Visual comparison of common silicate clays

1. Visual comparison of common silicate clays

3. Properties of common silicate clays http://en.wikipedia.org/wiki/Van_der_Waals_force: �the name van der Waals force is sometimes used as a synonym for the totality of non-covalent forces (also known as intermolecular forces). These forces, which act between stable molecules, are weak compared to those appearing in chemical bonding. To explain this, we refer to the article on intermolecular forces, where it is discussed that an intermolecular force has four major contributions. In general an intermolecular potential has a repulsive part, prohibiting the collapse of molecular complexes, and an attractive part. The attractive part, in turn, consists of three distinct contributions (i) The electrostatic interactions between charges (in the case of molecular ions), dipoles (in the case of molecules without inversion center), quadrupoles (all molecules with symmetry lower than cubic), and in general between permanent multipoles. The electrostatic interaction is sometimes called Keesom interaction or Keesom force after Willem Hendrik Keesom. (ii) The second source of attraction is induction (also known as polarization), which is the interaction between a permanent multipole on one molecule with an induced multipole on another. This interaction is sometimes measured in debyes after Peter J.W. Debye. [this is distinct from H-bonding: �The typical hydrogen bond is stronger than van der Waals forces, but weaker than covalent, ionic and metallic bonds.� - http://en.wikipedia.org/wiki/Hydrogen_bond] (iii) The third attraction is usually named after London who himself called it dispersion. This is the only attraction experienced by noble gas atoms, but it is operative between any pair of molecules, irrespective of their symmetry. � http://en.wikipedia.org/wiki/Van_der_Waals_force: �the name van der Waals force is sometimes used as a synonym for the totality of non-covalent forces (also known as intermolecular forces). These forces, which act between stable molecules, are weak compared to those appearing in chemical bonding. To explain this, we refer to the article on intermolecular forces, where it is discussed that an intermolecular force has four major contributions. In general an intermolecular potential has a repulsive part, prohibiting the collapse of molecular complexes, and an attractive part. The attractive part, in turn, consists of three distinct contributions (i) The electrostatic interactions between charges (in the case of molecular ions), dipoles (in the case of molecules without inversion center), quadrupoles (all molecules with symmetry lower than cubic), and in general between permanent multipoles. The electrostatic interaction is sometimes called Keesom interaction or Keesom force after Willem Hendrik Keesom. (ii) The second source of attraction is induction (also known as polarization), which is the interaction between a permanent multipole on one molecule with an induced multipole on another. This interaction is sometimes measured in debyes after Peter J.W. Debye. [this is distinct from H-bonding: �The typical hydrogen bond is stronger than van der Waals forces, but weaker than covalent, ionic and metallic bonds.� - http://en.wikipedia.org/wiki/Hydrogen_bond] (iii) The third attraction is usually named after London who himself called it dispersion. This is the only attraction experienced by noble gas atoms, but it is operative between any pair of molecules, irrespective of their symmetry. �

4. Types of charge Permanent pH-dependent

5. Isomorphous substitution The replacement of one ion for another of similar size within the crystalline structure of the clay takes eons � doesn�t change rapidly

6. Permanent charge

7. pH-dependent charge: on edges

8. Ion exchange The substitution of one ion for another on the surface or in the interstitial spaces of a crystal Cation exchange (e.g., Ca2+ for K+) Anion exchange (e.g., H2PO4- for NO3-)

9. What�s so great about ion exchange? Retards the release of pollutants to groundwater Affects permeability, with implications for landfills, ponds, etc. Affects nutrient availability to plants (constant supply, protection vs. leaching)

10. Definitions cation: An ion that carries a positive charge cation exchange: A process - cations in solution exchanged with cations on exchange sites of minerals and OM cation exchange capacity (CEC): The total amount of exchangeable cations that a particular material or soil can adsorb at a given pH

11. Controls on ion exchange Strength of adsorption Related to hydrated ionic radius and valence The smaller the radius and greater the valence, the more closely and strongly the ion is adsorbed. Strength ? valence/radius Relative concentration in soil solution

12. Cation Exchange Capacity The sum total of all exchangeable cations that a soil can adsorb Expressed in terms of positive charge adsorbed per unit mass If CEC =10 cmolc/kg ? soil adsorbs 10 cmol of H+ ? can exchange it with 10 cmol K+, or 5 cmol Ca2+ number of charges, not number of ions, what matters

13. Exchange affinity

14. Ion exchange vs. CEC

15. CEC depends upon Amount of clay and organic matter Type of clay minerals present

16. Examples of cation exchange Upper case takes place readily as Ca2+ binds more strongly than does K+ (lyotropic series) Second case: need more than 3 K+ for the reaction to take place even though the reaction is a charge-balanced one (I.e., only 3 of the K+ are involved). This is because the Al3+ is higher on the lyotropic series. Note also that these are REVERSIBLE (unless something precipitates, volatilizes, or is strongly adsorbed).Upper case takes place readily as Ca2+ binds more strongly than does K+ (lyotropic series) Second case: need more than 3 K+ for the reaction to take place even though the reaction is a charge-balanced one (I.e., only 3 of the K+ are involved). This is because the Al3+ is higher on the lyotropic series. Note also that these are REVERSIBLE (unless something precipitates, volatilizes, or is strongly adsorbed).

17. Charges on soil colloids

19. Source of charge on 1:1 clays ALL clay minerals have edge charges.ALL clay minerals have edge charges.

20. Source of charge for the smectites

21. Source of charge for the micas

22. Negative charges on humus

23. Surface charge comparison (a) 13 negative charges and 5 positive charges; (b) 3 negative charges and 6 positive charges(a) 13 negative charges and 5 positive charges; (b) 3 negative charges and 6 positive charges

24. Adsorbed cations by soil order

25. Organic matter and CEC

26. Adsorbed cations: area

27. CEC and pH

28. Influence of pH on the CEC of smectite and humus

29. Charge characteristics

30. A real-life application:

32. CEC and weathering intensity

33. Rule of thumbfor estimation of a soil�s CEC

34. Soil Order CEC (cmolc/kg)

35. Base saturation* A measure of the proportion of basic cations occupying the exchange sites Base cations are those that do not form acids Ca2+, Mg2+, K+, Na+, NH4+. . ., ions OTHER THAN H+ and Al3+

36. Equation for base saturation

37. Soil Order Base Saturation (%)