The magnetic properties of ilmenite-haematite crystals

The interpretation of both palaeomagnetic and geophysical prospecting data requires a better understanding of the magnetic properties of the iron oxide minerals found in rocks. The magnetic properties of ilmenite-haematite solid solutions have been investigated, the unusually large and pure haemo-ilmenite crystals from the Allard Lake region of Quebec being used. The magnetic component of these crystals is an ilmen-haematite phase, having a composition of about 10 mole % of ilmenite in haematite, and present in the form of exsolution lamellae that are roughly 5 $\mu$ long, 1 $\mu$ wide and 0.2 $\mu$ thick. The crystals have a very strong anisotropy causing magnetization in the basal plane and a weak anisotropy which produces an easy direction of magnetization within the basal plane. An improved ilmenitehaematite solvus curve has been produced by X-ray and Curie-point analysis of heat-treated crystals. Spontaneous reversal of magnetic polarity takes place with change in temperature in ilmen-haematite having between about 25 and 15% of ilmenite in haematite. This new reversing range of composition is quite different from that found to reverse by Uyeda. The reversal is due to a new antiparallel moment which grows as temperature falls. This has been interpreted as being due to an ordering of Fe$^{2+}$ ions on alternate cation layers by an electron transfer mechanism between trivalent and divalent iron atoms.