Aluminous-Ferruginous Oxide Mineral Nodules in Tropical Soils

Abstract

Secondary oxide mineral nodules occur in the ferruginous, ferruginous bauxitic, and bauxitic soils of the Hawaiian Islands. The concentration of these oxide nodule aggregates which are larger than 2 mm ranges from 5 per cent in some ferruginous soils to as high as 85 per cent in the ferruginous bauxitic soils of the Halii family. The nodules are formed by the process of induration in which the hydrated amorphous hydroxide and oxides are dehydrated to their crystalline oxide form. The resulting indurated oxide nodule becomes an independent unit in the soil system with corresponding loss of effective surface even though the nodule may contain friable clay material in its interior. The typical nodule produced in the ferruginous bauxitic soil has a dense iron oxide layer (hematite and maghemite) and, in the interior, soft to hard, lighter-colored material which contains gibbsite.