The magnetic properties of ilmenite-haematite crystals

Abstract

Abstract 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 µ long, 1 µ wide and 0.22 µ 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 ilmenite- haematite 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 Fe2+ ions on alternate cation layers b an electron transfer mechanism between trivalent and divalent iron atoms.