Natural “amorphous” ferric hydroxide

Rusty ferruginous precipitates deposited from soil-borne waters (in drainage ditches, from springs) at various localities, contain a ferric hydroxide rich in carbon and adsorbed water. It has up to 75% dithionite soluble Fe2O3, of which between 90 and 100% is oxalate soluble IR spectrograms do not show Fe-OH features in the OH stretching and bending range. X-ray diffraction reveals very broad lines at about 2.5 and 1.5 Å and somewhat sharper lines at 2.22, 1.97 and 1.71 Å, which are characteristic of ferrihydrite (name proposed by Chukhrov et al., 1972). These deposits are found in areas where water has percolated through acid soils rich in low molecular weight organic compounds. Furthermore, as similar material could be prepared in the laboratory by bacterial or H2O2 oxidation of ferric citrate solutions, it was concluded that the natural substance is formed by microbial decomposition of soluble iron—organic complexes. Transformation experiments suggest that aging under conditions corresponding to a humid temperate climate causes conversion to goethite. This aging process is greatly retarded by organic and other compounds retained by the hydroxide. No evidence of hematite formation could be found after 2 weeks at 70°C.