1. Silicate Clays

2. Rocks breakdown (weather) by physical means
Primary Minerals
Primary minerals are dominated by Si, Al, O
Primary minerals breakdown (weather) principally by chemical means
Secondary Minerals
Silicate clays Iron oxides Aluminum oxides
Silicate clay minerals are dominated by Si, Al, O

3. Silicate Clay minerals are crystalline
and formed by specific arrangement
of ionic forms of Si, Al, and O
Ions are stable forms of elements
that possess an electrical charge.
Cations are elements that have lost
electrons to become (+) charged
Anions are elements that have gained
electrons to become (-) charged.

4. Building a Crystal
(with cations and anions)

5. Building Blocks for Silicate Clays
Silicon Tetrahedron
Aluminum octahedron
hydroxide (OH-)
Oxygen (O2-)
Aluminum (Al3+)
silicon
(Si4+)

6. Fundamental Building Blocks
Graphite
Diamond
Silicon Tetrahedron Aluminum Octahedron
Crystalline Minerals
Oxygen (O2-)
silicon
(Si4+)
Hydroxide (OH-)
Aluminum (Al3+)

7. 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

8. 2:1 mineral {
Tetrahedra {
octahedra {
Tetrahedra

10. Silicate Clay minerals

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

12. Positive Charge = Negative Charge
Silicon
Aluminum
Positive Charge
Al3+ Si4+
Negative Charge
O2- OH-
Oxygen
Hydroxide
Positive Charge = Negative Charge

13. Talc Pyrophillite Uncharged silicate clay minerals AlSi2O5OH.
Positive Charge
Al3+ Si4+
Talc
Pyrophillite
Negative Charge
O2- OH-
AlSi2O5OH. +3
2 x (+4) = +8
5 x (-2) = -10 -1
Total = zero

14. 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.

15. Tetrahedral Substitution
Al3+ for Si4+

16. Positive Charge = Negative Charge
Silicon
Aluminum
Positive Charge
Al3+ Si4+
Positive Charge
O2- OH-
Oxygen
Hydroxide
Positive Charge = Negative Charge

17. Octahedral Substitution
Mg2+ for Al3+

18. Positive Charge = Negative Charge
Silicon
Aluminum
Magnesium
Positive Charge
Al3+ Si4+ Mg2+
Positive Charge
O2- OH-
Oxygen
Hydroxide
Positive Charge = Negative Charge

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

20. Na+
Charge
Na+
Na+
Na+
Na+
Na+
Na+

21. K+
Cation Exchange K+
Na+ K+
Na+ K+ K+
Na+
Na+
Na+
Na+
Na+

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

23. Concentration Soil Solution K+ K+ K+ Na+ K+ Na+ K+ K+ K+ K+ K+ K+ K+

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

25. Size
Soil Solution
Na+
Cs+
Stronger bond
(Cs+ preferred)

26. 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

27. What is a centimole? 1 mol of charge = 6.02 x 1023 charges
A centimole is 1/100 of a mole
6,000,000,000,000,000,000,000 charges