Reactions of Aluminosilcates

Building Blocks for Silicate Clays Silicon Tetrahedron Aluminum octahedron hydroxide (OH-) oxygen Aluminum (Al3+) silicon (Si4+)

Tetrahedra and Octahedra Sharing the Oxygens Linkage of thousands of silica tetrahedra and aluminum octahedra O { Tetrahedra Si O, OH { octahedra Al OH 1:1 Mineral

2:1 mineral { Tetrahedra { octahedra { Tetrahedra

1:1 minerals 2:1 minerals

Charge Balance = Si4+ O2- Al3+ OH- Positive charge Negative charge Al3+ Si4+ OH- O2-

Isomorphous Substitution Substitution of lower-charge cations for higher charge cations during mineral formation. Al3+ for Si4+ in tetrahedra Mg2+ for Al3+ in octahedra The result is a deficit of positive charge or a surplus of negative charge in the mineral structure.

Tetrahedral Substitution Al3+ for Si4+

Octahedral Substitution Mg2+ for Al3+

Al3+ for Si4+ Mg2+ for Al3+ Tetrahedral Substitution Octahedral Substitution Mg2+ for Al3+

Na+ Charge Na+ Na+ Na+ Na+ Na+ Na+

- - - - - - - - - - - - - - - Charged Surface Dense swarm of cations High cation concentration Ambient solution concentration - - - - - - - - - - - - - - -

Which Cations are preferred? K+ Cation Exchange K+ Na+ K+ Na+ K+ K+ Na+ Na+ Na+ Na+ Na+ Which Cations are preferred?

Factors Determining Cation Preference Concentration Charge (+1, +2,+3) Size

Concentration Soil Solution K+ K+ K+ Na+ K+ Na+ K+ K+ K+ K+ K+ K+ K+

Charge Soil Solution Ca+2 Ca+2 Ca+2 Ca+2 Ca+2 K+ K+ K+ K+ Ca+2 K+ Ca+2

Size Soil Solution Na+ Stronger bond (H+ preferred) H+

Mineral Cation Exchange Capacity The total quantity of cations a clay can adsorb. Related directly to the amount of Isomorphous substitution Equal to the amount of charge Units are cmolc/kg soil Range: 0 - 180 cmolc/kg

What is a Centimole? 1 cmol = 6,020,000,000,000,000,000,000 1/100 of a mole 1mole = 6.02 x 1023 charges 1cmol = 6.02 x 1021 charges 1 cmol = 6,020,000,000,000,000,000,000

Measuring Cation Exchange Capacity

- - - - - - - - - - - - - - - - - - - - - - Very High Concentration NH4+ NH4+ - - - - - - - - - - - - - - - - - - - - - - Na+ Mg2+ H+ NH4+ K+ Ca2+ Al3+ Mg+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

NH4+ K+ Ca2+ Na+ Mg2+ Soil NH4+ K+ Ca2+ Na+ Mg2+ beaker

Ba+2 Ba2+ NH4+ beaker The number of ammonium ions = number of charges Soil Ba2+ NH4+ NH4+ beaker The number of ammonium ions = number of charges

A clay’s negative sites are saturated With NH4+ and high concentrations of Ba2+ are used to displace them The displaced cations are collected in a beaker and the number of NH4+ cations is determined. The number of NH4+ cations = 2,000,000 How many exchange sites were on the clay?

A clay’s negative sites are saturated with Ca+2, and then high concentrations of Ba2+ are used to displace them The displaced cations are collected in a beaker and the number of Ca+2 cations is determined. The number of Ca+2 cations = 4,000,000 How many exchange sites were on the clay?

Ba+2 Ba2+ NH4+ beaker The number of ammonium ions = number of charges Soil Ba2+ NH4+ NH4+ beaker The number of ammonium ions = number of charges

1Kg of clay whose negative sites are saturated with NH4+. High concentrations of Ba2+ are used to displace them The displaced cations are collected in a Beaker. The volume of solution in the beaker is 1L. The concentration of NH4+ cations in the beaker is equal to 10 cmol/L 10 cmol NH4+ 1 Liter X 1 L of solution = 10 cmol NH4+ 10 cmolc/kg clay = CEC

Important Clay Minerals

Na Cl Hydration Hydration sphere size varies widely + - H O O H H H O

H O + H O Na H O H O

Hydroxide (OH-) O - “Electron greedy” H +

Kaolinite 1:1 Na+ CEC = 2 – 5 cmolc O - kg Limited isomorphous substitution in octahedra (Al3+ for Si4+ ) CEC = 2 – 5 cmolc kg O H + - Layers are H-bonded Non-expansible Adsorption is on external surfaces and edges Slightly Negative

2:1 Minerals Smectites: montmorillonite CEC = 80-120 cmolc kg Significant substitution in the octahedra (Al3+ for Si4+) Ca2+ Mg2+ Na+ CEC = 80-120 cmolc kg Cations satisfying charge Layers weakly held together by cations Highly expansible

Smectites Abundant water O Ca Ca Ca H H

Smectites Limited water O Ca Ca Ca H H

Vermiculite CEC = 100-180 cmolc kg Mg2+ Mg2+ Significant substitution in tetrahedra CEC = 100-180 cmolc kg Mg2+ Mg2+ Source of negative charge Is very close to the adsorbed cations Layers tightly bound Moderately expansible

Illite CEC = 20-40 cmolc kg K+ K+ Significant substitution in tetrahedra Source of negative charge Is very close to the adsorbed Cations Potassium fits into cavities on Clay surfaces clamping them shut. K+ K+ Non-expansible CEC = 20-40 cmolc kg

Tetrahedra Side View Octahedra Tetrahedra Tetrahedra Top View K+ K Clay Layer

{ Minerals Kaolinite Smectite Vermiculite illite Mineral Expansion CEC (cmol/kg) 1:1 None 2 – 5 2:1 High 80 - 120 2:1 Limited 100 – 160 2:1 None 20-40 { tetrahedral substitution Octahedral substitution tetrahedral substitution

K Clay Layer

Where do the minerals occur? Illites Vermiculites Smectites Kaolinite Fe, Al oxides Increased weathering 2:1 1:1