A 57Fe Mossbauer spectroscopic study of a selection of Australian and other goethites

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Abstract
The 57Fe Mossbauer spectra recorded at room temperature for seven Australian and two U.S.A. goethite mineral specimens of various composition and extent of aluminium substitution, particle size and origin are presented. Eight of these exhibit six-line magnetic hyperfine spectra with differing hyperfine magnetic splitting and asymmetric line broadening, resulting from super-paramagnetic relaxation effects. The results show that the relation between the decreasing magnitude of the hyperfine magnetic splitting with both increasing aluminium content and decreasing particle size, as previously reported for aluminium-substituted goethites, does not hold generally for the natural goethites studied here. For these, such a relation is more complex and appears to depend, not only on the extent of aluminium substitution, but also on the other impurity substituents in the goethite specimens. These combined effects make it difficult to use the magnitude of H to measure the particle size or to relate the type of Mossbauer spectrum with the mode of occurrence of the goethite.