Unusual thermomagnetic behaviour of haematites: neoformation of a highly magnetic spinel phase on heating in air

Abstract

The formation of traces of a magnetic phase with a Curie point of 470–475 °C is detected during routine thermomagnetic analysis of various haematite types without significant isomorphous substitution. Using heating and cooling rates of 10° min⁻¹, the formation temperature can be as low as 400 °C for synthetic haematite samples, whereas higher temperatures, 700–800 °C, are required for natural samples. The new phase appears to be persistent to prolonged heating at 1000 °C and has a cubic spinel structure with a unit cell length a₀ = 0.8350 ± 0.0005 nm, similar to pure maghemite. This suggests that the reverse reaction of the γ-Fe₂O₃→α-Fe₂O₃ transformation can occur under appropriate conditions. The low Tc of this particular maghemite variety suggests that the vacancy (and/or cation) ordering over the magnetic sublattices is different from usually occurring maghemite. In accordance with Takei & Chiba (1966), who also reported a pure maghemite variety with identical Tc, a cation-deficient spinel structure with part of the vacancies on tetrahedral sites is suggested. Thermally activated release of incorporated hydroxyl groups would trigger the formation of maghemite traces on the surface of well-crystalline haematite planes. Citrate–bicarbonate–dithionite extraction results support the idea that the maghemite is fine-grained and surficial on the haematite because low-field susceptibility values decrease according to the behaviour of fine-grained maghemite particles. The formation of traces of this highly magnetic mineral during routine stepwise thermal demagnetization or during annealing haematite at high temperatures may seriously affect NRM measurements or may be erroneously taken as haematite's defect moment.