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Silicate Clays
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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
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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.
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Building a Crystal (with cations and anions)
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Building Blocks for Silicate Clays
Silicon Tetrahedron Aluminum octahedron hydroxide (OH-) Oxygen (O2-) Aluminum (Al3+) silicon (Si4+)
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Fundamental Building Blocks
Graphite Diamond Silicon Tetrahedron Aluminum Octahedron Crystalline Minerals Oxygen (O2-) silicon (Si4+) Hydroxide (OH-) Aluminum (Al3+)
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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
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2:1 mineral { Tetrahedra { octahedra { Tetrahedra
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Silicate Clay minerals
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Charge Balance = Si4+ O2- Al3+ OH- Positive charge Negative charge
Al3+ Si4+ OH- O2-
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Positive Charge = Negative Charge
Silicon Aluminum Positive Charge Al3+ Si4+ Negative Charge O2- OH- Oxygen Hydroxide Positive Charge = Negative Charge
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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
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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.
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Tetrahedral Substitution
Al3+ for Si4+
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Positive Charge = Negative Charge
Silicon Aluminum Positive Charge Al3+ Si4+ Positive Charge O2- OH- Oxygen Hydroxide Positive Charge = Negative Charge
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Octahedral Substitution
Mg2+ for Al3+
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Positive Charge = Negative Charge
Silicon Aluminum Magnesium Positive Charge Al3+ Si4+ Mg2+ Positive Charge O2- OH- Oxygen Hydroxide Positive Charge = Negative Charge
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Al3+ for Si4+ Mg2+ for Al3+ Tetrahedral Substitution
Octahedral Substitution Mg2+ for Al3+
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Na+ Charge Na+ Na+ Na+ Na+ Na+ Na+
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K+ Cation Exchange K+ Na+ K+ Na+ K+ K+ Na+ Na+ Na+ Na+ Na+
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Factors Determining Cation Preference
Concentration Charge (+1, +2,+3) Size
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Concentration Soil Solution K+ K+ K+ Na+ K+ Na+ K+ K+ K+ K+ K+ K+ K+
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Charge Soil Solution Ca+2 Ca+2 Ca+2 Ca+2 Ca+2 K+ K+ Ca+2 K+ K+ Ca+2 K+
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Size Soil Solution Na+ Cs+ Stronger bond (Cs+ preferred)
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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: cmolc/kg
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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
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