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

• 2KAlSi3O8 + 2H+ + 9H2O H4Al2Si2O9 + 4H4SiO4 + 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• SO4

• 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

• CaCO3, CaSO4(H20) – 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 taxonomyeluviation

illuviation

B-Horizon

K-Horizon

A-Horizon

C-Horizon

Soil Horizons Photograph

O-Horizon Organic

Leached

Accumulation

Carbonate

Slightly-weath-ered parentmaterial

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 100ka

Figure 3-14

Textural Classification