Abstract
The bonding state and distribution of Si in five natural samples and one synthetic sample of “coprecipitated” siliceous ferrihydrite have been studied by X-ray photoelectron spectroscopy (XPS). For the natural samples, values of the Si 2s peak binding energy indicate that three-dimensional polymerisation of SiO4 tetrahedra is, at most, only poorly developed. In contrast, the value for the synthetic “coprecipitated” sample suggests a significantly higher degree of three-dimensional SiO4 polymerisation. For all of the natural samples the Fe/Si atomic ratio determined by XPS is comparable to, or slightly larger than, the average bulk ratio, indicating that the Si in these samples is well dispersed throughout or that the outer layers of the ferrihydrite aggregates are slightly depleted in Si. For the synthetic sample the Fe/ Si XPS ratio is markedly smaller than the corresponding bulk ratio indicating surface precipitation of a Si-rich phase. Synthetic samples prepared by coprecipitation of Si and Fe may not be good models for natural siliceous ferrihydrites.
