Abstract
Four Fe-rich deposits, two occurring at springs, one on boulders in a stream and the fourth in a stream bed on andesitic volcanoes in the North Island, New Zealand, have been investigated by mineralogical techniques. They have poorly-ordered structure with compositions intermediate between those of ferrihydrite and hisingerite. Electron microscopy revealed solid spheres of 30 Å dia which formed 0.1-0.5 μm aggregates. Surface areas, as measured by ethylene glycol monoethyl ether, were close to 600 m 2/g. X-ray diffraction gave weak patterns similar to both ferrihydrite and hisingerite. Infrared absorption showed a shift in the Si-O stretching band from 965 to 1020 cm −1 with decreasing amounts of Fe and increasing amounts of Si. These frequencies imply the presence of Fe-O-Si bonds in these materials. Desilication, by treatment with KOH in the laboratory, resulted in a conversion of these intermediates towards ferrihydrite.
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