✍ Scribed by D. E. Sawcer; B. A. Goodman; J. D. Russell; M. F. Thomas
No coin nor oath required. For personal study only.
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 M6ssbauer 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 N6el temperature of macaulayite is measured by extrapolation to be TN = 750__ 20K, substantially below that of hematite (TN = 950K), even when the effect of possible aluminum substitution is taken into account. This reduction in the magnetic ordering temperatures is discussed in terms of the proposed layer structure of the mineral.
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