• What are silicates? Formula....?
• Types & Classification of silicates with examples
• Practice questions

What are Silicates? Formula......?

Silicates are the minerals containing silicon and oxygen in tetrahedral SiO₄^4- units, which are linked together in several patterns. Depending on the way the tetrahedral units are linked, the silicates are classified into the following types.

TYPES & CLASSIFICATION OF SILICATES

• Ortho silicates (or Nesosilicates)
• Pyro silicate (or Sorosilicates)
• Cyclic silicates (or Ring silicates)
• Chain silicates (or pyroxenes)
• Double chain silicate (or amphiboles)
• Sheet or phyllosilicates
• Three dimensional (or tecto) silicates

1) ORTHO SILICATES

Ortho silicates (or Neso or Island silicates) are the simplest silicates which contain discrete SiO₄^4- tetrahedral units. The basic structural unit of ortho silicate unit is shown below.

The ortho silicate ion is the strong conjugate base of weak orthosilicic acid as well as it will not persist in aqueous solutions. Hence in nature, ortho silicate minerals are rare and only found with cations which form highly insoluble salts.

Examples of Ortho silicates:

 1) Phenacite (also known as phenakite) - Be₂SiO₄ 

2) Willemite - Zn₂SiO₄  - A minor silicate ore of zinc. Highly fluorescent (green) under shortwate UV.

Note: The Be²⁺ and Zn²⁺ ions are tetrahedrally surrounded by the oxygen atoms of silicate. 

 3) Olivine - (Fe/Mg)₂SiO₄ : Typically green in color. The cations are octahedrally coordinated to the oxygen atoms of the silicate. 

 4) Zircone - ZrSiO₄ : The oldest mineral on Earth.  The coordination number of Zr⁴⁺ is 8.

2) PYRO SILICATES

Pyro silicate (or Soro silicate or disilicate) contain Si₂O₇^6- ions which are formed by joining two tetrahedral SiO₄^4- which share one oxygen atom at one corner (one oxygen is removed while joining). Structure of pyrosilicate is shown below.

The pyrosilicate ion is less basic than orthosilicate ion. There only one mineral in nature containing pyrosilicate ion.

E.g.     1) Thortveitite - Sc₂Si₂O₇

3) CYCLIC SILICATES

Cyclic silicates 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. 

E.g.    

1) Benitoite - BaTi(SiO₃)₃ : containing three tetrahedra arranged cyclically [Si₃O₉)^6-]. 

2) Beryl - Be₃Al₂(SiO₃)₆ : containing six-silicate rings [Si₆O₁₈)^12-]. It is an aluminosilicate. Each aluminium is surrounded by 6 oxygen atoms octahedrally. Well-known varieties of beryl include emerald and aquamarine.

4) CHAIN SILICATES (PYROXENES)

Chain silicates or pyroxenes contain (SiO₃)_n^2n- ions which are formed by linking ‘n’ number of tetrahedral SiO₄^4- units linearly. Each unit shares two oxygen atoms with other units. 

 Examples of chain silicates:

 1) Spodumene - LiAl(SiO₃)₂  - a pyroxene mineral consisting of lithium aluminium inosilicate

 2) Diopsite - CaMg(SiO₃)₂ 

 3) Wollastonite - Ca₃(SiO₃)₃

  Note: The formula of cyclic silicates as well as chain silicates is (SiO₃)_n^2n-. Hence these are considered as an oligomers of the unknown SiO₃^2- ion.

5) DOUBLE CHAIN SILICATES (AMPHIBOLES)

The general formula of double chain silicates (or Amphiboles) is (Si₄O₁₁)_n^6n- . There are two types of tetrahedra: those sharing 3 vertices and those sharing only 2 vertices.

E.g.    

1) Asbestos - These are noncombustible fibrous silicates. They have been used for thermal insulation material, brake linings, construction material and filters. These are carcinogenic amphiboles affecting the lungs. Hence their applications are restricted nowadays. 

 2) Tremolite - Ca₂Mg₅(Si₄O₁₁)₂(OH)₂

 Note: The single and double chain silicates are collectively known as Inosilicates.

6) SHEET SILICATES (PHYLLO SILICATES)

The general formula of Sheet or Phyllo or two dimensional (2-D) 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. The layers are held together by weak van der Waal's forces.

E.g.    

