FERRIHYDRITE IN VOLCANIC ASH SOILS OF JAPAN

Eleven samples from a range of volcanic ash soils (Andisols or Andic Dystrochrepts) and one sample from a paddy soil formed in alluvium (Inceptisol) have been investigated for the presence of ferrihydrite using dissolution analysis and differential X-ray diffraction (DXRD). The presence of ferrihydrite in six samples, including that from the paddy soil, was established by DXRD. Except where low concentrations probably limited the distinguishability of some bands, five XRD bands (centered near 0.25, 0.22, 0.20, 0.17, and 0.15 nm) attributable to ferrihydrite were observed. The detection limit for ferrihydrite using DXRD was about 5-10% depending on the way XRD patterns were obtained and the nature of the soil matrix. Five of the samples with ferrihydrite concentrations too low to be identified by DXRD were also studied using Moessbauer spectroscopy. Changes in the Moessbauer spectra following acid-oxalate extraction were consistent, qualitatively, with the presence of ferrihydrite and, quantitatively, with the concentrations of ferrihydrite indicated by acid-oxalate-extractable Fe (Fe0) values. Concentrations estimated from Fe0 values were 1-16% for the volcanic ash soils and 5% for the paddy soil. This work supports the use of Fe0 values to indicate and estimate ferrihydrite in soils, and provides further evidence that ferrihydrite is commonly formed in volcanic ash soils and often occurs in association with allophane.