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The thermal behaviour of a ferrous doped kaolin has been studied by Mössbauer spectroscopy and electron paramagnetic resonance spectroscopy. From the observations it is concluded that the iron substitutes trioctahedrally as Fe2+ in the ‘gibbsite-like’ sheet in place of dioctahedral aluminium. The g = 2 EPR signal is shown to be associated with these ferrous ‘cells’ which appear to occur in clusters. It is suggested that these ferrous cells are trapped within the normal dioctahedral aluminium structure. Dehydroxylation of the ferrous iron cells takes place between 623 and 673 K leading to the formation of an iron-rich pyroxene and, by 723 K, a ferric oxide. At temperatures > 723 K the pyroxene itself oxidizes to a second ferric oxide. The EPR signal changes at 623 K and disappears at 723 K. The signal is attributed to a trapped hole induced by X-irradiation, located near a silicon atom on the boundary between normal dioctahedral cells and trioctahedral Fe2+ cells. It is possible to extend the model to explain some puzzling features concerning the g = 2 EPR signals reported by other authors and to propose other effects which might result from the presence of these cells.

The preparation of green rusts from sulphate solutions and representative M6ssbauer spectra are described. As the samples oxidized readily, attention focused on the M6ssbauer parameters at liquid nitrogen and helium temperatures. The spectra recorded at 77 K could be fitted satisfactorily with one ferrous iron quadrupole doublet with a separation of 2.93 mms 1 and one ferric iron quadrupole doublet with a separation of 0.45 rams -1. In some spectra a ferric iron magnetic hyperfine of strength 49.2 T was also apparent. At 4.2 K, the ferrous iron exhibited a hyperfine splitting with a field of 12.4 T whilst the ferric iron exhibited a hyperfine splitting with a field of strength 50-4 T. The ratio of ferrous to ferric ions was 2.25 + 0.25 at 77 K and at 4.2 K, and -1.6 with a large variation at room temperature. The liquid helium spectra did not always give a good chi-squared fit, the main reason being attributed to relaxation. The line-width of the ferrous iron site at 77 K is slightly larger than that for iron metal and could be explained by a variation in the number of near Fe 3+ neighbours at different Fe e+ sites, consistent with the assumption that the ferrous iron site is in the hydroxide sheet. The effect of different numbers of Fe 2+ and Fe 3+ neighbours probably contributed to the increase in line-widths at 4.2 K compared with those at 77 K. The ferrous iron doublet is marginally different to those of chloride and hydroxycarbonate green rusts and the aluminium analogues.Green rusts are unstable compounds containing a mixture of ferrous and ferric iron and were first described by Keller (1948) who produced a chloride and a sulphate green rust. The ratio of ferrous to ferric iron was found to lie between 0.8 and 4-0. A study by Bernal et al. (1959) of iron oxyhydroxides, which included green rusts, identified two forms of the sulphate species as well as the chloride form. These green rusts were prepared "by the partial oxidation of ferrous iron solutions" but the method was not described in sufficient detail to enable repeat studies to be made. Misawa et al. (1973Misawa et al. ( , 1974 made a study by ultra-violet spectroscopy and chemical analysis of the aerial oxidation of neutral and slightly alkaline ferrous sulphate solutions. An intermediate green complex was observed which was shown to be a precursor of green rust II and on that basis the ferrous to ferric ratio was estimated to be about unity. Gancedo et al. (1976) studied the corrosion products of iron in aqueous solutions of ammonium nitrate and found the room temperature M6ssbauer quadrupole doublets to have parameters similar to those of synthetic green rusts I and II, namely, 2.0 and 2-37 mms -1, respectively. It was noted that the ferrous to ferric iron ratios were 1. 88 and 0.63, respectively. McGill et al. (1976) in a study of the corrosion of cast iron in carbonate solutions reported the formation of a green rust compound which gave similar lines to green rust I but which was structurally different. Brindley & Bish (1976) poin...

Abstract--Previous studies by Electron Spin Resonance (ESR) have established the substitution of Fe 3+ and Mg 2+ in the kaolinite structure. It is shown that Fe E+ can substitute in kaolinite and stabilize defects which are detectable by ESR in a manner identical to Mg 2+. The development of methods of preparing a synthetic kaolinite doped with Fe 2+ is described in detail. It is shown that the main ESR signals, which occur at g = 2.0 in natural kaolinites and which previously have been interpreted in terms of iron and magnesium, can be attributed to iron alone.

A B ST RAC T:The structure of kaolin has been examined together with aspects of dosimetry and energy loss mechanisms of radiation to explain the formation ofg = 2 EPR centres. The analysis points to the formation of a trapped hole on the 'inner layer' oxygen atoms of kaolin located at the boundaries between divalent ion and trivalent ion 'cells', in particular at the boundaries with excess negative charge. Direct interaction of X-rays with atoms and the possibility of proton recoil are eliminated. The means of production appears to be by transfer of charge following ionization of atoms by secondary electrons, with transfer of vacancies ultimately to the oxygen ions. Mechanisms which result in a decrease in signal strength with increase in concentration are examined. It is concluded that the cell mechanisms discussed are consistent with the rates of production and that at 20 Mrad (air) the number of centres should be reaching saturation.