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

Abstract

Summary 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.

Magnetic discrimination between Al‐substituted hematites synthesized by hydrothermal and thermal dehydration methods and its geological significance

“…If so, higher unbalance of Al atoms in HFh* series samples results in a larger net magnetization than HG* series samples. In addition, the higher temperature during crystal formation probably induced some degree of recrystallization of the HG* series samples and annealed the defects out of the structure, diminishing the contribution of the defect moment to the magnetization [ Dunlop , 1971; de Boer and Dekkers , 1998, 2001]. …”

Thermal origin of continental red beds in SE China: An experiment study

“…6C). From the results of our heating experiments, it is inferred that when the temperature within the sedimentary sequence reached 150°C, authigenic ferrihydrite and goethite began to dehydrate and transform to hematite (Jiang et al, 2013;DeBoer and Dekkers, 2001). As shown in Fig.…”

Micro- to nano-scale characterization of martite from a banded iron formation in India and a lateritic soil in Brazil

“…7a) in the Brazilian sample, as well as the very slight decrease in the Indian rest fraction (Fig. 7d), indicates that another magnetic phase is present such as maghemite, which commonly transforms into hematite at that temperature (e.g., Tarling 1983;Thompson and Oldfield 1986;De Boer and Dekkers 2001). Magnetic hysteresis loops are presented in Fig.…”