Thermally induced transformation of magnetic minerals in soil based on rock magnetic study and Mössbauer analysis

The purpose of this study is to detect thermal transformations of magnetic minerals occurring during thermomagnetic susceptibility analysis based on the example of chernozem soil samples from Ukraine. Rock magnetic methods such as thermal decay of saturation remanent magnetization (SIRM), hysteresis loops and monitoring of magnetic susceptibility (k) during heating from temperature of liquid nitrogen (−196 °C) up to 700 °C were used as indicators of magnetic mineralogy, grain size and concentration. In addition, the changes in mineralogy caused by heating were monitored by Mössbauer analysis. The samples were taken from the topsoil and from the loess layer of the unpolluted chernozem profile from the Homutovsky Steppe (East-South Ukraine). SIRM(T) curves and Mössbauer analysis revealed maghemite, hematite and goethite in the topsoil and in the loess. Low- temperature susceptibility experiments showed superparamagentic (SP) – like behaviour in the topsoil and paramagnetic – like behaviour in the loess. The specimens were heated during susceptibility measurements in KLY-3 device from room temperature up to subsequent increasing temperatures: 250 °C, 400 °C, 500 °C, 600 °C and 700 °C. After heating to particular temperature, low-temperature experiments, SIRM(T) curves, hysteresis loops and Mössbauer analysis were performed. Additionally, the sample of topsoil and the sample of loess were heated several times to the increasing temperatures. Mössbauer analysis showed increase of Fe2+ ions indicating reduction process during heating. We suggest that in the topsoil, the prevailing transformations are inversion of hydroxides such as goethite and ferrihydrite to magnetite/maghemite which occur at temperature 200–450 °C, whereas in the loess reduction of lithogenic hematite to magnetite at temperature above 600 °C plays important role. The topsoil and loess do not differ significantly in such pedogenic parameters as pH, total iron content Fet or free iron Fed. The main differences are in humus and amorphous iron content Feo.