Distribution of manganese in synthetic goethite

A series of Mn-goethites was synthesized at highly alkaline conditions. The samples were aged for 15 days at a final [KOH] of 0.3 M. Products were washed free from non-goethite phases using 3 M H₂SO₄. The bulk mineralogy of the samples was determined by X-ray diffraction and verified on selected individual crystals by electron diffraction. The samples had a relatively low magnetic susceptibility (300 ≤ MS ≤ 400 × 10^-9m³/kg). As revealed by total acid dissolution, the Mn mole fraction X_Mn ranged from zero to 0.125. Five samples (X_Mn: 0.025, 0.050, 0.077, 0.099, 0.125) were selected to investigate the variability of the X_Mn value in single goethite needles (crystals) by analytical electron microscopy (AEM) using rastered and spot analyses. Linear regressions of both as a function of total Mn yielded unit slopes and zero intercepts, indicating that acid dissolution gave the same results as AEM. Spot AEM, however, revealed significant variation of Mn distribution within individual crystals which argues in favour of Mn zoning in goethite. Inhomogeneous transformation of ferrihydrite to goethite may partly explain the Mn zoning.