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

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Presentation on theme: "GY1004 Principles of Physical Geography B Lecture 2 Chemical Weathering DEPARTMENT OF GEOGRAPHY."— Presentation transcript:

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

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

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

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 NaClCaCO 3 Quartz SiO 2

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 H 2 0+ H + H 3 O + pH = -log 10 (H 3 O + ) H 2 0H + + (OH) - Concentration of H 3 O + in 1 litre pure water at 25 o C is (10 -7 ) mole

16 pH pH = -log 10 (H 3 O + ) pH = -log Thus for pure water at 298K (25 o C) pH = -(-7) = 7

17 Acids and bases Substances which increase the concentration of H + cause the pH to fall and are called acids Acids are, therefore, proton (H + ) donors Self ionisation of water H 2 0H + + (OH) -

18 Acids and bases Substances which decrease the concentration of H + cause the pH to rise and are called bases Bases are, therefore, proton (H + ) acceptors Self ionisation of water H 2 0H + + (OH) -

19 antacid tablets beer coffee The pH scale acidalkaline bleach neutral lemon juice

20 rivers and lakes rain water The pH scale acidalkaline oceanic water neutral peat 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

23 Carbonation Carbon dioxide + water carbonic acid H 2 CO 3 - H + + HC0 3 - bicarbonate ion CO 2 + H 2 OH 2 CO 3 - carbonic acid

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 Oxygen dissolved in water is the most common oxidising agent Reduction – the gaining of electrons Iron is the most commonly oxidised material

28 The oxidation of iron 4Fe O 2 2Fe 2 O 3 iron + oxygeniron oxide Bivalent (Fe 2+ ) Ferrous iron oxide FeO Trivalent (Fe 3+ ) Ferric iron oxide Fe 2 O 3 oxidationreduction

29 Bivalent (Fe 2+ ) Ferrous iron oxide FeO (Reduced form) Trivalent (Fe 3+ ) Ferric iron oxide Fe 2 O 3 (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 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 - ) In situ weathered granite Core stones

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. Fe 3+ and Al 3+ )

34 Chemical weathering processes 5 Desert varnish ( m thick) clay minerals oxides/hydroxides of iron and manganese detritus Libyan Desert

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 earths surface.


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