Formation of gibbsite in the presence of 2 : 1 minerals: an example from Ultisols of northeast India

There are two different Views regarding the genesis of gibbsite in tropical acid soils: (I) direct weathering of primary Al-silicate minerals; and (2) transformation through clay mineral intermediates. We investigated the gen esis of gibbsite in two representative Ultisols from northeastern India. Gi bbsite in these Ultisols appears to be the remnant of earlier weathering pr oducts of aluminosilicate minerals formed in a neutral to alkaline pedochem ical environment. The mere presence of gibbsite in these soils, therefore, does not indicate their advanced stage of weathering. The formation of typi cally rod-shaped and well-crystallized gibbsite in both the coarse and fine soil fractions in the presence of large amounts of 2:1 minerals indicates that the anti-gibbsite hypothesis may not be tenable in these tropical acid soils. A schematic model for the formation of gibbsite and kaolin in Ultis ols is proposed.