Clay Minerals and CEC: Clay Minerals Are Secondary Minerals

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Clay Minerals Are Secondary Minerals

These are divided into four groups

1. type mineral

2. type mineral

  • Expanding
  • Non-expanding

3. type minerals


  • Consists of 1 layer of silica tetrahedral + 1 alumina octahedral
  • Example- kaolinite, anauxite, hallosite, dickite


  • Two silica tetrahedral and one alumina octahedral
  • Example- montmorillonite, vermicullite, saponite


  • No. of silica and alumina sheets is same as expanding but layer within it.
  • Example- illite, hydrous mica
  • Here, 15 % silicone in silica sheets is substituted by al


  • It can be written as 2: 1: 1 that is si: ag: mg
  • Example- chlorite
  • It consists of layer of two silica tetrahedral and 1 alumina octahedral and 1 mg octahedral


  • An organic colloid consisting of various chains and loops of linked carbon atoms.
  • Fulvic acid- it has low molecular weight. It is part of humus soluble in acid and alkali.
  • Humic acid- it is fraction of humus which is soluble in alkali but not in acid. It has medium molecular weight.
  • Humin- it is insoluble in both alkali and acid. It has high molecular weight.

Ion Exchange

Reversible process by which cations and anions are exchanged between solid and liquid phases and between solid phases in close contact with each other.

Power of Replacement

Cation exchange capacity (CEC) :

Number of exchangeable cations per unit weight of dry soil. Its unit is Cmol (+) kg-1 or miliequivalent of cations per 100gm (me (+) /100g) . It is one of the most important chemical property of soil.

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