1. Chemical Weathering and Soils
Chapter 3

2. 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

3. 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

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

5. Redox Oxidization: Reduction 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

6. Oxidation – Soil in Costa Rica

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

8. 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)

9. Hydrolosis of Orthoclase
2KAlSi3O8 + 2H+ + 9H2O  H4Al2Si2O9 + 4H4SiO4 + 2K+
Orthoclase + water  kaolinite + silicic acid + potassium

10. 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

11. Ion Mobility Cl
SO4 Na Ca Mg K Si Fe Al
Most Mobile
Least Mobile

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

13. 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

14. Saprolite – Costa Rica Copyright © Richard Kesel 2002
Tropical weathering, Costa Rica. The slide shows alluvial fan/gravel weathered to clay.
Copyright © Richard Kesel 2002

15. Clay and Secondary Minerals
Clays (aluminum silicates with layered atomic structure)
Kaolinite most common
Illite, montmorillonite, smectite, micas

16. 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

17. Weathering as a proxy for relative age
Table 3-4

18. Weathering pits Olmec head, gulf coast of Mexico ~3000 years old
San Lorenzo Colossal Head 2, Museo Nacional de Antropologia, Mexico City

19. Soil Formation S or s = f (cl, o, r, p, t…)
S is Soil, s is some soil property
cl  Climate
o  organic (biologic) processes
r  topography (relief)
p  parent material
t  time

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

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

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

23. 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

24. 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

25. Climate-control of K horizon depth
Figure 3-24

26. 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
Fig. 8. pH chronofunctions of the Leptosols (black diamonds, bold regression lines and R2) and Regosols (white diamonds, thin regression lines and R2). The data points are weighted mean pH values of the upper 10 cm of the soils. The relationship could not be statistically proved.
Sauer et al., 2007, A soil chronosequence in the semi-arid environment of Patagonia (Argentina)

27. Extra slides

28. Saprolite – Phyllite weathering, Brazil
jpg

29. Saprolite – Costa Rica
Copyright © Matthew Lachniet 1999

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

31. Textural Classification
Figure 3-14