EPR STUDY OF MONTMORILLONITES FROM SEVERAL DEPOSITS

EPR spectroscopic and magnetochemical investigations of natural montmo rillonites originating from different deposits reveal the nature and l ocalization of paramagnetic centres contained in the samples. The EPR spectra show that all montmorillonites investigated especially contain Fe3+ ions. These ions occupy lattice sites of the montmorillonite str ucture of maximal and, to a larger extent, slight to moderate rhombic deformation in the region g' almost-equal-to 4 (g' = effective g-facto r). Effective magnetic interactions (dipole-dipole and exchange intera ctions) occur between the Fe3+ ions located in Al3+ sites of the montm orillonite structure and those located in the interlayer space or on t he surface of the crystallites. Analysis of the EPR spectra and of the dependence of magnetic susceptibility chi values on the strength of t he external magnetic field indicates that magnetically ordered phases are only rarely formed. Additionally, the EPR spectra of montmorilloni tes from Jelsovy Potok, Askangel, Borsa, and Pyzhevskii display hyperf ine structures of isolated Mn2+ ions in the g' almost-equal-to 2 range . In the case of the montmorillonite from Jelsovy Potok these Mn2+ ion s are localized in moderately deformed octahedral sites of the montmor illonite structure. Mn2+ ions in samples of montmorillonites from Aska ngel, Borsa, and Pyzhevskii are preferentially located in Ca2+ Sites o f calcite impurities. A resonance at g' almost-equal-to 2.009 indicate s the existence of paramagnetic oxygen defect centres in the samples. Measurements of the static magnetic susceptibility disclose strong par amagnetism for the samples from Borsa, Branany, and Taganskii. In cont rast, those from Askangel, Jelsovy Potok, and Pyzhevskii are only weak ly paramagnetic.