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Unit I - Chemistry of Weathering Processes.pptx
- 1. Chemistry of WeatheringProcesses: Weathering is the combination of processes by which pre-existing rocks physically disintegrate and chemically decompose into soil, loose clasts and dissolved components. Weathering products constitute the raw materials from which sedimentary rocks are made. Weathering is the simple consequences of exposing pre-existing rocks to the conditions at the earth’s surface. Low temperature and pressure , organic activity and atmospheric gases. Chemical weathering proceeds into two distinct ways . 1) Some constituents dissolve completely ,e.g. such minerals as calcite and halite . The constituents dissolved from such minerals are carried away by groundwater and runoff and can be precipitated elsewhere with or without the assistance of organisms.
- 2. 2. Other constituents,such as feldspar and micas, are altered into new minerals (especially clay minerals). These new minerals from which selected components are removed and carried away. Because they typically are fine grained than the original materials, they are more readily removed from weathering site. Chemical weathering of rocks and minerals involves several simultaneous chemical reactions: 1) Simple solution; 2) Hydration; 3) Hydrolysis and 4) Oxidation-reduction. These reactions proceed moret easily in the presence of both water and air. 1) Simple solution: (solid minerals +acid or water) Simple solution (dissolution) is the chemical reaction of solid rocks and minerals with water or acid . Bonds between ions in rigid crystalline lattices are broken and the freed ions are disseminated in solution. Solubility can be partial or complete. For example the minerals quartz are not very soluble. Less than 6 ppm is dissolved in normal fresh water. Crystals of quartz exposed in an outcrop of granite typically show little corrosion because of this minimum solubility. They appear fresh and unscatched by solution, standing in relief above more easily decomposed minerals such as feldspar.
- 3. Calcite is muchmore soluble than quartz. In natural world , exposure of limestone become pitted over periods of only days as they react with rainfall. Most natural rainfall becomes carbonic acid as raindrops fall through Earth’s atmosphere and absorb small amounts of carbondioxide gas: H2O+CO2= H2CO2 , Carbonic acid and acids in general contain abundant hydrogen ions . Due to their valancy and small size, they have a strong affinity for anions and will displace other cations in minerals structures . Limestone dissolved as hydrogen ions displace calcium ions , generally both dissolved Ca and bicarbonate: Ca CO3+H2CO3= Ca+2HCo3 Hydration and Dehydration: (Solid minerals+Water= New Hydrated minerals; Dehydrated is the reverse); Some weathering processes involve the chemical combination of pre-existing minerals with water (hydration) or removal of water from some pre-existing minerals (dehydration). These processes produce new minerals in greater equilibrium with environments. Two common reactions are the dehydration of gypsum to form anhydrite: CaSo4.2H2O= CaSo4+2H2O and Hydration of Iron oxide (hematite) to from limonite(this is essentially corrosive rusting) Fe2O3+3H2O= 2Fe (OH)3 (limonite; oxidized zone in soil).
- 4. 3) Hydrolysis: Hydrolysisis defined as the replacement of cations in a minerals structures by hydrogen ions derived either from water or more likely from acid . Hydrolysis releases to the solution . The cations replaced in the mineral structures by hydrogen and either converts the original mineral into a different minerals or dissolved it completely . Most silicate minerals weather primarily by a series of hydrolysis reactions, and silicate minerals such as pyroxene amphiboles, mica feldsaprs along with quartz, make the bulk of the earth’s primary crust.The specific of hydrolysis process and the extent to which an original mineral is decomposed depend on the material . Dark coloured (mafic) minerals such as olivine and pyroxene can dissolved completely : Mg2SiO4 (olivine)+4H= 2Mg+H4SiO4 (silicic acid) 2CaMgSi2O6 (Pyroxene)+16H=2Ca+2Mg+4H4SiO2 (dissolved silica). Light –coloured (felsic ) minerals, especially feldspar such as orthoclase and plagioclase, dissolved silcia and cations and leaving fine grained , easily transportable clay minerals: K AlSi3O8 K or Na or Ca feldspar +H=Al2Si2O5(OH)2 Clay mineral (Kaolinite)+K (ion in solution)+H4SiO4 (dissolved silica) (silicic acid).
