The Iron and Titanium Minerals in the Titaniferous Ferruginous Latosols of Hawaii

Abstract

Titaniferous Ferruginous latosols are an important group of Hawaiian soils. They have developed by pedogenetic weathering of the volcanic materials of basic and ultrabasic lava flows and their associated pyroclastic materials, under climatic conditions having definite alternating dry and wet seasons. The annual rainfall range for these soils is 30 to 60 inches. Under the native and undisturbed vegetation, these soils have a profile of very friable silt material with very little horizon differentiation. When exposed to dehydration by removal of the protective canopy cover of the forest, as occurs after forest fires, a tremendous change occurs in the soil profile morphology. The change is greater than the morphological differences which normally occur between the profiles of the Great Soil groups. A surface indurated horizon develops in which the bulk density and particle density have an approximate two-fold increase. The mineralogical changes are substantial. The amorphous hydrated titanium and iron oxides are converted into good crystalline forms of anatase, rutile, pseudo-brookite, titanohematite and titanomaghemite. In some cases these minerals develop from the weathering of titanomagnetite-ilmenite mixed crystals. The unusual characteristic of these soils is the apparent movement of the colloidal material in a very short space of time, which results in an accumulation of titaniferous minerals in the indurated surface horizon and the accumulation of amorphous silica giving weak X-ray diffraction patterns of alpha quartz on the very surface of the soil. Rutile is also identified along with the silica at the surface. Aluminum oxides are removed to the lower horizons and in some cases accumulated as irregularly shaped gibbsite nodules in scattered pockets below the clay horizon.