Lecture 11 Clay Minerals

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Lecture 11Clay Minerals

• Clay and organic matter in the soil provide the
  negative absorptive sites or Cation Exchange
  Capacity (CEC)

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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
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Clay minerals as a source of negative charge
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Clay as a source of negative charge

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

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Edge Effects are pH Dependent
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Source 2

• 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
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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

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Basic Building Blocks for Clay Minerals

• Silicon - Oxygen Tetrahedron

• SiO4-4

• Repeat Unit (Si2O5)-2

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Basic Building Blocks - 2
Al(OH)3

• Aluminum Octahedral (Gibbsite Sheet)
• Al(OH)6-3 Repeat Unit Al2(OH)42

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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
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11 layer
Basal O
Apical O, OH
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2 1 CLAY MINERALS

• 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
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kaolinite
illite
humus (fulvic acid)
mont- morillonite
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2. Vermiculite

• Same as Illite with all the K ions removed

Properties 1) expanding - large shrink swell 2)
High CEC150meq/100g
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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

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Go To Clay Minerals Web Page

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

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Dyad

• Where have you had contact with a clay ?

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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
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Clay Minerals Comparison
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The End