A study on the interaction between ferric ion and silicic acid in hydrosphere： Si-containing ferruginous deposits formed in neutral hot spring waters

Five ferruginous deposit samples formed from neutral hot springs were analyzed to determine whether they consisted of a mixture of silica, hydrous iron oxide or iron silicate by differential thermal analysis （DTA）, infra- red （IR） spectroscopy, powder X-ray diffraction （XRD）, and 57Fe M6ssbauer spectroscopy. The Si/Fe atomic ratios of the deposits ranged from 0.25 to 0.45, and were smaller than those of hisingerite （1-2）, but apparently close to those of siliceous ferrihydrite （0.25-0.5）. Si was confirmed to be present as monomeric or oligomeric silicate from the Si-O stretching vibration frequencies on the IR spectra. Judging from the results of DTA, which minerals starting to produce after heating, and a relationship between Si-O stretching vibration frequency and Si/Fe atomic ratio pro- posed by Henmi et al. （1981）, all the deposits in this study were concluded to be mixtures of various siliceous ferri- hydrites with low and high Si/Fe atomic ratios. Moreover, by comparing the chemical properties of hot spring wa- ters, the formation conditions of siliceous ferrihydrite were also discussed.