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Lecture 11 Clay Minerals

Lecture 11 Clay Minerals. Clay and organic matter in the soil provide the negative absorptive sites or Cation Exchange Capacity (CEC) Clay can hold positive cations from leaching

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Lecture 11 Clay Minerals

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  1. Lecture 11Clay Minerals Clay and organic matter in the soil provide the negative absorptive sites or Cation Exchange Capacity (CEC) Clay can hold positive cations from leaching Soil CEC is the most important indicator of the soil’s ability to hold onto and retain + cations like Ca, Mg, K, NH4+,

  2. Organic matter as a source of negative charge Acid soil (neutral molecule) • 1. Soil Humus - SOM • R-CO + OH <---> R-CO-O- + H+ • This charge is pH dependent or as the pH increases more OH- is available and therefore more CEC and conversely acid soils have a lower CEC from OM. Deprotonates Raise the pH Anion CEC site OH- or O- Ca++ or K+

  3. Clay minerals as a source of negative charge- internal and external

  4. Clay as a source of negative charge • 2. Source 1: 1) broken bonds at edge of clay mineral

  5. Edge Effects are pH Dependent

  6. Source 2 for negative charges: 2) internal charge not satisfied because of ionic substitution -called Isomorphous Substitution Al 3+ for Si 4+ or Mg2+ for Al 3+ or Zn2+ for Al3+ No substitution Zn++ for Al+++ Neutral -1 charge

  7. Silicate Clay Minerals • Aluminosilicates have a definite crystalline structure. Formed by the alteration of existing minerals or by synthesis from elements. • < .002 mm in size (only seen with aid of electron microscope) • Two basic kinds of clay minerals 1 : 1 and 2 : 1

  8. Basic Building Blocks for Clay Minerals • Silicon - Oxygen Tetrahedron • SiO4-4 • Repeat Unit: (Si2O5)-2 Four sides

  9. Basic Building Blocks - 2 Al(OH)3 • Aluminum Octahedral (Gibbsite Sheet) • Al(OH)6-3 Repeat Unit:Al2(OH)4+2 Eight Sided

  10. Types of Clay Minerals • 1) Kaolinite a 1: 1 clay mineral - 1 silica sheet and 1 gibbsite sheet • .000000072 cm thick Si Al Properties: 1) LARGE PARTICLES LOW CEC - 3-15 MEQ/100G 2) RIGIDLY FIXED- LOW SHRINK SWELL

  11. { 1:1 layer Basal O Apical O, OH

  12. 2 : 1 CLAY MINERALS 2-Silica tetrahedrons and 1 aluminum octahedral • 1. Hydrous Mica or Illite • replacement of Si4+ with Al+3 in 25% of tetrahedrals, each substitution leaves 1 neg charge site also K+ ions are a bridge between layers and hold them tightly together Properties 1) non-expanding 2) Low CEC 30 meq/100

  13. kaolinite illite humus (fulvic acid) mont- morillonite

  14. 2.Vermiculite • Same as Illite with all the K ions removed Properties 1) expanding - large shrink swell 2) High CEC=150meq/100g

  15. 3. Smectite or Montmorillonite • Mg++ replaces Al+++ in Octahedral layer • Properties: 1) expanding (common clay found in Minnesota’s soils. • 2) CEC = 80-150 meq/100g

  16. Go To Clay Minerals Web Page • http://www.soils.wisc.edu/virtual_museum/

  17. Dyad • Where have you had contact with a clay ?

  18. Sheets and Layers 1:1 non-expanding 2:1 non-expanding 2:1 expanding Al sheet Si sheet + + + + + + + Al sheet Si sheet Al sheet Si sheet + + + + + + + Al sheet Si sheet kaolinite illite smectite and vermiculite

  19. Clay Minerals Comparison

  20. The End

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