Influence of organic matter on the iron oxide mineralogy of volcanic soils

Samples of soils developing on volcanic materials that represent about 70% of the agriculture land area in Chile were studied by chemical methods, powder X-ray diffractometry (XRD) and Mossbauer spectroscopy. The soil samples were collected from native areas of Ultisols, Andisols, and Andisols presenting seasonal cycles of poor drainage, at depths between 0-15 cm and 15-30 cm from the top of the profile. The degree of crystallinity of iron oxides was evaluated from the ratio of amounts of Fe extracted with oxalate-oxalic acid (Fe-OX) and with bicarbonate-citrate-dithionite mixtures (Fe-DBC) Fe-OX/Fe-DBC. Results show that the specific saturation magnetization of soils and crystallinity of iron-rich minerals increase with decreasing organic matter contents of soils. From XRD and Mossbauer data, ferrihydrite (ideal formula, Fe5HO8 . 4H(2)O) could be the precursor either of the soil magnetite (Fe3O4) or hematite (alphaFe(2)O(3)). Older soils were found to be in an intermediate stage regarding the transformation of magnetite to maghemite (gammaFe(2)O(3)).