WEATHERING ENVIRONMENTS AND OCCURRENCE OF IMOGOLITE ALLOPHANE IN SELECTED ANDISOLS AND SPODOSOLS

Imogolite and allophane have been found in various soil environments, but most commonly occur in Spodosols and Andisols. Controversy exists regarding the origin of these short-range-order aluminosilicates, especially in Spodosols. The proto-imogolite theory suggests that imogolite forms in the surficial mineral horizons and is subsequently transported to the B horizons. In contrast, evidence from analyses of soil solutions collected from Spodosols in the Washington Cascades shows that Al is migrating primarily as an organic complex, with no proto-imogolite detected in the percolating solutions. The Andisols we studied in Japan also show an absence of imogolite/allophane in the A horizons and an abundance of imogolite in the B horizons. Extremely low levels of Al are transported from the A to the B horizons, indicating that imogolite was not translocated from the overlying A horizons. The occurrence of imogolite/allophane in the B horizons of both the Spodosol and Andisol is best explained by in situ formation. Indeed, soil solution composition from the Spodosol indicates that imogolite is unstable in the E and Bhs, but stable in the Bs, BC, and C horizons. The B horizons containing imogolite in the Spodosol and Andisol have a similar weathering environment characterized by non-Al-complexing inorganic proton donors (e.g., H2CO3, HNO3), low concentrations of dissolved organic C, a pH between 5 and 6.5 that promotes Al polymerization, and Si concentrations adequate to support the stability of imogolite. In situ formation of allophane and imogolite is proposed for both the Spodosol and Andisol.