Formation and stability of imogolite in a tephritic Spodosol, Cascade Range, Washington, U.S.A

Chemical equilibrium modeling was used to examine the stability of imogolite in a Spodosol influenced by volcanic ash in the Cascade Mountains of Washington State. Imogolite was found to be a major component of the clay fraction in the Bs and C horizons of these soils. Soil solutions were collected during the fall, winter and spring from the major genetic horizons. Following fractionation of Al into inorganic and organic complexes, saturation indices for various minerals were calculated. Weathering in the E and Bhs horizons was dominated by organic acids and Al transport occurred primarily as Al-organo complexes. Soil solutions in these horizons were highly undersaturated with respect to imogolite solubility. Therefore, imogolite sols would not be expected to form in these horizons and participate in transport of Al to the lower soil horizons. Soil solutions from the Bs and C horizons appeared to be in equilibrium with both the hydroxy aluminous interlayer of 2:1 layer silicates and imogolite. The hydrous aluminous interlayer appeared to control Al activities while imogolite regulates H 4SiO 4 activities. This pattern suggests that the availability of soluble H 4SiO 4 may limit the formation and dissolution of imogolite. Imogolite is a metastable mineral in the Bs and C horizons and its formation may occur under the present weathering environment which is dominated by H 2CO 3.