Chemical and Thermal Characteristics of Allophane in Andosols of the Tropics

Abstract Profile characteristics, chemical properties, and clay mineralogy of volcanic ash soils in the tropics were studied. The soils were classified as andosols or andepts. The properties studied varied somewhat depending on the pedogenic age. The younger soil had an AC profile and contained amorphous material, tentatively called pro‐allophane, mixed with pedogenic intergradient vermiculitic clay, especially in the subsoil. The older soils had thicker AC or ABC profiles with clay fractions dominated by allophane. Whereas pro‐allophane was inverted into mullite at 800C, allophane showed the normal thermal reaction without being inverted into mullite. Titration curves of the clay fractions also indicated differences in chemical behavior.