Occurrence and properties of lepidocrocite in some soils of New Zealand, South Africa and Australia

Abstract

Fifty-one samples, collected from 26 sites in New Zealand, South Africa and Australia, were tested for the presence of lepidocrocite (ã-FeOOH) and goethite (á-FeOOH). The samples were predominantly orange-coloured mottles, bands, crusts and pipestems from hydromorphic soils, but also included a placic horizon, iron-rich precipitates from water courses, altered pyrite cubes, and geode-like features in weathered saprolites. The iron oxides were identified and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Lepidocrocite was identified in 49 of the samples, and in 44 was present in concentrations exceeding I%, with the highest being approximately 70%. Crystallinities of the lepidocrocite were assessed from the widths and positions of XRD peaks, and, in some cases, from TEM. Goethite was also present in most samples, and predominated over lepidocrocite in some. In all three countries, the most common occurrence of lepidocrocite was associated with the gleyed soil materials commonly found in wet, poorly drained pseudogley soils (mostly 'humic slope gley') of humid temperate climate. Under these climatic conditions the high degree of water supply, relatively lower soil temperatures, lower evaporation and slower water movement caused reductomorphic conditions. At these sites soil iron oxides are reduced and, on reoxidation, lepidocrocite and goethite are generally formed. The colour generally associated with lepidocrocite in iron-rich segregations in hydromorphic soils is 7.5YR6-7/4-8 Lepidocrocite was also identified in all of the other samples mentioned above. A study of the relative proportions of lepidocrocite and goethite, and the crystallinity of these associated minerals in the various morphological concentrations (profile trends), suggests that their formation is strongly governed by soil microenvironmental factors (pH, Eh, and ionic environment). However, climatic and pedogenic factors such as podzolization and high organic matter contents may modify these mineral phases or induce further transformation. These findings indicate that lepidocrocite occurs in a remarkably wide range of materials and weathering environments. Moreover, the morphology, crystallinity, differential XRD line broadening and line shift of the lepidocrocite, and the commonly associated goethite also vary markedly in the respective materials. This suggests that other factors associated with particular weathering environments, such as those involved in pseudomorphous alteration of pyrite and the influence of chloride ions, may be more important in the formation of lepidocrocite than the conditons associated with hydromorphy. The morphology and characteristics of lepidocrocite and goethite, together with field associations, appear to be useful indicators of soil genesis.