chemical weathering and soils chapter 3. weathering igneous minerals formed out of equilibrium with...
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Chemical Weathering and Soils
Chapter 3
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Weathering
• Igneous minerals formed out of equilibrium with Earth’s surface
• WEATHERING converts less-stable minerals to more-stable via… – Chemical processes (Decomposition)– Physical processes (Disintegration)– and Biologic processes
• Soils are the by-product of weathering
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Decomposition
• Acidic soil water dissolves grain surfaces• Rainwater (pH <5.6)• Organic acids• high temperature = higher weathering
rates
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Chemically-weathered grain
Etch pits formed parallel to cleavage planes on hornblende grain
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Processes of Decomposition
• Soil zone processes– Oxidation/Reduction (Redox)– Solution– Hydrolosis– Ion Exchange
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Redox
• Oxidization:– Oxic environments, e.g. above water table – Iron minerals are typically red and brown
• Reduction– Anoxic environments – e.g., below the water
table– Minerals are usually grey in color
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Oxidation – Soil in Costa Rica
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Solution
• Dissolution is removal of atoms from minerals and into dissolved aqueous form
• Minerals have varying solubilities
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Hydrolosis
• “The reaction between mineral elements and the hydrogen ion of dissociated water”
• H+ replaces cation (e.g. K+) in original mineral; K goes in aqueous phase
• Breaks apart silicate minerals to produce clay minerals and other compounds– Orthoclase feldspar kaolinite (clay)
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Hydrolosis of Orthoclase
• 2KAlSi3O8 + 2H+ + 9H2O H4Al2Si2O9 + 4H4SiO4 + 2K+
• Orthoclase + water kaolinite + silicic acid + potassium
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Ion Exchange
• Cations in solution are exchanged with cations on mineral surfaces
• Most effective in clay minerals• Cation Exchange Capacity (CEC) is quantitative
estimate of this ability for different minerals
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Ion Mobility
• Cl• SO4
• Na• Ca• Mg• K• Si• Fe• Al
Most Mobile
Least Mobile
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Goldich Mineral Stability Series
• Instability related to initial temperature and pressure conditions of primary minerals
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Saprolites product of chemical weathering
• Saprolite formation = f (cation leaching)
• Leaching = f (rainfall, percolation through material, temperature, pH) – 30+ m thick in humid tropics– High in Fe-oxides – High in insoluble Al-oxides
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Copyright © Richard Kesel 2002
Saprolite – Costa Rica
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Clay and Secondary Minerals
• Clays (aluminum silicates with layered atomic structure)
• kaolinite most common
• illite, montmorillonite, smectite, micas
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Other Secondary Minerals
• Al, Fe, Si, and Ti hydrous oxides– Common in saprolites– Orange to brown color
• CaCO3, CaSO4(H20) – Common in arid climates where leaching is minimal– White to tan color
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Table 3-4
Weathering as a proxy for relative age
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Weathering pits• Olmec head, gulf coast of Mexico
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Soil Formation
• S or s = f (cl, o, r, p, t…)– S is Soil, s is some soil property– cl Climate– o biologic (organic) processes– r topography (relief)– p parent material– t time
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Soil Classification
• Texture (grain size + organic matter)• Structure• Color• Organic Matter• Mineralogy (primary and secondary)• Many others
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Table 3-5
Soil Horizons• Infinite combinations!• Soil taxonomyeluviation
illuviation
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B-Horizon
K-Horizon
A-Horizon
C-Horizon
Soil Horizons Photograph
O-Horizon Organic
Leached
Accumulation
Carbonate
Slightly-weath-ered parentmaterial
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Arid soils
• Lack strong zonation found in humid soils• 1) thin, organic-poor, silt rich vesicular A horizon
(Av horizon)• 2) Red argillic B horizon (on Pleist. Soils)• 3) secondary carbonate (calcrete)
accumulations– Micropendents or lamallae on ped and clast bottoms
• Groundwater flow is upward via capillary action
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Figure 3-17
The K or Bk Horizon• Arid to semi-arid soils• “Calcification”
– 1) dissolution of carbonate at surface
– 2) downward migration through soil– 3) Precipitation of carbonates from
evaporation as coatings
• Carbonate accumulation• Aka caliche, calcrete
– Why your house doesn’t have a basement!
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Figure 3-24
Climate-control of K horizon depth
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Soils Applications
• Factor of time• Profile Development Index (PDI)
– Relative age differences
• Chronofunctions– Quantitative relation between soil development and age
• Paleosols– Buried, relict, and exhumed
• Soils can be used to relative-date landforms
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Extra slides
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Saprolite – Phyllite weathering, Brazil
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Saprolite – Costa Rica
Copyright © Matthew Lachniet 1999
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Rates of Chemical Weathering• 0.5 to 1.5 mm
per 100ka
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Figure 3-14
Textural Classification