Phase transformation of soil iron minerals for identification by thermomagnetic analysis

Phase transformation of goethite, hematite, and lepidocrocite was investigated by thermomagnetic analysis under oxidative and reducing condition. It is shown that the minerals reveal clearly diverse thermomagnetic behaviors. Hematite under oxidative condition does not change the magnetization, while under reducing condition, the magnetization of hematite increases significantly. The starting temperature of hematite to magnetite reduction is about 360 °C. Goethite under oxidative conditions has two changes in magnetization – the first at 50-100 °C is attributed to dehydration of the sample, the second at 270 °C indicates goethite to hematite transformation. Under reducing condition, goethite starts to transform to magnetite at 260 °C that is 100 °C lower than the temperature of hematite to magnetite reduction. Lepidocrocite under oxidation condition transforms to maghemite at 220 °C resulting in an increase in magnetization. Maghemite in turn transforms to hematite at 400 °C. Reducing condition leads to lepidocrocite transformation to magnetite. Obtained data can be useful for iron oxide and hydroxide identification in soils and sediments.