Lecture 11 Clay Minerals

1
Lecture 11Clay 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
  soils 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 lt---gt 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.

Acid soil (neutral molecule)
Deprotonates
Raise the pH
Anion CEC site
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

• 2. Source 1
• 1) broken bonds at edge of clay mineral

5
Edge Effects are pH Dependent
6
(No Transcript)
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 -1 charge
8
Silicate Clay Minerals

• Aluminosilicates have a definite crystalline
  structure. Formed by the alteration of existing
  minerals or by synthesis from elements.
• lt .002 mm in size (only seen with aid of electron
  microscope)
• Two basic kinds of clay minerals 1 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)6-3 Repeat Unit Al2(OH)42

Eight Sided
11
Types of Clay Minerals

• 1) Kaolinite a 1 1 clay mineral - 1 silica
  sheet and 1 gibbsite sheet
• .000000072 cm thick

Si
Al
Properties 1) LARGE PARTICLES LOW CEC - 3-15
MEQ/100G 2) RIGIDLY FIXED- LOW SHRINK SWELL
12

11 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 Al3 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
14
(No Transcript)
15
kaolinite
illite
humus (fulvic acid)
mont- morillonite
16
2. Vermiculite

• Same as Illite with all the K ions removed

Properties 1) expanding - large shrink swell 2)
High CEC150meq/100g
17
3. Smectite or Montmorillonite

• Mg replaces Al in Octahedral layer
• Properties 1) expanding (common clay found in
  Minnesotas soils.
• 2) CEC 80-150 meq/100g

18
(No Transcript)
19
(No Transcript)
20
Go To Clay Minerals Web Page

• http//www.soils.wisc.edu/virtual_museum/

21
Dyad

• Where have you had contact with a clay ?

22
Sheets and Layers
11 non-expanding
21 non-expanding
21 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 0 20 Al3 10 Al3 2.5 Al3
Octahedral 0 0 15 Mg2 15 Mg2
Tetrahedral --- 20 Al3 10 Al3 2.5 Al3
CEC me/100g 3-15 (edges) 30 150 80 - 150
Shrink-swell Low None Mod to High High
Interlayer H-bonds Fixed K Exch. cations 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