Silicates

A large number of silicate minerals exist in nature. Some of the important minerals are : feldspar i.e. Albite

NaAlSi₃O₈, zeolites e.g. chabazite Ca₂[(AlO₂)₄ (SiO₂)₈.H₂O, micas [KAl₂(Si₃AlO₁₀(OH)₂] and asbestos [Mg₃(Si₂O₅)(OH)₄].

The basic structural unit in silicate is the SiO₄^4- terahedra.

The two important man-made silicates are glass and cement.

Silicates are the compounds in which the anion present are either discrete SiO₄^4-  tetrahedra or a number of such units joined together through corners.

Types Of silicates

Silicates are classified into the following different types depending upon the number of corners of SiO₄^4-  tetrahedron shared with other tetrahedra through oxygen atoms. The negative charge on silicate structure is neutralized by positively charged metal ion.

1) Orthosilicates : These are simple silicates containing discrete SiO₄^4- tetrahedra i.e. there is no sharing of corners with the other.

For examples : Zircon (ZrSiO₄) , Forestrite (Mg₂SiO₄) , wellimite (Zn₂SiO₄) , Phenacite (Be₂SiO₄).

2) Pyrosilicates : When two SiO₄^4- tetrahedra share one corner, Si2O7 6-  anion is formed. Silicates containing this anion are called pyrosilicates. The structures possessed by them are also called island structures.

For example : Thortveitite Sc₂(Si₂O₇) , Hemimorphite Zn₃(Si₂O₇)Zn(OH)₂·H₂O

3) Cyclic or ring silicates : If two oxygen atoms per tetrahedra are shred to form closed rings such that the structure with the general formula (SiO₃^2-)_n  or (SiO3)_n^2n- are obtained, the silicate containing these anions are called cyclic silicates.

For example: Si₃O₉^6- , Si₆O₁₈^12- , Wollastonite (Ca₃Si₃O₉) , Beryl (Be₃Al₂Si₆O₁₈)

4) Chain silicates : If two oxygen atoms per tetrahedron are shred such that a linear single strand chain of the general formula (SiO₃^2-)_n or (SiO₃)_n^2- is formed, then the silicates containing these anions are called chain silicates.

For example:  Spodumene ( LiAl(SiO₃)₂) , Diopside (CaMg(SiO₃)₂)

If two chains are cross linked, the resulting double-stranded silicates having the formula [(Si₄O₁₁)_n^6-] are called amphiboles.

For ex: Tremolite ( Ca₂Mg₅(Si₄O₁₁)(OH)₂) , asbestos (CaMg₃O(Si₄O₁₁)

5) Sheet silicates: The sharing of three corners results in an infinite two dimensional sheet structure of the formula (Si₂O₅)_n^2n- or (Si₂O₅^2-)_n

Silicates containing these anions are called sheet silicates.

6) Three dimensional silicates:  If all the four corners are shared with other tetrahedra,  three-dimensional structure network is obtained.

For example: quartz, tridymite and cristobalite

Zeolites

If some of the Silicon atoms in a three-dimensional network silicates are replaced by Al³⁺ ions, the overall structure thus obtained carries a negative charge and is called and aluminosilicates. Therefore to balance the negative charge ,some cations such as Na⁺ , K⁺ and Ca²⁺ are incorporated into structure.

Such three dimensional aluminosilicates are called feldspar (KalSi₃O₈) and zeolites(NaAlSi₂O₆·H₂O).

Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerisation.

For example: ZSM-5 is used to convert alcohols directly into gasoline. Hydrated zeolites called permutit are used as ion exchangers in softening of hard water.