Ferrihydrite and allophane in four Andepts from Hawaii and implications for their classification

Abstract The mineralogy and composition of the Waimea, Kukaiau, Akaka and Hilo soils on the island of Hawaii were studied using methods which included Moessbauer spectroscopy and chemical dissolution. The Hilo soil (which experiences 3000 mm/year rainfall) and the Akaka soil (3800 mm/year rainfall) both contain large amounts (about 25%) of iron oxides of short-range order, together with other iron oxides, gibbsite and allophane. The iron oxides of short-range order have characteristics similar to those of the reactive mineral ferrihydrite, which is known to occur in a wide range of soils. These characteristics, however, in two respects (X-ray diffraction and acid-oxalate solubility) do not rule out the presence or admixture of the much less known feroxyhite, another closely related iron oxide of short-range order. Most samples from these two soils dry irreversibly to give hard aggregates, a property probably related to the presence of the iron oxides of shortrange order. Both the Hilo and the Akaka soils are classified as Hydrandepts in the current USDA Soil Taxonomy. In the new proposed order of Andisols these two soils would become Hydrudands. It is desirable to recognise the presence of short-range-order iron oxides in these soils. The Kukaiau soil (currently classified as a Dystrandept) which occurs in a lower rainfall zone (1500 mm/year) contains about 25% allophane and lesser amounts of ferrihydrite, other iron oxides and gibbsite. The Waimea soil occurs in the lowest rainfall zone (1000 mm/year) and the dominant mineral is halloysite but some allophane, ferrihydrite and hematite are also present. This soil is currently classified as a Eutrandept. Under the Andisol proposal the Kukaiau and Waimea soils would be a Hapludand and a Haplustand, respectively. Quartz, mica and other layer silicates were present in all four of the soils suggesting that there have been additions of eolian material.