Silicate Clays

Rocks breakdown (weather) by physical means Primary Minerals Primary minerals are dominated by Si, Al, O Primary minerals breakdown (weather) principally by chemical means Secondary Minerals Silicate clays Iron oxides Aluminum oxides Silicate clay minerals are dominated by Si, Al, O

Silicate Clay minerals are crystalline and formed by specific arrangement of ionic forms of Si, Al, and O Ions are stable forms of elements that possess an electrical charge. Cations are elements that have lost electrons to become (+) charged Anions are elements that have gained electrons to become (-) charged.

Building a Crystal (with cations and anions)

Building Blocks for Silicate Clays Silicon Tetrahedron Aluminum octahedron hydroxide (OH-) Oxygen (O2-) Aluminum (Al3+) silicon (Si4+)

Fundamental Building Blocks Graphite Diamond Silicon Tetrahedron Aluminum Octahedron Crystalline Minerals Oxygen (O2-) silicon (Si4+) Hydroxide (OH-) Aluminum (Al3+)

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

Silicate Clay minerals

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

Positive Charge = Negative Charge Silicon Aluminum Positive Charge Al3+ Si4+ Negative Charge O2- OH- Oxygen Hydroxide Positive Charge = Negative Charge

Talc Pyrophillite Uncharged silicate clay minerals AlSi2O5OH. Positive Charge Al3+ Si4+ Talc Pyrophillite Negative Charge O2- OH- AlSi2O5OH. +3 2 x (+4) = +8 5 x (-2) = -10 -1 Total = zero

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+

Positive Charge = Negative Charge Silicon Aluminum Positive Charge Al3+ Si4+ Positive Charge O2- OH- Oxygen Hydroxide Positive Charge = Negative Charge

Octahedral Substitution Mg2+ for Al3+

Positive Charge = Negative Charge Silicon Aluminum Magnesium Positive Charge Al3+ Si4+ Mg2+ Positive Charge O2- OH- Oxygen Hydroxide Positive Charge = Negative Charge

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

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

K+ Cation Exchange K+ Na+ K+ Na+ K+ K+ Na+ Na+ Na+ Na+ Na+

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+ Ca+2 K+ K+ Ca+2 K+

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

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 mol of charge = 6.02 x 1023 charges A centimole is 1/100 of a mole 6,000,000,000,000,000,000,000 charges