Iron oxide minerals in soils of the Ha'apai group, Kingdom of Tonga

Abstract

The Group consists mainly of an archipelago of low coral atolls. Soils are formed in the andesitic tephra which overlie the coral. Accessions of marine salts are high and give rise to high soil pH values (> 7) and high concentrations of exchangeable cations. The soils are dark reddish brown to reddish brown in color, with limited color differentiation with soil depth, and they are not hydromorphic. The results of study of whole soils and size fractions by chemical extraction, X-ray diffraction, differential thermal analysis, Moessbauer spectroscopy, and by measurement of variation of magnetic attraction following heating to various temperatures, lead to the following conclusions: iron oxide minerals comprise 7-12% by weight of the soils; the predominant clay size iron oxide is very poorly crystalline aluminum-substituted lepidocrocite (.gamma.-Fe0.8Al0.2OOH); ferrihydrite is probably also present in the clay fraction; lepidocrocite and ferrihydrite together account for 50-60% of the total Fe in the soils studied; magnetite, probably partially oxidized towards maghemite, is present, particularly in sand and silt fractions, and accounts for about 10% of the total Fe in the soils. The remainder of the total Fe occurs in unweathered minerals in the silt and sand fractions and in the structure of halloysite in the clay fraction. The common soil iron oxides, goethite and hematite, were not detected. Although the occurrence of lepidocrocite is usually linked with hydromorphic soil conditions, this and other reports suggest that it is also found under non-hydromorphic conditions.