1. GY1004 Principles of Physical Geography B
Lecture 2
Chemical Weathering
DEPARTMENT OF GEOGRAPHY

2. Chemical weathering
The formation of granite
Open cast Kaolinite mine, Devon.
Chemical weathering, therefore, reflects the tendency for new minerals to be formed which are stable under conditions prevailing at the Earth’s surface.

3. Approaches to studying chemical weathering
Kinetic approach
Considers the factors that control the rate of chemical reactions
Thermodynamic approach
Considers the ultimate stable forms by analysing the energy changes involved in chemical reactions

4. Chemical weathering processes 1
Solution – the process by which ions are detached by water molecules and then carried away from the weathering zone

5. Chemical weathering processes 2
Hydration – the absorption of water into the mineral structure

6. Chemical weathering processes 3
Oxidation and reduction – the process by which minerals gain and lose electrons

7. Chemical weathering processes 4
Hydrolysis – the replacement of metal cations in a mineral structure by hydrogen ions (H+) and the combining of these released cations with hydroxyl ions (OH-) .

8. Chemical weathering processes 5
Organic processes e.g. chelation – the release and mobilisation of metal cations (e.g. Fe3+ and Al3+)

9. Chemical weathering processes 6
Cation exchange – the substitution of one cation for another of a different element in a mineral structure.

10. Chemical weathering processes 1
Solution – the process by which ions are detached by water molecules and then carried away from the weathering zone

11. Why is water a good solvent?

12. Mineral solubility Halite Calcite Rock salt Limestone Quartz SiO2
NaCl CaCO3
Quartz
SiO2

13. Equilibrium solubility
The extent to which a mineral will dissolve in water (ppm; mg l-1)
Determined by the types of chemical bonds
Ionically bonded minerals are more susceptible to solution than covalently bonded minerals -

14. Equilibrium solubility and ionic potential

15. Equilibrium solubility and pH
Self ionisation of water
H20 H+ + (OH)-
H20 + H+ H3O+
Concentration of H3O+ in 1 litre pure water at 25oC is (10-7) mole
pH = -log10 (H3O+)

16. pH pH = -log10 (H3O+) Thus for pure water at 298K (25oC)

17. Acids and bases Self ionisation of water H20 H+ + (OH)-
Substances which increase the concentration of H+ cause the pH to fall and are called acids
Acids are, therefore, proton (H+) donors

18. Acids and bases Self ionisation of water H20 H+ + (OH)-
Substances which decrease the concentration of H+ cause the pH to rise and are called bases
Bases are, therefore, proton (H+) acceptors

19. The pH scale 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 acid neutral alkaline
acid
neutral
alkaline
lemon juice
beer
coffee
antacid tablets
bleach

20. The pH scale 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 acid neutral alkaline
acid
neutral
alkaline
rivers and lakes
peat water
rain water
oceanic water

21. Equilibrium solubility and pH

22. Carbonation
The dissolution and disassociation of carbon dioxide and water in a reversible reaction to produce bicarbonate ions
The solution of carbonate rocks such as limestone (CaCO3) is enhanced through a process of carbonation.
Carbonation describes the dissolution and disassociation of carbon dioxide and water in a reversible reaction to produce bicarbonate ions (HCO3-)
Two stages

23. Carbonation Carbon dioxide + water carbonic acid CO2+ H2O H2CO3-
H2CO3- H+ + HC03-
bicarbonate ion

24. Carbonation and the weathering of calcareous rocks

25. Chemical weathering processes 2
Hydration – the absorption of water into the mineral structure

26. Chemical weathering processes 3
Oxidation and reduction – the process by which minerals gain and lose electrons

27. Oxidation and reduction
Oxidation – the loss of electrons
Reduction – the gaining of electrons
Oxygen dissolved in water is the most common oxidising agent
Iron is the most commonly oxidised
material

28. The oxidation of iron 4Fe2+ +3O2 2Fe2O3 iron + oxygen iron oxide
Bivalent (Fe2+)
Ferrous iron oxide
FeO
Trivalent (Fe3+)
Ferric iron oxide
Fe2O3
reduction
oxidation

29. Bivalent (Fe2+)
Ferrous iron oxide
FeO (Reduced form)
Trivalent (Fe3+)
Ferric iron oxide
Fe2O3 (Oxidised form)
Gley soil
Laterite

30. Redox potential (Eh) Eh is measured in millivolts (mV)
Positive Eh = oxidising environment
Negative Eh = reducing environment
Eh varies with pH

31. Chemical weathering processes 4
In situ weathered granite
Core stones
Hydrolysis – the replacement of metal cations in a mineral structure by hydrogen ions (H+) and the combining of these released cations with hydroxyl ions (OH-)

32. The weathering of feldspar by hydrolysis

33. Chemical weathering processes 5
Organic processes e.g. chelation – the release and mobilisation of metal cations (e.g. Fe3+ and Al3+)

34. Chemical weathering processes 5
Libyan Desert
Desert varnish (10-30 mm thick)
clay minerals
oxides/hydroxides of iron and manganese
detritus

35. Chemical weathering processes 6
Cation exchange – the substitution of one cation for another of a different element in a mineral structure.

36. The chemical weathering of silicate minerals
The formation of silicate minerals in cooling magma
Silicate minerals are the constituent minerals of nearly all rocks

37. The chemical weathering of silicate minerals

38. Summary
Chemical weathering is the chemical process of mineral and rock decay;
The relative stability of silicate minerals is roughly the reverse order of crystallisation of a granitic melt;
Carbonate rocks are prone to solution.

39. Summary
The extent to which a mineral will dissolve is termed its equilibrium solubility;
Calcium is the most soluble mineral followed by sodium and potassium. These are all more soluble than silica, aluminium and iron;
Other important chemical weathering processes include hydrolysis, hydration, oxidation and reduction.

40. Summary
Overall, chemical weathering arises from the chemical instability of silicate minerals and the solubility of carbonate minerals at or near the earth’s surface.