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Lecture 12 Clay Minerals

Lecture 12 Clay Minerals. Clay and organic matter in the soil provide the negative absorptive sites or Cation Exchange Capacity (CEC). Organic matter as a sources of negative charge. 1. Soil Humus - SOM R-CO + <---> R-CO-O- + H+

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Lecture 12 Clay Minerals

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  1. Lecture 12Clay Minerals Clay and organic matter in the soil provide the negative absorptive sites or Cation Exchange Capacity (CEC)

  2. Organic matter as a sources of negative charge • 1. Soil Humus - SOM • R-CO + <---> 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. OH- or O- Ca++ or K+

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

  4. Source 2: 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

  5. 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

  6. Basic Building Blocks for Clay Minerals • Silicon - Oxygen Tetrahedron(Si2O5-2)

  7. Basic Building Blocks - 2 • Aluminum Octahedral (Gibsite Sheet) • Al(OH)6-3

  8. Types of Clay Minerals • 1) Kaolinite a 1: 1 clay mineral • 1 silica sheet and 1 gibsite sheet • .0072 cm thick Properties: 1) LARGE PARTICLES LOW CEC - 3-15 MEQ/100G 2) RIGIDLY FIXED- LOW SHRINK SWELL

  9. 2 : 1 CLAY MINERALS • 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 K Space

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

  11. 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

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

  13. Where have you had contact with a clay ?

  14. The End

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