Presentation on theme: "GY1004 Principles of Physical Geography B"— Presentation transcript:
1 GY1004 Principles of Physical Geography B Lecture 2Chemical WeatheringDEPARTMENT OF GEOGRAPHY
2 Chemical weatheringThe formation of graniteOpen 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 approachConsiders the factors that control the rate of chemical reactionsThermodynamic approachConsiders 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
12 Mineral solubility Halite Calcite Rock salt Limestone Quartz SiO2 NaCl CaCO3QuartzSiO2
13 Equilibrium solubility The extent to which a mineral will dissolve in water (ppm; mg l-1)Determined by the types of chemical bondsIonically bonded minerals are more susceptible to solution than covalently bonded minerals-
22 CarbonationThe dissolution and disassociation of carbon dioxide and water in a reversible reaction to produce bicarbonate ionsThe 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 electronsReduction – the gaining of electronsOxygen dissolved in water is the most common oxidising agentIron is the most commonly oxidisedmaterial
28 The oxidation of iron 4Fe2+ +3O2 2Fe2O3 iron + oxygen iron oxide Bivalent (Fe2+)Ferrous iron oxideFeOTrivalent (Fe3+)Ferric iron oxideFe2O3reductionoxidation
29 Bivalent (Fe2+)Ferrous iron oxideFeO (Reduced form)Trivalent (Fe3+)Ferric iron oxideFe2O3 (Oxidised form)Gley soilLaterite
30 Redox potential (Eh) Eh is measured in millivolts (mV) Positive Eh = oxidising environmentNegative Eh = reducing environmentEh varies with pH
31 Chemical weathering processes 4 In situ weathered graniteCore stonesHydrolysis – the replacement of metal cations in a mineral structure by hydrogen ions (H+) and the combining of these released cations with hydroxyl ions (OH-)
38 SummaryChemical 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 SummaryThe 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 SummaryOverall, chemical weathering arises from the chemical instability of silicate minerals and the solubility of carbonate minerals at or near the earth’s surface.