Influence of Manganese Oxide Minerals on the Formation of Iron Oxides1

G. S. R. Krishnamurti and P. M. Huang
Department of Soil Science, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0W0, Canada
1 Contribution No. R. 568, Saskatchewan Institute of Pedology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.

Abstract: The influence of manganese oxide minerals (cryptomelane, hausmannite, and pyrolusite) on the formation of iron oxides was studied in the FeCl2-NH4OH system at different Mn/Fe molar ratios (0, 0.01, 0.1, and 1.0) and pHs (3.0, 4.0, 5.0, and 6.0) by X-ray powder diffraction, infrared absorption, transmission electron microscopic, and chemical analyses. In the absence of Mn minerals, lepidocrocite (γ-FeOOH) precipitated at pHs 5.0 and 6.0; however, no precipitate formed at lower pHs. All the Mn minerals studied promoted the precipitation of iron oxides and oxyhydroxides. In the presence of Mn oxides, Fe2+ was oxidized to Fe3+, which hydrolyzed and precipitated as noncrystalline and/or different crystalline iron oxides and oxyhydroxides, depending on the nature of the Mn oxides present in the system. Simultaneously, Mn2+ was detected in solution after the reaction by electron spin resonance spectroscopy. The presence of cryptomelane and hausmannite resulted in the formation of åkaganeite (β-FeOOH) and magnetite (Fe3O4), respectively. Thus, the effect of Mn oxides on the formation of Fe oxide minerals in the weathering zone merits attention.

Key Words: Åkaganeite • Cryptomelane • Feroxyhyte • Hausmannite • Iron oxides • Manganese • Oxidation-reduction • Pyrolusite • Synthesis

Clays and Clay Minerals; October 1988 v. 36; no. 5; p. 467-475; DOI: 10.1346/CCMN.1988.0360513
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)