Chemical Weathering and Soils Chapter 3

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

Decomposition Acidic soil water dissolves grain surfaces Rainwater (pH <5.6) Organic acids high temperature = higher weathering rates

Chemically-weathered grain Etch pits formed parallel to cleavage planes on hornblende grain

Processes of Decomposition Soil zone processes –Oxidation/Reduction (Redox) –Solution –Hydrolosis –Ion Exchange

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

Oxidation – Soil in Costa Rica

Solution Dissolution is removal of atoms from minerals and into dissolved aqueous form Minerals have varying solubilities

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)

Hydrolosis of Orthoclase 2KAlSi 3 O 8 + 2H + + 9H 2 O  H 4 Al 2 Si 2 O 9 + 4H 4 SiO 4 + 2K + Orthoclase + water  kaolinite + silicic acid + potassium

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

Ion Mobility Cl SO 4 Na Ca Mg K Si Fe Al Most Mobile Least Mobile

Goldich Mineral Stability Series Instability related to initial temperature and pressure conditions of primary minerals

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

Copyright © Richard Kesel 2002 Saprolite – Costa Rica

Clay and Secondary Minerals Clays (aluminum silicates with layered atomic structure) kaolinite most common illite, montmorillonite, smectite, micas

Other Secondary Minerals Al, Fe, Si, and Ti hydrous oxides –Common in saprolites –Orange to brown color CaCO 3, CaSO 4 (H 2 0) –Common in arid climates where leaching is minimal –White to tan color

Table 3-4 Weathering as a proxy for relative age

Weathering pits Olmec head, gulf coast of Mexico

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

Soil Classification Texture (grain size + organic matter) Structure Color Organic Matter Mineralogy (primary and secondary) Many others

Table 3-5 Soil Horizons Infinite combinations! Soil taxonomy eluviation illuviation

B-Horizon K-Horizon A-Horizon C-Horizon Soil Horizons Photograph O-Horizon Organic Leached Accumulation Carbonate Slightly-weath- ered parent material

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

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!

Figure 3-24 Climate-control of K horizon depth

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

Extra slides

Saprolite – Phyllite weathering, Brazil

Saprolite – Costa Rica Copyright © Matthew Lachniet 1999

Rates of Chemical Weathering 0.5 to 1.5 mm per 100ka0.5 to 1.5 mm per 100ka

Figure 3-14 Textural Classification