Aluminum in soil forms the structure of primary and secondary minerals, especially aluminosilicates,
such as feldspars, micas, kaolins, smectites, and vermiculites (275)
. As the soils continue to
weather (especially under conditions of high rainfall and warm climates), silicon is leached away,
usually as Si(OH)4 in solution, leaving aluminum behind in the solid forms of aluminum oxyhydroxides,
such as boehmite and gibbsite, as shown below (276)
The soils themselves become 'older,' more acidic, and more aluminum toxic and would be
classified as Oxisols or Ultisols.