1. Lecture 11 Clay Minerals
Clay and organic matter in the soil provide the negative absorptive sites or Cation Exchange Capacity (CEC)
Clay can hold positive cations from leaching
Soil CEC is the most important indicator of the soil’s ability to hold onto and retain + cations like Ca, Mg, K, NH4+,

2. Organic matter as a source of negative charge
1. Soil Humus - SOM
R-CO OH <---> R-CO-O- + H+
This charge is pH dependent or as the pH increases more OH- is available and therefore more CEC and conversely acid soils have a lower CEC from OM.
(neutral molecule)
Acid soil
Raise the pH
Deprotonates
CEC site
Anion
OH- or
O Ca++ or K+

3. Clay minerals as a source of negative charge- internal and external

4. Clay as a source of negative charge
1) broken bonds at edge of clay mineral

5. Edge Effects are pH Dependent

7. Source 2 for negative charges:
2) internal charge not satisfied because of ionic substitution -called Isomorphous Substitution
Al 3+ for Si 4+ or Mg2+ for Al 3+ or Zn2+ for Al3+
No substitution Zn++ for Al+++
Neutral charge

8. Silicate Clay Minerals
Aluminosilicates have a definite crystalline structure. Formed by the alteration of existing minerals or by synthesis from elements.
< .002 mm in size (only seen with aid of electron microscope)
Two basic kinds of clay minerals : 1 and 2 : 1

9. Basic Building Blocks for Clay Minerals
Silicon - Oxygen Tetrahedron
SiO4-4
Repeat Unit: (Si2O5)-2
Four sides

10. Basic Building Blocks - 2
Al(OH)3
Aluminum Octahedral (Gibbsite Sheet)
Al(OH) Repeat Unit: Al2(OH)4+2
Eight Sided

11. 1) Kaolinite a 1: 1 clay mineral - 1 silica sheet and 1 gibbsite sheet
Types of Clay Minerals
1) Kaolinite a 1: 1 clay mineral - 1 silica sheet and 1 gibbsite sheet
cm thick Si Al
Properties:
1) LARGE PARTICLES LOW CEC MEQ/100G
2) RIGIDLY FIXED- LOW SHRINK SWELL

12. {
1:1 layer
Basal O
Apical O, OH

13. 2 : 1 CLAY MINERALS 2-Silica tetrahedrons and 1 aluminum octahedral
1. Hydrous Mica or Illite
replacement of Si4+ with Al+3 in 25% of tetrahedrals, each substitution leaves 1 neg charge site also K+ ions are a bridge between layers and hold them tightly together
Properties
1) non-expanding
2) Low CEC 30 meq/100

15. kaolinite
illite
humus
(fulvic acid)
mont-
morillonite

16. 2. Vermiculite Properties 1) expanding - large shrink swell
Same as Illite with all the K ions removed
Properties
1) expanding - large
shrink swell
2) High CEC=150meq/100g

17. 3. Smectite or Montmorillonite
Mg++ replaces Al+++ in Octahedral layer
Properties: 1) expanding (common clay found in Minnesota’s soils.
2) CEC = meq/100g

20. Go To Clay Minerals Web Page

21. Dyad
Where have you had contact with a clay ?

22. Sheets and Layers kaolinite illite smectite and vermiculite
1:1 non-expanding
2:1 non-expanding
2:1 expanding
Al sheet
Si sheet + + + + + + +
Al sheet
Si sheet
Al sheet
Si sheet + + + + +
Al sheet + +
Si sheet
kaolinite
illite
smectite and
vermiculite

23. Clay Minerals Comparison
Kaolinite
Illite
Vermiculite
Smectite
Tetrahedral
20% Al3+
10% Al3+
2.5% Al3+
Octahedral
15% Mg2+
---
CEC me/100g
3-15 (edges) 30
150
Shrink-swell
Low
None
Mod to High
High
Interlayer
H-bonds
Fixed K+
Exch. cations
Origin
Recrystallization under intense acid weathering
Early alteration of micas
Intermediate alteration of micas
Recrystallization under moderate neutral to alkaline weath.

24. The End