Formation and properties of a continuous crystallinity series of synthetic ferrihydrites (2-to 6-line) and their relation to FeOOH forms

Ferrihydrite is a poorly crystalline Fe oxide of which 2-(XRD)line and 6-line varieties are commonly used in experiments, although species with intermediate numbers of peaks have been found in nature. To simulate nature, we synthesized two continuous series of ferrihydrites with between 2 and 6-7 peaks at room temperature in two different ways: (1) by varying the rate of hydrolysis of an Fe(NO3)(3) solution (HR series); and (2) by oxidizing an FeCl2 solution containing up to 73 mmol Si/L (Si series), both at pH 7. Mossbauer spectra at 4.2 K showed that the ferrihydrites of the FIR series had a constant magnetic hyperfine field (B-hf) at 4.2 K of 48.8 T whereas B-hf in the Si series dropped from 49.4 to 46.7 T as the Si content of the ferrihydrites increased from 0 to 74.7 g/kg of Si. Temperature scans between 4 and 170 K illustrate that the magnetic order breaks down at a temperature which is lower the higher the hydrolysis rate and the Si concentration in the ferrihydrite.