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Tonight

Tonight. Module 1 exam Mineralogy Soil colloids. Sand. May be rounded or irregular Not sticky, even when wet Water holding capacity is low Quite inactive chemically. Silt. Irregularly fragmental Diverse in shape Seldom smooth or flat Really is microsand Has an adhering film of clay.

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Tonight

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  1. Tonight • Module 1 exam • Mineralogy • Soil colloids

  2. Sand • May be rounded or irregular • Not sticky, even when wet • Water holding capacity is low • Quite inactive chemically

  3. Silt • Irregularly fragmental • Diverse in shape • Seldom smooth or flat • Really is microsand • Has an adhering film of clay

  4. Physical properties of silt • By itself, acts like sand • Quite inactive chemically • Coated with clay: • some plasticity • some cohesion • some adsorption

  5. Soil Colloids • Phyllosilicate clays • Hydrous oxides of Al and Fe • Amorphous minerals • Organic soil colloids

  6. Importance of Soil Colloids • Shrinking • Swelling • Plasticity (Stickiness) • Cohesion • Dispersion • Flocculation

  7. Properties of Soil Colloids • Small size • High specific surface area • Surface charge

  8. What is clay? • Size of <2 m • Layered structure, thin plates • Tetrahedral and octahedral layers • Covalent bonding

  9. Clay • High surface area • High adsorption power • Highly plastic when wet • High heat of wetting • Chemically active

  10. Permanent charge • Due to charge imbalance in the crystal structure • Charge imbalance due to isomorphous substitution • Charge due to broken edges of crystals

  11. Cation exchange capacity • A cation has a positive charge • Once cation can be exchanged for another • CEC = cation exchange capacity • CEC is pH dependant

  12. Cation exchange • Cations attracted to the negatively charged clay • Cations concentrated next to the clay surface • Less concentration of cations as move away from the clay surface

  13. Adsorbed cations • H+, Al3+ and Ca2+ • Mg2+ • K+ and Na+

  14. Lyotrophic Series • How close a cation can get to the surface of a clay depends on the hydrated size and charge of the cation • Al3+>Ca2+>Mg2+>NH4+>K+>Na+

  15. 1:1 and 2:1 layers • 1:1 One sheet of tetrahedral and one of octahedral • 1:1 (open faced sandwich) • 2:1 two tetrahedral sheets (bread) with an octahedral sheet in the middle (filling) • (closed sandwich)

  16. Swelling clay • Swelling in the interlayer space is due to water. • Not all clays swell • Some have their layers bonded too tightly to swell

  17. Surface area of clay • Clays have a high surface area • The area depends on their structure • 2:1 clays have more surface area than 1:1 clays • More area = more opportunity for reactions

  18. Effect of surface area • Adsorbing power • Swelling • Plasticity and cohesion • Heat of wetting Brady 1974 SAND SILT CLAY COLLOIDAL CLAY

  19. Iron and aluminum oxides • pH dependant charge • Al or Fe bonded to OH2+ at low pH • Al or Fe bonded to OH at neutral pH • Al or Fe bonded to O- at high pH

  20. Organic matter • Starts off as litter • Decomposed to humus • Humus has colloidal properties

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