Clay Minerals

All clay minerals have a similar chemical composition, a layered structure, and a great affinity for water. Some swell easily and may double in thickness when wet. They reversibly bind ions, particularly metal cations. These properties result from the structure of clay minerals.



Structure

All clay minerals consist of particles that contain anionic layered silicates and metal cations. They are part of the phyllosilicate group of minerals. Most have the "sandwich" structure with 2 layers of sheet silicates bonded to octahedral cations. Other, weakly bonded cations are located between layers and are solvated by water.

Clay particles in soil result from the physical and chemical weathering of silicate-containing rock.

There are 4 main classes of clay minerals.
  1. The Kaolinite Group consists of polymorphs of formula Al2Si2O5(OH)4. The repeat unit is a single silicate sheet condensed with alumina octahedra.

  2. The Montmorillonite/Smectite Group includes talc, vermiculite, montmorillonite and others. The general formula is (Ca,Na,H)(Al,Mg,Fe,Zn)2(Si, Al)4O10(OH)2-xH2O, where x represents a variable amount of water. These minerals all have the sandwich structure with tetrahedral silicate layers strongly bonded to octahedral aluminum or magnesium atoms.

  3. The Illite Group clays have a general formula (K,H)Al2(Si,Al)4O10(OH)2-xH2O, where x represents a variable amount of water. The structure of this group is similar to the montmorillonite group with a sandwich-type structure.

  4. The Chlorite Group also contains sandwich-type units. In many of these minerals, there is another weakly-attached, octahedrally coordinated Mg2+ between sandwiches. The general formula is X4-6Y4O10(OH, O)8. The X represents either aluminum, iron, lithium, magnesium, manganese, nickel, zinc or rarely chromium. The Y represents either aluminum, silicon, boron or iron but mostly aluminum and silicon.

The picture below shows the structure of the smectite-illite clays with the sandwich-type repeat unit.




The next picture shows the structure of kaolinite clays with a single silicate sheet bonded to an octahedral metal oxide layer in the repeat unit.





Ion Exchange

The space between the "sandwiches" contains water and weakly bound cations. The cations are there to balance the overall negative charge of the silicate-aluminate sandwich unit.

When the concentration of a particular cation is greater outside the clay particle than inside, that cation will diffuse into the particle and some other cation will diffuse out.

In this way, clay minerals pick up ions that are essential to plant growth when plants decompose at the end of the growing cycle and they slowly give up those nutrients to the growing plants during the growing season.



Particularly important to growing plants are K+, Ca2+, Mg2+. Plants also get smaller amounts of Fe2+, Mn2+, Cu+, Zn2+, B3+, Mo3+. Anions, such as nitrate and phosphate, can also be stored and exchanged in the inter-sandwich area.


Retention of Water

The same area that holds the weakly bound cations, salts, and polar molecules can also retain large amounts of water. Some clays can double their volume by increasing the water between the sandwich layers. This water is released slowly back into the soil around plant roots.




Professor Patricia Shapley, University of Illinois, 2010