Al-substituted goethite—An indicator of pedogenic and other weathering environments in South Africa

Abstract

Over 200 goethite-containing samples from widely diverse environments along the eastern seaboard of South Africa were examined and described, mainly by X-ray powder diffraction (XRD). Aluminium-substituted goethites ranging from 0 to about 32 mole % AlOOH were identified mainly by XRD line-shift and chemical extraction. They occur in every soil zone and major soil type in one or other form from sources such as: (1) non-hydromorphic- highly weathered (strongly acid soils, ferruginous bauxites, saprolites) and moderately weathered (moderately acid soils) environments; (2) hydromorphic environments (mottles, concentrations, pipe stems, crusts, ferricretes, geodes); and (3) crusts in calcareous environments. These goethites, formed in different climatic and weathering environments were grouped according to their amount of Al-substitution and crystallinity (as determined by XRD line broadening). Al-substitution in goethite formed in hydromorphic, moderately acid soils and calcareous environments was generally low (0–15 mole %). Goethite formed in non-hydromorphic highly weathered environments was generally high in Al-substitution (15–32 mole %) and increased in the following order: saprolites and strongly acid, freely drained soils < ferruginous bauxites < strongly acid soils overlying plinthite. This study showed that the degree of Al-substitution and crystallinity of goethites may reflect the environment in which they have formed and serve as an indicator of soilforming processes.