- 5. 4) Oxidation-reduction:(Atmospheric oxygengains electorns and is reduced as mineral constiuents lose electrons and are oxidised, producing new “rusted minerals”. Oxidation and reduction are inexorably linked . Oxidation does not occur without reduction, and vie versa. Oxidation is the processes by which an atom or ion loses electrons. Reduction is the process by which an atom or ion gains electrons. The best oxidizing agent is atmosheric oxygen , O2; non-ionized atom of oxygen (zero valancy) in the atmosphere combine readily with other existing ions and gain electrons to become anions of oxygen (O2). As a result, oxygen is reduced , but the ion from which the oxygen atoms gain electrons is oxidized . The most obvious example of this process involve the oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) . In the natural world, this is the process of rusting, so called because it changes, dull metalic ferrous iron to reddish –orange ferric iron. For example: Fe2Sio6 (pyroxene) +O2 (atmospheric oxygen)+ H2O=4Fe(OH)3 +H4Sio4 (dissolved silica); 2FeS2 (pyrite)+O2(atmospheric oxygen)=Fe2O3 (hematite)+2S (dissolved sulfur)
- 6. Controls of chemicalweathering • Climate: climate is of paramount importance. Higher temperature promote chemical weathering because kinetic thermal energy facilitates any reaction. More extensive in warmer climates (lower latitutes) than in colder climates (higher latitude/higher elevation). • Moisture (rainfall) is also important because most chemical weathering requires water-thus humid climates is most favourable. In desert –temperature but water for hydrolysis/solution and hydration. Arctic region and higher mountain terrain –water in the from of snow and ice not liquid water. • Hydrolysis and simple solution depend upon the ready availability of hydrogen ion. The abundance of hydrogen ion –more specifically the activity of hydrogen ions in solution is controlled by the acidity or alkalinity of a solution ; that is, the pH. The pH is a way to designate the concentration of H ions in solution or (H+) . The PH of 7 is termed as neutral solution. Solutions with an excess of hydrogen ions have pH values from 1 to just below 7 are called acids. • Most natural waters have pH value between 4 and 9.The likelihood of oxidation depends largely on the availability of free atmospheric oxygen. The term Eh expresses the potential for either oxidation or reduction. Eh, short for redox potential is measured using an electolytic cell.
- 7. Mechanical Weathering • Frostaction – Mechanic effect of freezing (and expanding) water on rocks • Pressure release – Removal of overlying rock allows expansion and fracturing • Plant growth – Growing roots widen fractures • Burrowing animals • Thermal cycling – Large temperature changes fracture rocks by repeated expansion and contraction
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- 10. Surface Area &Weathering
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- 12. CHEMICAL WEATHERING (LARGELYCONTROLLED BY CLIMATE) *ROLE OF WATER *DISSOLUTION 1. WATER DISSOLVES HALITE AND GYPSUM 2. CARBONIC ACID DISSOLVES LIMESTONE (CO2+H2O----HCO3) i. CALCIUM CYCLE ii. ACID RAIN
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- 15. 3.OXIDATION (REACTION OFCERTAIN CHEMICALS WITH O2) i. IRON OXIDES ii. COPPER OXIDES 4. HYDROLYSIS (REPLACEMENT OF MAJOR POSITIVE IONS WITH PROTONS) OF POTASSIUM FELDSPAR INTO i) CLAY: BECOMES PART OF SOIL ii) SILICIC ACID: CEMENTS SEDIMENTS OR FORMS ANIMAL SHELLS AND SKELETONS iii) POTASSIUM IONS: PROVIDE PLANT NUTRIENTS
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- 17. FACTORS THAT INFLUENCECHEMICAL WEATHERING 1.CLIMATE i) MOISTURE ii) HEAT iii) VEGETATION 2. LIVING ORGANISMS 3. TIME
- 18. MINERAL COMPOSITION: RELATIONSHIPBETWEEN A MINERAL’S TEMPERATURE AND PRESSURE OF CRYSTALLIZATION AND ITS SUSCEPTIBILITY TO WEATHERING (HIGH TEMP CRYSTALLIZATION----- LESS STABLE AND EASILY WEATHERED-EXAMPLE: OLIVINE & PYROXENE) REGOLITH: LOOSE, FRAGMENTED MATERIAL THAT COVERS MUCH OF THE EARTH’S SURFACE SOIL: UPPERMOST ORGANIC-RICH PORTION OF THE REGOLITH Mineral composition
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- 20. D.SOME PRODUCTS OFCHEMICAL WEATHERING: a) CLAY MINERAL i) KAOLINITE ii) SMECTITE iii) PRACTICAL USES FOR CLAYS b) METAL ORES i) FORMATION OF BAUXITE ii) OTHER ORES
- 21. Soil • Soil -a layer of weathered, unconsolidated material on top of bedrock – Common soil constituents: • Clay minerals • Quartz • Water • Organic matter • Soil horizons – O horizon - uppermost layer; organic material – A horizon - dark layer rich in humus, organic acids – E horizon - zone of leaching; fine-grained components removed by percolating water – B horizon - zone of accumulation; clays and iron oxides leached down from above – A horizon - partially weathered bedrock
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- 23. Soils and Climate •Soil thickness and composition are greatly affected by climate – Wet climates: • More chemical weathering and thicker soils • Soils in moderately wet climates tend to have significant clay-rich layers, which may be solid enough to form a hardpan – Arid climates: • Less chemical weathering and thinner soils • Subsurface evaporation leads to build-up of salts • Calcite-rich accumulation zones may form, cementing soil together into a hardpan – Extremely wet climates (e.g., tropical rainforest) • Highly leached and unproductive soils (laterites) • Most nutrients come from thick O/A horizons
- 24. Vegetation and soildevelopment
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- 26. CLASSIFYING SOILS 1. OLDCLASSIFICATION SYSTEM • PEDALFERS • PEDOCALS • LATERITES 2. MODERN CLASSIFICATION SYSTEM a) BASED ON MANY PHYSIAL & CHEMICAL CHARACTERISTICS b) EXAMPLES OF SOIL TYPES 1) ENTISOL 2) VERTISOL 3) OXISOL 4) ULTISOL 3. PALEOSOLS (“OLD SOILS”)
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