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 was hed free from non-goethite phases using 3 M H2SO4 The bulk mineralogy of th e samples was determined by X-ray diffraction and verified on selected indi vidual crystals by electron diffraction. The samples had a relatively low m agnetic susceptibility (300 less than or equal to MS less than or equal to 400 x 10(-9) m(3)/kg). As revealed by total acid dissolution, the Mn mole f raction 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-M n value in single goethite needles (crystals) by analytical electron micros copy (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, reve aled significant variation of Mn distribution within individual crystals wh ich argues in favour of Mn zoning in goethite. Inhomogeneous transformation of Ferrihydrite to goethite may partly explain the Mn zoning.