EPR characterisation of iron in various clay minerals : Montmorillonites and Layered Double Hydroxides

The local environment of Fe3+ in natural clays has been analysed by using electronic paramagnetic resonance (EPR) and Mossbauer spectroscopies, in addition to classical methods of investigation. For various raw montmorillonites, the X-band-EPR (approximate to9.5GHz) allows the identification of structural Fe3+ and other phases (iron oxides and hydroxides), as well as other defect species, (E' centres in quartz, 3d-elements...). We also used high frequency EPR (95GHz and 285GHz) and Mossbauer spectroscopies to obtain accurate values of the fine structure parameter D, which is sensitive to the local ordering. Distribution laws P(D), correlated with the distribution of distortions of iron sites, were deduced from this analysis. The electric field gradient model, appropriate for amorphous or glassy solids was not suitable for simulations of P(D) in clays. The experimental laws P(D), obtained from the distribution of Mossbauer quadrupole splittings, reproduced correctly the EPR spectra. These distributions are narrower than in glasses, due to the lamellar organisation of clays and suggest the presence of only one family of distortions from octahedral symmetry In order to avoid the paramagnetism of secondary phases or species (quartz, Mn2+, etc), we have studied similar synthetic lamellar compounds. Layered double hydroxides, which are anionic clays, were selected for this comparison. X-band spectra of compounds with various Fe3+ molar contents, show a wide absorption around g=2 and no significant absorption near g=4. Iron is in a more symmetrical situation than in natural clays and strong spin-spin interactions were observed. Multi-frequency EPR and Mossbauer spectroscopies at room and low temperatures, were used to describe the actual nature of couplings between d-elements.