Thermally induced transformation of magnetic minerals in soil based on rock magnetic study and Mossbauer 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 degrees C) up to 700 degrees C were used as indicators of magnetic mineralogy, grain size and concentration. In addition, the changes in mineralogy caused by heating were monitored by Mossbauer 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 Mossbauer 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 degrees C, 400 degrees C, 500 degrees C, 600 C and 700 degrees C. After heating to particular temperature, low-temperature experiments. SIRM(T) curves, hysteresis loops and Mossbauer analysis were performed. Additionally, the sample of topsoil and the sample of loess were heated several times to the increasing temperatures. Mossbauer 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 degrees C, whereas in the loess reduction of lithogenic hematite to magnetite at temperature above 600 degrees C plays important role. The topsoil and loess do not differ significantly in such pedogenic parameters as pH, total iron content Fe-t or free iron Fe-d The main differences are in humus and amorphous iron content Fed. (C) 2009 Elsevier B.V. All rights reserved.