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Weathering: breaking rocks down Example of an external (surface) process (vs. internal process) ► Weathering occurs because many minerals in rocks are not stable at temperatures and pressures that exist on earth’s surface. ► Weathering: breaking rocks down Mechanical weathering: physical disintegration of rock (into smaller pieces). ► Chemical Weathering: chemical destruction of primary minerals, creation of secondary minerals (residuals) and dissolved ions. Air and water react with minerals. ► Mechanical Weathering Mechanical Weathering Mechanical Weathering What does this have to say about susceptibility of minerals to weathering? Chemical Weathering KAlSi3O8 + H+ + HCO3- + H2O ►► Al2Si2O5(OH)4 + K+ + HCO3- + SiO2 Chemical Weathering KAlSi3O8 + H+ + HCO3- + H2O ►► Reactants Al2Si2O5(OH)4 + K+ + HCO3- + SiO2 Products Chemical Weathering KAlSi3O8 + H+ + HCO3- + H2O ►► Potassium feldspar Introduced in solution Al2Si2O5(OH)4 + K+ + HCO3- + SiO2 Clay mineral residual Carried away in solution Weathering Chemical Weathering Processes: • Solution • CO2 + H20 ◄► H2CO3 • (formation of carbonic acid) • Source(s) of CO2? • in atmosphere • in soil air from organic decomposition 12 Weathering Chemical Weathering Processes: • Solution of calcite • CO2 + H20 ► H2CO3 • CaCO3 + H2CO3 ► Ca 2+ + 2 (HCO3)solid solution solution solution 13 Big Spring ,Missouri Chemical Weathering Solution Chemical Weathering Solution Weathering Chemical Weathering Processes: • Oxidation-reduction ► Often earliest form of chemical weathering. ► affects ferrous silicates—such as? • pyroxene, olivine, amphibole, biotite ► affects ferrous carbonates and sulfides. ► Example: Fe2SiO4 + 4 H2O + O2 ► 2 Fe2O3 + H4SiO4 FeS2 + O2 + H2O = Fe2+ H2SO4 (AMD) 16 Diversion ditch Sediment pond BLGP Sludge pond Treatment ponds Trend of upper seepage front 100m Chemical Weathering Oxidation—Acid Mine Drainage (AMD) Chemical Weathering Oxidation—Acid Mine Drainage (AMD) Chemical Weathering Oxidation— Acid Mine Drainage (AMD) Weathering of Landscapes Fig. 5.12 26 Weathering and Ores 1. Over time a granite mountain is reduced to boulder-sized pieces of granite. This is an example of ________________. a. mechanical weathering b. chemical weathering c. hydrolysis d. solution 2. In the chemical reaction shown below, a. where would this reaction occur? b. what are the reactants? c. what are the products? d. which reactants and products are minerals? Ions? e. which of the products is a residual? Which are in solution? KAlSi3O8 + H+ + HCO3- + H20 ►►► Al2Si2O5(OH)4 + K+ + HCO3- + SiO2
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