1) Talc - Mg₃Si₄O₁₀(OH)₂ : It is the main ingredient of soap stone. It​ is the softest material with a smooth and greasy touch.

2) Micas : General formula is X₂Y_4–6Z₈O₂₀(OH,F)₄  

 Where   X = K, Na, or Ca 

             Y = Al, Mg, or Fe 

             Z = Si or Al 

 E.g. Muscovite mica - KAl₂(AlSi₃O₁₀)(F,OH)₂ - split into thin layers extremely easily

and Lepidolite - KLi₂Al(Al,Si)₃O₁₀(F,OH)₂ 

3) Clay : It is an aluminosilicate with sheet structure. 

4) Kaolinite - AlSiO₅(OH)₅

7) THREE DIMENSIONAL SILICATES (TECTO SILICATES)

The general formula of three dimensional (3-D) or tecto or Framework 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.,

  A FEW IMPORTANT POINTS ABOUT SILICATES

* Silicate minerals are very common in the Earth crust since Oxygen and Silicon are the most abundant elements.

 * The degree of polymerization is denoted by Oxygen to Silicon ratio (O/Si). Greater the degree of polymerization, lower will be the O/Si ratio. The values of O/Si for ortho silicates (lease polymerized) = 4:1 while for tecto silicates (most polymerized silicate) = 2:1.

* With increase in the degree of polymerization, there is decrease in the charge per silicon atom as well as the basicity of silicate mineral. Indeed, silica (SiO₂) is an acidic oxide.

* The basic silicate minerals readily react with weak acids and undergo weathering.

SILICATES - PRACTICE MCQ QUESTIONS & ANSWERS  CSIR NET GATE IIT JAM JEE NEET

35) Name of the structure of silicates in which three oxygen atoms of SiO₄^4- are shared is :

1) Pyrosilicates

2) Pyroxenes

3) Tectosilicates

4) Amphiboles

36) The type of silicate unit present in beryl silicate Be₃Al₂[Si₆O₁₈] is  :

1) sorosilicate

2) cyclosilicate with six units

3) sheet silicate

4) cyclosilicate with three units

37) How many oxygen atoms of SiO₄^4- ​units are shared in the continuous 3d frame work of silicates?

1) 2

2) 3

3) 4

4) 1

38) Beryl is a silicate. Its anion part contains [Si_nO_3n]^12-, then the value of n is :

1) 3

2) 6

3) 9

4) 18

39) Name the type of the structure of silicate in which one oxygen atom of SiO₄^4- is shared :

1) Nesosilicate

2) Sorosilicate

3) Phyllosilicate

4) Tectosilicate

40) Silica is a network solid with Si−O−Si bonds. In many aluminosilicates, a few Si atoms in the silica structure are replaced by Al atoms. Zeolites, clays, asbestos etc. are examples of aluminosilicates having different structures. The statement which is not correct is:             (NEST 2017)

(A) The basic structural unit in silicates is tetrahedral (SiO₄)⁴⁻.

(B) Replacement of a Si atom in silica by an Al atom results in negative charge on the Al atom.

(C) Silica dissolves in aqueous NaOH to form sodium silicate in which Na⁺ is bonded to oxyanions.

(D) Silicones are examples of silicates.

41) The most likely geometry for a silicate unit is:             (NEST 2012)

(A) square planar.

(B) square pyramidal.

(C) octahedral.

(D) tetrahedral

42) Egyptian blue CaCuSi₄O₁₀ is an example of:

A) sheet silicate

B) Cyclic silicate

C) Pyrosilicate

D) Chain silicate

43) The most used acid catalyst in oil industry and the relevant process are respectively: (CSIR NET JUNE 2012 Solved - AdiChemistry)

1) Aluminophosphate and reforming

2) Aluminosilicate and cracking

3) Aluminosilicate and reforming

4) Aluminophosphate and cracking.

44) The species that result by replacing one quarter of Si(IV) in pyrophyllite [Al2(OH)2Si4O10] with Al(III) Charge balance by K(I)] is  (CSIR NET JUNE 2019 Solved - AdiChemistry)

(1) Muscovite

(2) Phlogopite

(3) Montmorillonite

(4) Talc

Question-45) silicates with continuous 3d framework are known as:

(1) Tectosilicates

(2) amphiboles

(3) Pyroxenes

(4) Cyclic silicates

Answer: 1