• Sesquioxides are weathering products of Fe/Al-rich primary silicates. They form through oxidation (chemical weathering). The secondary clay minerals weather farther and loose all of its Si and most of the other ions with exception of Al and Fe.
  
• Sesquioxides are non-silicate clay minerals or colloids. They consist of modified octahedral sheets with either Fe³⁺ or Al³⁺ in the cation position (central cation). They have neither tetrahedral sheets nor silicon in their structures.
  
• They posses little or no isomorphous substitution, i.e. little negative surplus charge and hence little cation exchange capacity ( CEC) .
  
• At the surface they posses covalently bound OH-ions which may cause strong adsorption of certain anions like phosphate.
  
• Gibbsite [Al(OH)₃]: most common Al-Oxide in highly weathered tropical soils such as Ferralsols (see fig.1).
  
• Further reading see: " http://www.gly.uga.edu/schroeder/geol6550/CM06.html"
  

Fig.1 Gibbsite

( Source: Brady and Weil 2002, p. 331.)

• Ferrihydrite (Fe₂O₃ . nH₂O): in temperate regions, yellow-brownish color.
  
• Goethite (FeOOH): brownish soil color, very abundant.

  Fig.2 Goethite

  ( Source: http://webmineral.com/data/Goethite.shtml)

  
  
• red hematite (Fe₂O₃): posses Fe²⁺ instead of Al³⁺ in the O-sheet. Formation: through dehydration of ferrihydrite; gives redder colors to well-drained soils, especially in hot, dry climates.

  Fig.3 Hematite

  ( Source: http://www2.volstate.edu/svinson/geo100/minerals.html)