Some cautions on the interpretation of Mossbauer spectra in mineralogical studies of volcanic soils

Room temperature Mossbauer spectra of bulk Chilean soil samples derived fro m volcanic materials (Ultisols and Andisols) are very complex and least-squ ares fitting not seldom leads to many different interpretations. In most ca ses the presence of commonly occurring iron oxides in those soils, namely m aghemite (ideal formula, gamma Fe2O3), hematite ( gamma Fe2O3) and goethite (alpha FeOOH) in Andisols, and maghemite, magnetite (Fe3O4), ferrihydrate (Fe5HO8. 4H(2)O) and/or goethite in the Ultisol can only be speculated. In the present paper we present X-ray diffraction and Mossbauer analyses of pa rticle size fractions (soil, sand, silt and clay), which were submitted to magnetic extraction (sand fractions) and subsequently to NaOH selective che mical treatment (silt and clay fractions), in laboratory procedures which w ere conveniently adapted to such volcanic soil-samples. When chemically tre ated soil separates of Ultisol are analyzed, the presence of partially oxid ized magnetite is confirmed, and the existence of hematite is also inferred . The magnetic fraction separation and chemical treatments are to be essent ial steps in sample preparation in the mineralogical assessment of Chilean soils derived from volcanic materials.