A Mössbauer effect study of the mineral macaulayite

The mineral macaulayite, which is thought to consist of layers equivalent to two hematite unit cells flanked by silicate sheets, has been investigated in the temperature range 4.2K–650K by Mössbauer spectroscopy. At low temperatures there is a close resemblance between the spectra of macaulayite and that of hematite in its weakly ferrimagnetic state. At higher temperatures, however, the spectra of macaulayite show two distinct magnetically-ordered components corresponding to surface and bulk iron in the hematite units. Superparamagnetism is observed in the spectra resulting from the microcrystalline nature of the sample. The Néel temperature of macaulayite is measured by extrapolation to be T _N=750 ± 20K, substantially below that of hematite (T _N=950K), even when the effect of possible aluminium substitution is taken into account. This reduction in the magnetic ordering temperatures is discussed in terms of the proposed layer structure of the mineral.