GY1004 Principles of Physical Geography B

Lecture 2Chemical Weathering

DEPARTMENT OF GEOGRAPHY

Chemical weathering

Chemical weathering, therefore, reflects the tendency for new minerals to be formed which are stable under conditions prevailing at the Earth’s surface.

The formation of granite

Open cast Kaolinite mine, Devon.

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

Chemical weathering processes 1

Solution – the process by which ions are detached by water molecules and then carried away from the weathering zone

Chemical weathering processes 2

Hydration – the absorption of water into the mineral structure

Chemical weathering processes 3

Oxidation and reduction – the process by which minerals gain and lose electrons

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-)

Chemical weathering processes 5

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

Chemical weathering processes 6

Cation exchange – the substitution of one cation for another of a different element in a mineral structure.

Chemical weathering processes 1

Solution – the process by which ions are detached by water molecules and then carried away from the weathering zone

Why is water a good solvent?

Mineral solubility

Halite Calcite

Rock salt Limestone

NaCl CaCO3

Quartz

SiO2

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

Equilibrium solubility and ionic potential

Equilibrium solubility and pH

Self ionisation of water

H20 + H+ H3O+

pH = -log10 (H3O+)

H20 H+ + (OH)-

Concentration of H3O

+ in 1 litre pure water at 25oC is 0.0000001 (10-7) mole

pH

pH = -log10 (H3O+)

pH = -log10 10-7

Thus for pure water at 298K (25oC)

pH = -(-7) = 7

Acids and bases

Substances which increase the concentration of H+ cause the pH to fall and are called acidsAcids are, therefore, proton (H+) donors

Self ionisation of water

H20 H+ + (OH)-

Acids and bases

Substances which decrease the concentration of H+ cause the pH to rise and are called basesBases are, therefore, proton (H+) acceptors

Self ionisation of water

H20 H+ + (OH)-

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 antacid

tabletsbeer

coffee

The pH scale

acid alkaline

bleach

neutral

lemon juice

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 rivers

and lakes

rain water

The pH scale

acid alkaline

oceanic water

neutral

peat water

Equilibrium solubility and pH

Carbonation

The dissolution and disassociation of carbon dioxide and water in a reversible reaction to produce

bicarbonate ions

Carbonation

Carbon dioxide + water carbonic acid

H2CO3- H+ +

HC03-

bicarbonate ion

CO2+ H2O H2CO3-

carbonic acid

Carbonation and the weathering of calcareous

rocks

Chemical weathering processes 2

Hydration – the absorption of water into the mineral structure

Chemical weathering processes 3

Oxidation and reduction – the process by which minerals gain and lose electrons

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

The oxidation of iron

4Fe2+ +3O2 2Fe2O3

iron + oxygen iron oxide

Bivalent (Fe2+)

Ferrous iron oxide

FeO

Trivalent (Fe3+)

Ferric iron oxide

Fe2O3

oxidation

reduction

Bivalent (Fe2+)

Ferrous iron oxide

FeO (Reduced form)

Trivalent (Fe3+)

Ferric iron oxide

Fe2O3 (Oxidised form)

Gley soil Laterite

Redox potential (Eh)

Eh is measured in millivolts (mV)

Positive Eh = oxidising environment

Negative Eh = reducing environment

Eh varies with pH

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

The weathering of feldspar by hydrolysis

Chemical weathering processes 5

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

Chemical weathering processes 5

Desert varnish (10-30 m thick)

clay minerals

oxides/hydroxides of iron and manganese

detritus

Libyan Desert

Chemical weathering processes 6

Cation exchange – the substitution of one cation for another of a different element in a mineral structure.

The chemical weathering of silicate minerals

The formation of silicate minerals in

cooling magmaSilicate minerals are the constituent minerals of nearly all rocks

The chemical weathering of silicate minerals

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.

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.

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.