Weathering environments and occurrence of imogolite/allophane in selected andisols and spodosols
Ugolini, F.C. | Dahlgren, R.A.
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 analysis of soil solutions collected from Spodosols in the Washington Cascades shows that Al is migrating primarily as in 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, 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 ad 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.
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