CLAY MINERALS

SSC 051

FERTILIZERS AND SOIL FERTILITY

 

Clay minerals are crystals formed from soluble products of primary minerals. The kind of clay formed depends on the proportion of different ions in solution and the weathering environment.

Most clays are crystalline with a definite repeating arrangement of atoms. they are made up of layers of oxygen atoms bonded together by silicon or aluminum.

The basic building blocks of clay minerals are shown below.

 

 

 

Kaolinite

1:1 type clay

Characteristics

  1. Composed of layers each of which consists of 1 silica and 1 alumina sheet.
  2. CEC of approximatley 10 meq/100g. The source of the charge is due to ionizationof the Al-OH or Si-OH portion of the clay. Loss of the H leaves a temporary negative charge ( Al-O- or Si-O- ). The extent of ionized H depends on the pH. More ionization occurs in more basic solutions.
  3. Nonexpanding - Low swelling due to the bonding of oxygen and hydrogen between the sheets. Used for pottery, tile and bricks.
  4. Relatively low surface area (7-30 m2/g) and large in size compared to other clays.
  5. The dominant clay in humid, warm, well drained, soils such as the Southeastern U.S. Much of the silica is removed from the soil due to leaching. Soils are high in aluminum and have a low pH.

 

 

Montmorillo-

nite (2:1 type clay)

Characteristics

  1. A 2:1 clay mineral is composed of layers each of which consists of 1 alumina and 2 silica sheets.
  2. CEC of approximately 100 meq/100g. The high charge is due to isomorphic substitution or the substitution of one ion for another of similar size but lower positive valence. Si4+ and Al3+ cations may be replaced during clay formation by Fe2+, Fe3+, Mg2+ or Zn2+ resulting in a net increase in negative charge. More internal surface area between layers is available for cation exchange.
  3. Expanding - high shrink swell capacitybecause waster can go between layers.
  4. High surface area (600-800 m2/g)
  5. Found in soils that have had little or no leaching such as soils of arid regions, poorly drained soils, soils developed from alkaline parent material.
  6. Used to seal ponds for well-drilling muds, and for thickeners in paints and lipstick. Very plastic and cohesive.

 

 

 

 

 

 

 

 

 

 

Illite

2:1,non-

expandable

Characteristcs

  1. A 2:1 structure similar to montmorillonite.
  2. CEC of approximately 30 meq/100g
  3. Nonexpanding - Interlayer K+ and isomorphic substitution of Al3+ for Si4+ in the tetrahedral layer forms a strong bond between the three layer units.
  4. Medium surface area (65-120 m2/g).
  5. Formed from weathering of micas

 

Vermiculite

2:1 type clay

Characteristics

  1. Similar in structure to illite, but K has been lost due to weathering. The layers are held more weakly by hydrated magnesium.
  2. CEC of approximately 150 meq/100g.
  3. More isomorphic substitution of Al3+ for Si4+ than illite results in the higher CEC.
  4. Limited expansion.
  5. Forms from weathering of micas. More weathered than illite.