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

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.

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

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 Quartz SiO2 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+ + (OH)- H20 + H+ H3O+ Concentration of H3O+ in 1 litre pure water at 25oC is 0.0000001 (10-7) mole pH = -log10 (H3O+)

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

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

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

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

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

Equilibrium solubility and pH

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

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

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 Reduction – the gaining of electrons Oxygen dissolved in water is the most common oxidising agent 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 reduction oxidation

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 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-)

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 Libyan Desert Desert varnish (10-30 mm thick) clay minerals oxides/hydroxides of iron and manganese detritus

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 magma Silicate 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.