Structural studies on the manganese for iron substitution in the synthetic goethite–jacobsite system

Abstract

A set of synthetic oxides were prepared from Fe(III) and Mn(II) nitrate solutions in alkaline media with a Mn/(Mn+Fe) ratio up to 0.5. The structural characterization of the solid phases obtained was carried out by X-ray diffraction (XRD). XRD analyses showed that in preparations with a Mn mol fraction (xMn) lower than 0.14 (samples G1–G4) a Mn-substituted goethite was the only crystalline phase present. A second phase, jacobsite appeared together with goethite in the range 0.45≥xMn≥0.14 (samples G5–G8). For xMn=0.55 the majority and minority phases detected were jacobsite and hausmannite. Atomic and cell parameters for the nine samples synthesized were obtained by the Rietveld refinement of the XRD data. The relative proportion of the different phases in the system was also estimated by the refinement program and indicates that at xMn=0.28, jacobsite and goethite-like phases are in the same proportion. Refined cell parameters show that the unit cell in the goethite-like phase is distorted as a function of xMn. Deviations from the Vegard are observed and suggest a non-homogeneous substitution of Mn in the goethite phase.