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
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
Saprolite – Costa Rica
Copyright © Richard Kesel 2002
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
Weathering as a proxy for relative age
Table 3-4
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
Soil Horizons
eluviation
• Infinite combinations!
• Soil taxonomy
illuviation
Table 3-5
Soil Horizons Photograph
O-Horizon
A-Horizon
Organic
B-Horizon
Accumulation
K-Horizon
Carbonate
C-Horizon
Leached
Slightly-weathered 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
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
Figure 3-17
Climate-control of K horizon depth
Figure 3-24
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
Textural Classification
Figure 3-14