Clay is a particle size

Clay is a particle SIZE

• Clay is a particle SIZE

• Predominant make-up is SECONDARY minerals

• Example: SiO2

• crystalline QUARTZ (SiO2) : resistant to weathering

• Amorphous silica (SiO2) : 10x more soluble

Silicate Clays (crystalline)

• Silicate Clays (crystalline)

• Sesquioxide/oxidic clays

• Amorphous clays (non-crystalline)

Micelle: particle of silicate clay

• Micelle: particle of silicate clay

• Composed of tetrahedral and octahedral “sandwiches”

• Tetrahedron: central cation (Si+4, Al+3) surrounded by 4 oxygens

• Octahedron: central cation (Al+3,Fe+2, Mg+2) surrounded by 6 oxygens (or hydroxyls)

Connected tetrahedra, sharing oxygens

• Connected tetrahedra, sharing oxygens

Connected octahedra, sharing oxygens or hydroxyls

• Connected octahedra, sharing oxygens or hydroxyls

1000s of tetrahedra and octahedra connect in clay minerals to give:

• 1000s of tetrahedra and octahedra connect in clay minerals to give:

• Planes of Si, Al, Mg
• Planes of Oxygen, hydroxyl groups

• Sheets combine to form layers

• Layers are separated by interlayer space

• Water, adsorbed cations

Lower charge cations replace higher charge cations as central cation

• Lower charge cations replace higher charge cations as central cation

• E.g., Mg+2 replaces Al+3

• leaves net negative charge

Different types of silicate clays are composed of sandwiches (combinations) of layers with various substances in their interlayer space.

• Different types of silicate clays are composed of sandwiches (combinations) of layers with various substances in their interlayer space.

• 2:1 two tetrahedral sheets to one octahedral sheet

• 1:1 one tetrahedron sheet to one octahedral sheet

1:1

• 1:1

• Hydrogen bonds in interlayer space

• strong

• Nonexpandable

• Low CEC

• Particles can grow very large (0.2 – 2 µm)

• Effective surface area = 10 – 30 m2/g

• External surface only

Kaolinite

• Kaolinite

• good road base

• good foundation

• good for pottery; China clay (porcelain)

• easy to cultivate, but need manure or fertilizer

• Dominant clay mineral in highly weathered soils

2:1

• 2:1

• Weathering product

• Always negative due to isomorphous substitution

• Layers weakly held together by weak O-O bonds or cation-O bonds

• Cations adsorbed in interlayer space

• Expandable

• High CEC

Interlayer cations hold layers together

• Interlayer cations hold layers together

• In dry soils, bonding force is strong and hard clods form; deep cracks

• In wet soils, water is drawn into interlayer space and clay swells.

• Montmorillonite

• Vertisols

• Dominant clay mineral of most MN soils

High effective surface area = 650 – 800 m2/g

• High effective surface area = 650 – 800 m2/g

• Internal surface area >> external

• Particles small

• Most expandable of all clays

• Click here, Pat

2:1

• 2:1

• As mica crystallizes from magma:

• Isomorphous substitution of Al+3 for Si+4 in tetrahedra
• high net negative charge

• K+ ions in interlayer space

• Strongly binds layers

• Non-expandable

• Illite

• Surface area 70 -175 m2/g

2:1

• 2:1

• Forms from alteration of mica

• Weathering removes some K+ ions
• Replaced by hydrated cations in interlayer space

• Water molecules and cations bridge layers, so not as expandable as smectites

Still have very high net negative charge

• Still have very high net negative charge

• High CEC (highest of all clays)

• Expandable

• Octahedral ions are Al, Mg, Fe

• Surface area 600 – 800 m2/g

• Internal >> external

2:1

• 2:1

• Central cations in octahedral sheets is Fe or Mg

• Interlayer space occupied by a stable, positively charged octahedral sheet

• Non-expandable

• 70 -100 m2/g surface area

Ultimate weathering products

• Ultimate weathering products

• Ultisols and Oxisols

• Very stable; persist indefinitely

• Yellow, red, brown

• Fe or Al as central cations

• Lack negative charge

• Don’t retain adsorbed cations

• Non-expandable

• Low CEC

Often are net positive

• Often are net positive

• Often have enough Al or Mn to be toxic to plants

• High capacity to fix phosphorous so it is not available to plants

• Highly weathered so no more nutrients to release in weathering

In heavily leached soils, sheets decompose into component Si tet. and Al oct.

• In heavily leached soils, sheets decompose into component Si tet. and Al oct.

• Al oct. often weather into gibbsite Al(OH)3

silicates

• silicates

• Allophane and imogolite

• Common in volcanic ash

• High internal negative charge

• High CEC

• High water-holding capacity

• Surface area 100 – 1000 m2/g

refers to cation exchange capacity (CEC)

• refers to cation exchange capacity (CEC)

• Ability to retain and supply nutrients
• Fertility

• High activity clays:

• Less weathered ; high effective surface area
• smectite, vermiculite, mica (illite), chlorite

• Low activity clays:

• More weathered; less effective surface area
• kaolinite

• Example:

• “Fine-loamy, mixed, superactive, frigid Typic Hapludalfs”

Usually a mixture

• Usually a mixture

• Climate

• Parent material

• Degree of weathering