World Reference Base for Soil ResourcesMineral Soils conditioned by Parent MaterialMineral Soils conditioned by TopographyMineral Soils conditioned by a wet (sub) Tropical Climate
Ferralsols
Alisols
Nitisols

Excursus: Termites
Excursus: Clay eluviation
Acrisols
Lixisols

Process of ferralitization

Table of contents

  1. Introduction
  2. Net loss of Silicium (Si) and formation of kaolinite
    a. Example: hydrolysis of feldspar
  3. Relative accumulation of sesquioxides
    a. Oxidation of primary minerals
1. Introduction

Oxisol
Fig.1 Ferralic B horizon (here: Bo1 and Bo2)
( Source: www.soils.umn.edu/.../soil2125/ doc/s5chp2.htm )

  • Soils containing a ferralic B horizon could develop on old continental platforms due to their geo-morphological stability over long period of time and the exposure to wet and hot climate for extended periods of time. Ferralitization is a slow process, also in the humid tropics.

  • Formerly this process was named laterization, kaolinization or desilication.
  • The process comprises net loss of Si, formation of kaolinite and relative accumulation of sesquioxides.
2. Net loss of Si and formation of kaolinite
  • Generally, hydrolysis reactions are most important processes during chemical weathering. Water molecules split into their H and OH components and the H often replaces a cation from the mineral structure.
  • The net loss of Si and the formation of kaolinite during the process of ferralitization will be explained by using the hydrolysis of feldspars as an example.
  • Feldspars are highly abundant silicate minerals and are stable in Earth‘s interior under high temperatures and little water. However, they are very instable at Earth‘s surface (low temperature, abundant water).

     
    Fig.2 Non-weathered compact granite: quartz (gray) and feldspar (green) are tightly interlocked


  • An extreme example of feldspar weathering is found in granite blocks in the humid tropics due to heavy rainfalls, high temperatures and high biological activity.

 
Fig.3 Grains of feldspar
( Source: Press and Siever, 1995, p. 119.)

a. Hydrolysis of feldspar

 
Fig.4 Feldspar in granite

  • Water is used and is integrated into the lattice structure of kaolinite. The H+ - ions of the water react with the oxygen of the feldspar (water is adsorbed). The primary mineral disintegrates, K and part of the Si are washed out in the soil solution resulting in the formation of the secondary clay mineral kaolinite.
  • However, the reaction of feldspar with water under laboratory conditions is a very slow process, i.e. this reaction cannot be responsible for the rapid weathering in nature.
  • The addition of acid accelerates the process through the release of H+-ions which may combine chemically with other substances.
  • Carbon dioxide (CO2) is the most abundant and natural occurring acid and responsible for the increase of the weathering rate in nature.
  • It develops through solution of CO2 in the rain water: H2O + CO2 to H+ + HCO3- -> H2CO3
  • In rain water only very little CO2 is dissolved because the concentration in the atmosphere is rather low (0,035 %). However, it is sufficient for the weathering of feldspars. In comparison to tropical soils, values up to 5 % were found through:

 
Fig.5 CO2 concentration (%, x-axis) in soil under old-growth tropical rainforest (Costa Rica)
( Source: Veldkamp, Göttingen.)

  1. The activity of microorganisms and plant roots which release CO2 into soil air (acceleration of the process). Plants and microorganisms are more active under warm and moist conditions and produce more CO2.
  2. Weathering rate increases with increasing temperature, i.e. the chemical reactions take place faster with increasing temperature in the humid tropics.
  3. This high concentrations are also found in deeper layers, leading to an acceleration of weathering of the parent materials. Under humid tropical conditions, water percolates through the soil and takes dissolved weathering products along. As a consequence no reaction equilibrium will be reached and the rock disintegration continues.

 
Fig.6 Weathering reaction

Relative accumulation of sesquioxides

 
Fig.7 Banded Fe formation, South Africa.
( Source: www.iml.rwth-aachen.de/ Dmg/fe.html.)

  • The formation of sesquioxides (Al/Fe oxides) from primary minerals is a result of oxidation.
  • Oxidation is the chemical combination of an element with oxygen.
  • It is an important process where rocks contain Fe and Mn (2-divalent; for example pyroxene).
  • Fe exists in the primary mineral as divalent (ferrous form). If exposed to air and during soil formation Fe becomes oxidized (looses an electron) and becomes trivalent (ferric form).
  • In the air Fe reacts instantaneously with oxygen leading to the release of electrons. Iron changes from Fe2+ to Fe3+.
  • In the soil or water, the oxidation of Fe2+ to Fe3+ takes place in the lattice of the mineral. The silicate structure dissolves and Fe2+ oxidizes to Fe3+. The formed Fe2O3 is largely insoluble.
  • This changes (valence electrons, ionic radius) leads to a de-stabilization of the lattice structure of the mineral.
  • The increase of positive charges is partly counterbalanced by the release of other cations (K, Mg...) out of the lattice, and the lattice becomes instable.
  • Further disintegration follows and H+ -ions hydrolytically disrupt Si-O-M bindings in the lattice elsewhere.
  • Because many minerals contain Fe2+, the weathering of a rock is mostly accompanied by a brown or reddish coloration.
  • Fe-oxides and hydroxides are small particles. They often coat bigger particles like quartz. When Fe-oxide particles are removed by weathering the quartz grains become uncovered giving the soil the typical gray color of an albic E horizon (see fig.8).

 
Fig.8 Albic E horizon
( Source: http://soils.ag.uidaho.edu/soilorders/index.htm)

a. Oxidation of primary minerals

 
Fig.9 Example olivine

 
Fig.10 Example pyroxene

  • For further pictures on primary minerals please click  here