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 soil’s ability to hold onto and retain + cations like Ca, Mg, K, NH4+,

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.

OH- orO- Ca++ or K+

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

Clay as a source of negative charge

2. Source 1:1) broken bonds at

edge of clay mineral

Edge Effects are pH Dependent

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

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 : 1 and 2 : 1

Basic Building Blocks for Clay Minerals

Silicon - Oxygen Tetrahedron

Repeat Unit: (Si2O5)-2

SiO4-4

Four sides

Basic Building Blocks - 2

Aluminum Octahedral (Gibbsite Sheet) Al(OH)6

-3 Repeat Unit: Al2(OH)4+2

Al(OH)3

Eight Sided

Types of Clay Minerals

1) Kaolinite a 1: 1 clay mineral - 1 silica sheet and 1 gibbsite sheet

.000000072 cm thick

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

Si

Al

Basal O

Apical O, OH

{1:1 layer

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

Properties1) non-expanding2) Low CEC 30 meq/100

kaolinite illite

mont-morillonite

humus (fulvic acid)

2. Vermiculite

Same as Illite with all the K ions removed

Properties1) expanding - largeshrink swell2) High CEC=150meq/100g

3. Smectite or Montmorillonite Mg++ replaces Al+++ in Octahedral

layer Properties: 1) expanding (common clay

found in Minnesota’s soils. 2) CEC = 80-150 meq/100g

Go To Clay Minerals Web Page http://www.soils.wisc.edu/virtual_museum/

Dyad

Where have you had contact with a clay ?

Sheets and Layers

Al sheet

Al sheet

Al sheet

Al sheetSi sheet

kaolinite

Si sheet

Si sheet

Si sheet

illite smectite and vermiculite

1:1 non-expanding 2:1 non-expanding 2:1 expanding

+ +

+

+

++

+

+

+

+

++

+ +

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

OriginRecrystallization under intense acid weathering

Early alteration of micas

Intermediate alteration of micas

Recrystallization under moderate neutral to alkaline weath.