56 jenny. ORIGIN OF SOILS [Ch. 2 



further OH ions to assume its preferred octahedral configuration of 

 hydroxyl ions. Coupled with the absence of stabilizing intertetra- 

 hedral K ions, the surface layer becomes unstable and polyhedra peel 

 off. 



Tetra- and octahedra liberated from feldspars and other minerals 

 aggregate among themselves to form clusters of colloidal size, namely, 

 colloidal silica, colloidal aluminum hydroxide, and mixtures, the col- 

 loidal alumino-silicates. Although young colloidal particles are prob- 

 ably amorphous, upon aging the polyhedra orient themselves to defi- 

 nite crystal lattices such as cristobalite, diaspore, bauxite, goethite, 

 gibbsite (hydrargillite), clay minerals of the montmorillonite-beidel- 

 lite-nontronite group, clay minerals of the hydrous mica-illite-vermicu- 

 lite group, and clay minerals of the kaolinite-halloysite group. (Com- 

 pare Chapter 25.) Fe ions of the original minerals tend to be excluded 

 from incorporation into clay particles. They form the stable oxyhy- 

 droxides, such as limonite, goethite, and hematite. 



It does not appear necessary that primary minerals undergo com- 

 plete breakdown into individual polyhedra. Fragments of chains and 

 sheets of tetra- and octahedra may recombine. Sometimes mere ionic 

 substitutions bring about fundamental alterations, such as the conver- 

 sion of biotite into vermiculite. 



The specific conditions which control the formation of the various 

 clay minerals in soils are not well known. Long ago Mattson showed 

 that the Si02:Al 2 03 ratio of colloidal alumino-silicates is influenced 

 by the pH of the medium in which they are formed. As the reaction 

 changes from alkaline to acid, the ratio increases. In general, as 

 stated by Ross and Hendricks (1945) , "Alkaline feldspars and the micas 

 tend to alter to kaolin minerals, whereas ferromagnesian minerals, cal- 

 cic feldspars, and volcanic glasses commonly alter to members of the 

 montmorillonite group." The roles played by climate, time of weath- 

 ering, vegetation, and drainage conditions have not yet been eluci- 

 dated. 



Formation of Claypan Soils and the Migration of Colloidal Clay 

 Particles 



Numerous soils exhibit accumulation of colloidal clay particles in 

 the B horizon (Fig. 1). Extreme cases of clay accumulation produce 

 soils known as claypan soils. Their B horizons may be so tight and 

 sticky that they are nearly impermeable to water and air. 



The source of clay in the B horizon is twofold. First, clay is formed 



