Abstract
The transformation of two natural siliceous ferrihydrites to hematite has been observed by X-ray diffraction, Moessbauer spectroscopy, weight loss and colour change. Three synthetic ferrihydrites (a 2-line pure sample, a 7-line Si-containing sample and a 7-line Ge-containing sample) have been used for comparison. The inhibitory effect of Si on the transformation, as observed previously, has been confirmed, and Ge has been found to exert a similar effect. Adsorption of soluble silicate onto the 2-line pure ferrihydrite also produced an inhibitory effect. It was possible to reduce OH: Fe ratios in ferrihydrites to the order of 1: 10 or less without an apparent phase change, and the essentiality of H2O and OW in the ferrihydrite structure is questioned. The results are consistent with a model in which Si-or Ge-hydroxy anions are bonded to, and bridge, the surfaces of crystalline domains within each primary particle of ferrihydrite.
