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Reactions of Aluminosilcates

Reactions of Aluminosilcates. Building Blocks for Silicate Clays. Silicon Tetrahedron. Aluminum octahedron. hydroxide (OH - ). oxygen. Aluminum (Al 3+ ). silicon. (Si 4+ ). Tetrahedra and Octahedra Sharing the Oxygens. Linkage of thousands of silica tetrahedra and

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Reactions of Aluminosilcates

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  1. Reactions of Aluminosilcates

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

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

  4. 2:1 mineral { Tetrahedra { octahedra { Tetrahedra

  5. 1:1 minerals 2:1 minerals

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

  7. 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.

  8. Tetrahedral Substitution Al3+ for Si4+

  9. Octahedral Substitution Mg2+ for Al3+

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

  11. Na+ Charge Na+ Na+ Na+ Na+ Na+ Na+

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

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

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

  15. K+ K+ K+ K+ K+ K+ K+ Concentration Soil Solution K+ K+ Na+ K+ Na+ K+ K+ K+ K+ K+ K+ K+ Na+ K+ Na+ K+ K+

  16. Ca+2 Ca+2 Ca+2 Ca+2 Charge Soil Solution Ca+2 Ca+2 Ca+2 Ca+2 K+ K+ K+ K+ Ca+2 K+ Ca+2 K+ K+ K+ K+ Ca+2 Ca+2 K+ Ca+2 K+ K+

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

  18. 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

  19. What is a Centimole? 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

  20. Measuring Cation Exchange Capacity

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

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

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

  24. 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?

  25. 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?

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

  27. 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

  28. Important Clay Minerals

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

  30. O O O O H H H H H H H H + Na

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

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

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

  34. Smectites Abundant water O Ca Ca Ca H H

  35. Smectites Limited water O Ca Ca Ca H H

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

  37. K+ K+ Illite 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. Non-expansible CEC = 20-40 cmolc kg

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

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

  40. Clay Layer K K K Clay Layer

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

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