphiboles are similar to pyroxenes though they contain some OH groups which are attached to the cations. Tremolite, Ca2Mg5(Si4O11)2(OH)2 is an example of amphibole.