1. Reactions of Aluminosilcates

3. Building Blocks for Silicate Clays
Silicon Tetrahedron
Aluminum octahedron
hydroxide (OH-)
oxygen
Aluminum (Al3+)
silicon
(Si4+)

4. Tetrahedra and Octahedra Sharing the Oxygens
Linkage of thousands of silica tetrahedra and
aluminum octahedra O {
Tetrahedra Si
O, OH {
octahedra Al OH
1:1 Mineral

5. 2:1 mineral {
Tetrahedra {
octahedra {
Tetrahedra

7. 1:1 minerals
2:1 minerals

8. Charge Balance = Si4+ O2- Al3+ OH- Positive charge Negative charge
Al3+ Si4+
OH- O2-

9. Isomorphous Substitution
Substitution of lower-charge cations for
higher charge cations during mineral formation.
Al3+ for Si4+ in tetrahedra
Mg2+ for Al3+ in octahedra
The result is a deficit of positive charge or a
surplus of negative charge in the mineral structure.

10. Tetrahedral Substitution
Al3+ for Si4+

11. Octahedral Substitution
Mg2+ for Al3+

12. Al3+ for Si4+ Mg2+ for Al3+ Tetrahedral Substitution
Octahedral Substitution
Mg2+ for Al3+

13. Na+
Charge
Na+
Na+
Na+
Na+
Na+
Na+

14. - - - - - - - - - - - - - - - Charged Surface Dense swarm of cations
High cation concentration
Ambient solution concentration

15. Which Cations are preferred?K+
Cation Exchange K+
Na+ K+
Na+ K+ K+
Na+
Na+
Na+
Na+
Na+
Which Cations are preferred?

16. Factors Determining Cation Preference
Concentration
Charge (+1, +2,+3)
Size

17. Concentration Soil Solution K+ K+ K+ Na+ K+ Na+ K+ K+ K+ K+ K+ K+ K+

18. Charge Soil Solution Ca+2 Ca+2 Ca+2 Ca+2 Ca+2 K+ K+ K+ K+ Ca+2 K+ Ca+2

19. Size
Soil Solution
Na+
Stronger bond
(H+ preferred) H+

20. Mineral Cation Exchange Capacity
The total quantity of cations a
clay can adsorb.
Related directly to the amount of
Isomorphous substitution
Equal to the amount of charge
Units are cmolc/kg soil
Range: cmolc/kg

21. What is a Centimole? 1 cmol = 6,020,000,000,000,000,000,000
1/100 of a mole 1mole = 6.02 x 1023 charges
1cmol = 6.02 x 1021 charges
1 cmol =
6,020,000,000,000,000,000,000

22. Measuring Cation Exchange Capacity

23. - - - - - - - - - - - - - - - - - - - - - -
Very High Concentration
NH4+
NH4+
Na+
Mg2+ H+
NH4+ K+
Ca2+
Al3+
Mg+

24. NH4+ K+
Ca2+
Na+
Mg2+
Soil
NH4+ K+
Ca2+
Na+
Mg2+
beaker

25. Ba+2 Ba2+ NH4+ beaker The number of ammonium ions = number of charges
Soil
Ba2+
NH4+
NH4+
beaker
The number of ammonium ions = number of charges

26. A clay’s negative sites are saturated
With NH4+ and high concentrations of
Ba2+ are used to displace them
The displaced cations are collected in a
beaker and the number of NH4+ cations
is determined.
The number of NH4+ cations = 2,000,000
How many exchange sites were on the clay?

27. A clay’s negative sites are saturated
with Ca+2, and then high concentrations of
Ba2+ are used to displace them
The displaced cations are collected in a
beaker and the number of Ca+2 cations
is determined.
The number of Ca+2 cations = 4,000,000
How many exchange sites were on the clay?

28. Ba+2 Ba2+ NH4+ beaker The number of ammonium ions = number of charges
Soil
Ba2+
NH4+
NH4+
beaker
The number of ammonium ions = number of charges

29. 1Kg of clay whose negative sites are
saturated with NH4+. High concentrations
of Ba2+ are used to displace them
The displaced cations are collected in a
Beaker.
The volume of solution in the beaker is 1L.
The concentration of NH4+ cations in the
beaker is equal to 10 cmol/L
10 cmol NH4+
1 Liter
X 1 L of solution = 10 cmol NH4+
10 cmolc/kg clay = CEC

30. Important Clay Minerals

31. Na Cl Hydration Hydration sphere size varies widely + - H O O H H H O

32. H O + H O Na H O H O

33. Hydroxide (OH-) O -
“Electron greedy” H +

34. Kaolinite 1:1 Na+ CEC = 2 – 5 cmolc O - kg
Limited isomorphous substitution
in octahedra (Al3+ for Si4+ )
CEC = 2 – 5 cmolc kg O H + -
Layers are H-bonded
Non-expansible
Adsorption is on external surfaces
and edges
Slightly
Negative

35. 2:1 Minerals Smectites: montmorillonite CEC = 80-120 cmolc kg
Significant substitution in the octahedra
(Al3+ for Si4+)
Ca Mg Na+
CEC = cmolc kg
Cations satisfying charge
Layers weakly held together by cations
Highly expansible

36. Smectites
Abundant water O Ca Ca Ca H H

37. Smectites
Limited water O Ca Ca Ca H H

38. Vermiculite CEC = 100-180 cmolc kg Mg2+ Mg2+
Significant substitution in tetrahedra
CEC = cmolc kg
Mg2+
Mg2+
Source of negative charge
Is very close to the adsorbed
cations
Layers tightly bound
Moderately expansible

39. Illite CEC = 20-40 cmolc kg K+ K+
Significant substitution in tetrahedra
Source of negative charge
Is very close to the adsorbed
Cations
Potassium fits into cavities on
Clay surfaces clamping them shut. K+ K+
Non-expansible
CEC = cmolc kg

40. Tetrahedra Side View Octahedra Tetrahedra Tetrahedra Top View K+ K
Clay Layer

41. { Minerals Kaolinite Smectite Vermiculite illite Mineral Expansion
CEC (cmol/kg)
1:1
None
2 – 5
2:1
High
2:1
Limited
100 – 160
2:1
None
20-40 {
tetrahedral
substitution
Octahedral
substitution
tetrahedral
substitution

43. K
Clay Layer

44. Where do the minerals occur?
Illites
Vermiculites
Smectites
Kaolinite
Fe, Al oxides
Increased
weathering
2:1
1:1