Mossbauer spectra of proto-ferrihydrite at 77 K and 295 K, and a reappraisal of the possible presence of akaganeite in New Zealand soils

Abstract

The Mossbauer spectra of a synthetic proto-ferrihydrite and a synthetic aluminous proto-ferrihydrite are similar broad paramagnetic doublets at both 77 K and 295 K, with Ä ~ 0.75 mm s-1 and ä ~ 0.33 mm s-1 (295 K), ~ 0.43 mm s-1 (77 K). Each spectrum can be fitted by two Lorentzian doublets, having Ä ~ 0.60mm s-1, 0.95 mm s-1, but the iron nuclei probably exist in a range of very similar sites, related to the poorly ordered nature of the material, rather than in two specific sites. An earlier assignment of akagankite as one of the iron-containing minerals in some soil and soil concretion samples, from Mossbauer evidence, is now considered to be an error. Those peaks attributed previously to akagankite can equally well be accounted for in terms of ferrihydrite phases, and this is supported by evidence from other techniques.