Phyllosilicate Clay Minerals

Structure and PropertiesPresented by Leah Brueggeman

Common Experiences with Clays

– Slippery–Poorly drained–Crack when dry–Hard to dig–Moldable

Soil fractions

Clay Characteristics

• Small particle size (clay size fraction) less than 002mm (2 microns)

• Large surface area (range from 10m2/g to 800m2/g)

• Carry a negative charge

Phyllosilicate Structure

– Phyllon – meaning leaf, Silic meaning flint– Basic building blocks are

Silicate Tetrahedron Aluminum Octahedron

Basal oxygens

Apical oxygen

Clay Structure

Basal oxygens

Apical oxygens

Makes one layer

Two Layer Types• 1:1 one tetrahedral:

one octahedral• 2:1 two tetrahedral:

one octahedral

Apical oxygen

Apical oxygen

Basal oxygen

Molecular and Structural

Origin

Fluid and Rock Interaction

Alteration Or Decomposition

Weathering is a continual Process

Isomorphic Substitution

• Ions substitution in the basic mineral structure– Al3+ for Si4+ in Tetrahedral layer– Fe2+ , Fe3+, Mn2+ , Mg2+ for Al3+ in Octahedral layer

Results in charge imbalance (permanent charge)

Isomorphic “same shape” refers to the substitution of one ion for another without changing the morphology or structure of the mineral

Isomorphic Substitution

4+

4+

3+

3+

Clay Types

• The type and amount of substitution creates clay minerals with different properties

• Properties affected include stickiness, plasticity, swelling, and cation exchange capacity

• 2:1 clay minerals fine grained micas, smectites and vermiculites,chlorites

• 1:1 clay mineralskaolinite

Smectite Vs. Kaolinite

Kaolinite• Layers are electrically neutral because there is little cation substition in the structure

Layers are held together by hydrogen bonds (Non Expansive)

Charges are unsatisfied only on broken edges and surface(pH dependent)

H+bonds

Smectite

Cation Exchange Capacity

1:1 Kaolinite Clay

2:1 Smectite clayAt least as 20x greater CECNon pH dependent because structural

pH Dependent AECLow CEC No Interlayer

Role in Importance to Ag

• Nutrient Retention – Many plant nutrients are cations (Ca, Mg, K,Na,H)

• Water Holding Capacity: opposite charged end of the polar water molecule attracted to the internal and external surfaces

• CEC– Soils with high CEC hold and retain important

plant nutrients

Other actions of clay in soil

• Retention of contaminants• Sorption of metals• Soil Structure

Swelling Clay in ND

Hydrated Interlayer

• Shrinks and swells

• Can expand up to 30%

• Most Surface Area

Sodic Soils in ND

DispersionIncreasing Na+ in solution

Na+

Na+Na+

Need twice as many Na+ than Ca2+

Ca2+

Ca2+

Ca2+

Slippery, Swells, Shrinks, Hard

BUT

Holds Water, Retains NutrientsStructure, Reduces Contaminants