Thermal generation of ferromagnetic minerals from iron-enriched smectites |
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| Authors: | A M Hirt A Banin A U Gehring |
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| Institution: | Institut für Geophysik, ETH-Hönggerberg, 8093 Zürich, Switzerland;Department of Soil and Water Sciences, The Hebrew University, Rehovot 76100, Israel;Department of Soil Science, University of California, Berkeley, CA 94720, USA |
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| Abstract: | In recent years remagnetization of orogenic belts has been explained by fluid migration through rocks undergoing deformation. A laboratory study of remagnetization is presented in which varying amounts of iron (0-13.5 weight per cent Fe2O3) are adsorbed onto smectite surfaces. All smectite samples contain structural Fe (III) which is located in octahedral sites and is thermally stable up to 700  C. An increase in the amount of iron adsorbed onto the clay surface leads to the formation of ferric nanophases in which parts are magnetic. Mineralogical changes that occur during thermal treatment between room temperature and 700 C were monitored using electron spin resonance (ESR), bulk susceptibility, acquisition of isothermal remanent magnetization (IRM) and Curie temperature analysis. After heating the samples to 250 C, a new ferrimagnetic phase is created as indicated by ESR and IRM acquisition. ESR spectra, IRM acquisition and Curie analyses suggest that magnetite is the predominant phase that is being created. These grains continue to be created and grow with heating up to 500 C. Above this temperature a decrease in the intensity of the IRM at 1T suggests that the phase is being transformed into haematite. The thermal experiments on iron-loaded smectites show that surface-induced processes can lead to the formation of new magnetic minerals under conditions characteristic of low-grade metamorphism. |
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| Keywords: | Key words: magnetite mineralogy remanent magnetization |
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