Abstract
A general study of the oxidation states of iron and manganese in selected polymetallic oxide/oxyhydroxide crusts was undertaken by means of 57Fe Mössbauer X-ray and photoelectron spectroscopies, X-ray diffraction, and electron spin resonance. Spectroscopic analyses allowed the differentiation of these crusts from their origins: “hydrogenous” or “hydrothermal”. This paper shows that polymetallic oxide/oxyhydroxide crusts with high Mn:Fe ratios (⩾34.4) contain manganese in the form of todorokite. When the Mn:Fe ratios in the crusts are ⩽ 1, spectroscopic data indicate the occurrence of Fe(III) and Mn(IV) in the forms of goethite and δ-MnO 2, respectively. In addition, Mn(III) ions incorporated inside the goethite structure to give α-Fe(III) 1−xMn(III) xOOH (where x represents the molar fractions: x=Mn/(Mn+Fe)) are strongly suspected in these ferromanganese deposits. This problem is addressed in the present work from crystallographic considerations.
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