Chemical Weathering and Soil Development

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Physical weathering Types of physical weathering are: - Frost wedging - Expansion and contraction due to diurnal temperature changes - Release of overburden pressure -- (sheeting and exfoliation) - Biological process -- ie. rooting insolation (thermal) weathering expansion and contraction with wetting and drying the surface temperature of dark colored rock can vary from 0-50 o C between day and night, since rock (especially jointed rock) has low thermal conductivity the differential stresses of expansion and contraction of the outer 1-5 cm of rock causes separation of concentric shallow layers called spalling or spheroidal weathering in boulders frost shattering the force of water in rock fractures as it freezes and expands, or is forced into the rock by the pressure of freezing water the most common physical weathering process, given the widespread distribution of frost (even in the tropic at high elevations) most effective in coastal arctic and alpine environments where there are hundreds of frost cycles per year the specific volume (vol./unit mass) of water increases by 9% upon freezing producing stress that is greater than the strength of all common rock Chemical weathering Alteration to cause chemical or mineralogical changes: weakens rocks The reactive components in the Earth's atmosphere include: – Oxygen O 2 – Carbon Dioxide - CO 2 –Water -H 2 O Nitrogen, the most abundant consistent in the atmosphere has little effect on the chemical weathering of rocks and minerals.
Main schema of chemical weathering Hydrolysis mineral cations (e.g., Ca + , Fe + , Na + , K + , Al + ) are replaced by hydrogen ions (H + ) from acidic water Mg 2 SiO 4 (Olivine) + 4 H + 2 Mg 2+ + H 4 SiO 4 KAlSi 3 O 2 (Feldspar) + H + Al 2 Si 2 O 5 (OH) 4 (Kaolinite) + K + + H 4 SiO 4 • the most common weathering process • pure water is a poor H + donor, however biogenic CO 2 dissolves in water to produce carbonic acid: • the weathering products are in solution or a residue is clay, that is, the first stage of soil development • the soil water solution becomes more basic as H + is consumed Hydrolysis is the weathering reaction that occurs when the two surfaces of water and compound meet. It involves the reaction between mineral ions and the ions of water (OH - and H + ), and results in the decomposition of the rock surface by forming new compounds, and by increasing the pH of the solution involved through the release of the hydroxide ions. Hydrolysis is especially effective in the weathering of common silicate and alumino-silicate minerals because of their electrically charged crystal surfaces. Muscovite mica K 2 Al 6 Si 6 O 20 (OH) 4 ↓ Illite clay KAl 5 Si 7 O 20(OH) 4 ↓ Kaolinite clay Al 4 Si 4 O 10 (OH) 8 ↓ Gibbsite Al(OH) 3 (Bauxite) Hydrolysis involves the progressive loss of the more soluble cations (Na, K, Ca, Mg) and under extreme conditions Si The most common chemical weathering processes • Hydrolysis • Oxidation • Reduction • Hydration • Carbonation • Chelation/Complexation • Solution Oxidation and Reduction Oxidation is loss of an electron (e - ) to dissolved oxygen - Fe is the most commonly oxidized mineral element: Fe 2+ (ferrous iron) Fe 3+ (ferric iron) 8 FeO + 2 O 2 4 Fe 2 O 3 4 FeSiO 3 + O 2 2Fe 2O 3 (hematite) + 4 SiO 2 (aq) 2 FeS 2 (pyrite) + O 2 Fe 2O 3 (hematite) + 2 S 4FeS 2 (pyrite) + 15 O 2 +14H 2 O-> 4Fe(OH) 3 (Limonite) + 8 H 2 SO 4 - other readily oxidized mineral elements include Mg, S, Cr Oxidation is the reaction that occurs between compounds and O 2. The net result of this reaction is the removal of one or more e - from a compound, which causes the structure to be less rigid and increasingly unstable. The most common oxides are those of Fe and Al, and their respective red and yellow staining of soils is quite common in tropical regions which have high temperatures and precipitation. Atmospheric O 2 is the most abundand oxidizing agent on Earth, and has been since about 1.8 billion years. Reduction is the reverse of oxidation: is thus caused by the addition of one or more e - resulting in more stable products
Page 1: Chemical Weathering and Soil Develo
Page 5 and 6: Solution Water and the ions it carr
Page 7 and 8: Limitierende Faktoren: Temperatur (
Page 9 and 10: Example: Feldspar weathering 2 KAlS
Page 11 and 12: Schematic presentation of basic and

Chemical Weathering and Soil Development

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