ESR of clays

Electron spin resonance (ESR) spectroscopy has contributed significantly to the identification and characterization pf paramagnetic impurities associated with clays, Following a brief discussion of the general principles of the technique, a review is given of the application of ESR to the study of those paramagnetic species (chiefly iron and radiation-induced lattice defects) located either within the aluminosilicate structure or present as an external impurity phase. In recent years a number of relatively modern physical techniques have been increasingly utilized in investigations of structural and physicochemical properties of clay minerals. One of these, electron spin resonance (ESR) spectroscopy, has now become a significant tool in clay mineral research and a large number of papers have been wholly or partially devoted to the interpretation of ESR spectra arising from paramagnetic ions or radicals associated with clays. Though the ESR method is restricted to a certain range of paramagnetic species, it can yield a wide range of detailed information relating to: (a) the occurrence and location of paramagnetic substitutional impurities and lattice defects (trapped hole or electron centres) in clays; (b) the extent of structural disorder and nature of thermal phase transitions of clay minerals; (c) the nature of paramagnetic impurities associated with clay surfaces, including organic radicals and transition metal oxides and hydroxides; (d) the mobility of hydrated ions in the interlamellar space of expanding lattice clays utilizing transition metal ions or organic radical cations as 'spin probes'; (e) the distribution of lattice charge sites in clays; (f) the reactions of adsorbed molecules on the external and interlamellar surfaces involving the formation and stabilization of free radicals. It should not be forgotten, however, that natural materials are far from the spectroscopist's ideal of well-characterized, pure single crystals. Apart from work reported here on micas and vermiculites, most studies have been made on powder samples containing paramagnetic species either isomorphously substituted in the aluminosilicate lattice or present as a surface-adsorbed or dispersed impurity phase. In these circumstances care is required in the interpretation of experimental data. Moreover, it is desirable to compar e 0009-8558/80/12~)0-0321502.00 9 1980 The Mineralogical Society