Lec.2 Clay minerals

1. Lec.2 Clay minerals

2. WEATHERING

3. Weathering Sedimentary clays may have various origins. One of the most expected المتوقّعةِorigins is the erosion, transportation and deposition of geological and pedological formations that are exposed at the surface of landmasses. The surficial processes resulting from the interaction between the rocks (lithosphere), the water (hydrosphere), the air (atmosphere) and the organisms (biosphere) are referred to as weathering. The net result of the weathering process is to transform solid rocks that are largely composed of silicate minerals formed at relatively high temperatures and pressures (as compared with conditions existing at the earth's surface) into three main fractions of disintegrated rock materials: • Materials in solution (solutes). • Resistates, i.e., material present in the origin rock that remain chemically more or less unaltered in the weathering product. •New minerals produced by weathering

4. 1- Physical weathering: It is also called mechanical weathering. It is the physical breakup or disintegration of rocks without changes in their composition. a-Frost wedgingالشَقّ:It is also called freeze-and-thaw الذوبان. The physical basis of frost wedging is the 9.2% volume increase in H2O during the phase change from water to ice. Frost wedging is most effective in areas of recurrent freezing and thawing rather than in areas characterized by permanently بشكل دائم freezing temperatures.

5. b-Diurnal temperature change:تغيير درجةِ الحرارة النهاريِThe basis for this type is the fact that rocks are poor conductors of heat and, therefore, surface exposed to the sun's rays will expand to a far greater extent than the rock a few centimeters below the surface, This would cause the outer mass to become detached from the main body of rock, giving rise to phenomena الظاهرة known as exfoliation. c-Sheetingالشراشف:Many types of rocks, particularly granites, are sometimes seen in the field jointed into a series of massive sheets, roughly تقريباًparallel to the topographic surface. These fractures are caused by release of pressure that accompanies يُرافقُ erosion of overlying strata. d- Hydration-dehydration: Although the process itself-is chemical but it induces some physical effect. Some minerals like anhydrite, iron oxides and bauxites take water in their composition and expand. This expansion causes a physical effect on the neighboringالمُجَاوَرَة rocks and helps in their disintegration.

6. e- Crystal growth: Crystal growth; is the weathering process of rock disintegration by growth of salt crystal especially in dry climate'. During long drought periods, ground water is drawn يسْحُبto the surface of the rock by capillary force القوة الشعرية and evaporative pumping. As evaporation of the water takes place in the porous outer zone of the rocks, tiny crystals is capable of producing exfoliation or granular disintegration of rocks, especially when the salt crystals partially or completely dissolve. f- Swelling and shrinkage of clays: الأنتفاخ وإنكماش الطينِ This type occurs by continual swelling and shrinkage of soils and mud rocks as the particles of fine silt and clay absorb water in alternate periods of rain and drought. Shrinkage forms soil cracks in dry periods, making the infiltration of rainfall much rapid in early stages of an ensuing rain. َعْملُ التسرّب من سريعِ مطرِ الكثيرِ في المراحلِ المبكّرةِ لمطرِ

7. 2- Chemical weathering: Implying mineralogical and chemical changes. At the earth surface, the new chemical conditions may become highly aqueous, and the high-temperature silicate minerals become unstable. The sequence of minerals weathering: the first to crystallize at depth are- the first to decompose at the surface. . ., (Bowens reaction series) [Stability of the ferromagnesian minerals in weathering environments: (Olivine, Amphibole, Pyroxene, Muscovite , Biotite) ] At slow rates of weathering, the mineral constituents of the source rocks arechemically altered, and mechanically disintegrated. But when erosion rates are rapid.minerals may be transported and buried before much alteration and disintegration can take place. This is determined by two factors: a- Climate b- Topographic elevation Through the chemical weathering, part of the mineral material is dissolved away; part is changed in to another mineral phase by loosing some ions in aqueous solution. This may leave behind clay minerals and iron oxides.

8. a- Hydrolysis:The processes whereby common silicate minerals break down to form hydrated minerals is called hydrolysis. The principal result of hydrolysis are thus the removal of metals of the alkalis and alkaline earths, and the libration of silica in a soluble form due to gain H+1 and (OH) -1 in aqueous solution. eg. Hydrolysis of microcline: 3KAlSi3O8 + 2H + 12H2O —----- KAl3Si3O10(OH)2 + 6H4 SiO4 + 2K+ (microcline) (muscovite) (dissolved silica) Under humid conditions: الرطبةِ 2KAl3Si3O10(OH)2 + 2H + 3H2O —------- 3Al2Si2O5(OH)4 + 2K+ (kaolinite) Under severe weathering: 3Al2Si2O5(OH)4 + 5 H2O —----- 2A1(OH)3 + 2H4 SiO4 (kaolinite)(gibbsite) (dissolved silica) Bauxite: Gibbsite and other aluminum hydroxides formed by leaching of kaoiinitic soils from their silica.

9. b –Hydration: The process which convert the minerals to the hydrated substances. Anhydrite tends to take up water of crystallization, and is converted to gypsum near the surface: CaSO4 + 2H20 —------- CaSO4 . 2H20 (40% increasing size) c- Oxidation: It principally affects sulphides, minerals containing ferrous iron, and the organic material of carbonaceous rocks. In pyrite and marcasite, both iron and sulphur are oxidized, forming ferric hydroxide, which often remains as an ochreous residual deposit, and sulphuric acid, which is washed away and reacts with other substances.

10. d- Carbonation: The process of carbonation consists mainly in the substitution of carbonic from silicic acid in the silicates. It has been shown experimentally بشكل تجريبيthat CO2 in aqueous solution is capable of attacking many minerals قادرة على المُهَاجَمَة العديد مِنْ المعادنِ such as quartz, feldspars, hornblende, olivine, and muscovite at ordinary temperatures and pressures. e- Solution: The simplest of all weathering processes is that of solution. Rainwater and most ground waters may gain CO2 and are thus capable of dissolving calcium carbonate. This is the principal method of weathering of limestone.

13. C- Effect of annual rainfall on clay mineralogy: effect of annual rainfall on clay mineralogy in soils of acidic igneous rocks parents • D- Chemical weathering of basalts and. andesites: • These crystallize at higher temperature; contain more iron and glass, very poor in (OH): • 1- in humid warm climates (tropical)إستوائي: the ferrous iron may be leached away, into the soil pore waters, leaving behind gibbsite Al (OH)3 residues. • 2- in climate of alternating wet and dry seasons: • a. ferrous iron is leached away • b. at dry season, this is .oxidized and precipitated as goethite FeO(OH),which is extremely insoluble, forming laterite horizon.

14. Laterite:Hydrated iron oxides (+kaolinite), forms through extreme weathering of iron rich soils in humid climate. Erosion:Clays formed in the weathering process, may eventually be removed by rain waters, through erosion. Transportation:Due to the small grain size of clay particles, they can remain in suspension, and move for long distance in river flow. Aquatolysis:On transportation by rivers, clay minerals may loose K+1. Halmyrolysis:Modification in the chemistry of clay minerals, on entering seawater, or on the seafloor.