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Occurrence Of Lepidocrocite And its Association With Goethite in Natal Soils¹

ABSTRACT

Hydromorphic soils (pseudogleys) in a highland-montane climate of Natal (South Africa) contain appreciable amounts of lepidocrocite. Its occurrence appears to be restricted to imperfectly drained lower positions on southern (cooler) slopes at an altitude between 1,300–1,900 m. In these positions, Fe is reduced under anaerobic Conditions leading to lepidocrocite formation on reoxidation. Goethite is closely associated with lepidocrocite and increases on approaching better aerated microzones such as root channels and textural boundaries where it usually forms crusts. Both FeOOH forms were determined quantitatively by XRD. Simultaneous formation of the two Fe-oxides is inferred from synthesis experiments in which an increasing partial pressure of CO₂ in an O₂-CO₂ gas mixture to oxidize a FeCl₂ solution favored goethite at the expense of lepidocrocite. The crystallinity of lepidocrocite (as measured by XRD line broadening) decreased when approaching root channels. This is attributed to a decreased in pH and agrees again with synthesis experiments where the same effect was observed. Goethite is generally less crystalline than lepidocrocite.

¹ Contribution from the Dep. of Soil Sci., Univ. of Natal, Pietermaritzburg, Rep. of South Africa.

² Professor of Soils on study leave from the Institut für Bodenkunde, Techn. Universität, München, Freising 12, West Germany and Pedologist, Soil and Irrigation Res. Instit., Pretoria, located at Natal Region, P/B X9059, Pietermaritzburg, Natal, Rep. S. Africa, respectively.

Received for publication October 13, 1976. Accepted for publication June 6, 1977.

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