The clay mineralogy and chemistry were determined for five pedons derived from tephra and volcaniclastic materials in northern California. These soils are believed to have formed under vegetation and climatic conditions similar to those at present; coniferous vegetation and a xeric soil moisture regime with approximately 1000 mm of annual precipitation. The five pedons showed a surface enrichment of organic carbon (3.6-6.7%), an acidic to neutral pH (5.3-6.9), and a generally low base saturation (10-40%). The high concentrations of free iron oxides (2-10% Fe) and clay (20-50%) indicate a relatively high degree of weathering. Clay mineralogy was dominated by imogolite and/or halloysite with lesser amounts of gibbsite and Al-humus complexes. Concentrations of Si in solutions equilibrated with the soils ranged between 10-3.85 and 10-3.45 M. Solubility calculations and data from the literature indicate that imogolite or Al-rich allophane should be preferentially formed at these Si concentrations. However, there was no relationship between the Si concentration and the predominance of imogolite versus halloysite. The coexistence of gibbsite, imogolite, and halloysite may be due to their differential formation as the soil solution Si concentrations fluctuate between the intense winter-spring leaching period and the pronounced summer period of desiccation and/or to the transformation of amorphous products to more crystalline minerals upon desiccation. In addition, the summer desiccation has led to the dehydration of halloysite in the surface horizon while halloysite in the lower horizons remains hydrated. Classification of the pedons according to Soil Taxonomy indicates an Andisol-Inceptisol-Alfisol transition that results from the transformation of active forms of Al and Fe, such as imogolite and ferrihydrite, to more highly crystalline phases. The two pedons dominated by imogolite were classified as Andisols, while the remaining three pedons were dominated by halloysite and did not meet andic soil property criteria because of a deficiency of active Al of active Al and Fe.
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