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Soil Mineralogy and Chemistry

Lecture 4. Soil Mineralogy and Chemistry. Phyllosilicate Minerals. tet. tet. tet. tet. tet. tet. oct. oct. oct. Phyllosilicates, NO layer charge. trioctahedral. dioctahedral. T:O (class). oct. octahedra. gibbsite. brucite. 1:1. oct. kaolinite. serpentine. oct. 2:1.

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Soil Mineralogy and Chemistry

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  1. Lecture 4 Soil Mineralogy and Chemistry

  2. Phyllosilicate Minerals

  3. tet tet tet tet tet tet oct oct oct Phyllosilicates, NO layer charge trioctahedral dioctahedral T:O (class) oct octahedra gibbsite brucite 1:1 oct kaolinite serpentine oct 2:1 talc pyrophyllite

  4. Building of Tetrahedral Sheets * Linked SiO4 tetrahedra

  5. SiO4 Tetrahedron

  6. z y x

  7. Si6O18

  8. Linking Rings to FormTetrahedral Sheets

  9. Tetrahedral Sheet Si:O 2:5

  10. Building of Octahedral Sheets

  11. Al(OH)6 or Mg (OH)6 Octahedra OH

  12. Octahedral Sheet OH

  13. A Georgian Ultisol

  14. Octahedral-Tetrahedral Linkage

  15. Sharing of Apical Oxygens in Tetrahdral Sheet with Hydroxyls of Octahedral Sheet Serpentine (1:1 trioctahedral mineral) tet oct

  16. Sharing of Apical Oxygens in Tetrahdral Sheet with Hydroxyls of Two Octahedral Sheets Talc (2:1 trioctahedral mineral) tet oct tet

  17. Short-hand Symbols tet oct Short hand, Block notation

  18. tet tet tet tet tet tet oct oct oct Phyllosilicates, NO layer charge trioctahedral dioctahedral T:O (class) oct octahedra gibbsite brucite 1:1 oct kaolinite serpentine oct 2:1 talc pyrophyllite

  19. Charge Development Isomorphic Substitution  permanent (layer) charge

  20. Isomorphic Substitution tet Al3+ Si4+ oct Al3+, Fe3+ Mg2+

  21. NET Charge - - - - - - - 0 ++ ++ ++ ++ ++ ++ ++ - - - - - - - Isomorphic substitution - - - - - - - -2 ++ + ++ ++ ++ + ++ - - - - - - -

  22. Charge Development Terminal Bonds • pH-dependent charge (ionizable functional groups)

  23. Terminal Bonds terminal bonds

  24. Terminal Bonds: Ionizable Functional Groups O Al Al-OH2+ Al-OHo + H+  Al-O- + H+ Low pH High pH • pH-dependent charge • edges of phyllosilicates; all surface on Fe- and Al-oxides

  25. tet tet tet tet tet tet tet tet oct oct oct oct Phyllosilicates: 2:1 with layer charge micas 2:1 clay minerals K+ K+ K+ Ca2+ H2O K+ H2O H2O 1 unit of (-) layer charge per formula unit < 1 unit of (-) layer charge per formula unit

  26. Swelling Potential?

  27. Interlayer Status: d-spacing d-spacing Structure K+ (and mica)

  28. CEC cmol/Kg Properties of Clay Minerals

  29. Accessory Minerals Kaolin and Oxide Rich Soil Western GA Ultisol

  30. Smectite Soil Iron and Al-oxide Rich ? NE Montana Vertisol Hawaiian Oxisol

  31. Clay mineralogy reflects weathering processes Micas  Vermiculite  Smectite  Kaolinite  Al,Fe-Oxides • Young, weakly weathered soils • = fine-grained mica, chlorite, vermiculite • (Entisol, Inceptisol) • Intermediate weathering • = vermiculite, smectite, kaolinite • (Mollisol, Alfisol, Ultisol) • Strong weathering • = kaolinite, hydrous oxides • (Ultisol--> Oxisol)

  32. Mol E/I E/I Alf Ult

  33. Organic Matter - reactive functional groups: carboxyl, hydroxyl, phenolic * Humus, Humic Acid, Fulvic Acid

  34. + Flocculation (chemical) Aggregation Flocculation and Aggregation (organic gluing)

  35. Organic Matter Promoted Aggregation

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