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The effect of clay minerals on the formation of goethite and hematite from ferrihydrite after 16 years’ ageing at 25°C and pH 4–7

¹ Lehrstuhl für Bodenkunde der Technischen Universität München, D-85350 Freising, Germany, and ² Agronomy Department, Purdue University, 1150 Lilly Hall, West Lafayette, IN 47907-1150, USA

^* E-mail: uschwert{at}pollux.edv.agrar.tu-muenchen.de

(Received 17 May 1999; revised 23 February 2000)

In soils and other weathering environments, hematite and goethite are usually formed in the presence of clay minerals. Two-line ferrihydrite was aged in the presence of six different clay minerals in aqueous suspension at 25°C and pH 4, 5, 6 and 7 for 16 years. The transformation into hematite/goethite mixtures was complete in the systems with gibbsite, kaolinite, illite and smectite, but incomplete in those with soil smectite and allophane. Soil smectite and allophane, as well as increasing pH, favoured hematite over goethite. The grainy and multi-domainic hematite crystals in the allophane system contained Si and Al (EDX) and showed a Si–O vibration at ~920 cm^–1 (FTIR). The unit-cell contraction due to structural Al was counteracted by structural OH, which was removed at 200°C. Heating also increased the magnetic hyperfine field at RT by 1.2–1.6 T. Hematite crystals with irregular shape and grainy interior have also been observed in soils.

KEYWORDS: weathering, soil, hematite, goethite, kaolinite, illite, smectite

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