When interpreting thermomagnetic curves of non‐saturated magnetic minerals, irreversible heating and cooling curves need not necessarily imply chemical or structural changes. Increased aligning of magnetic moments on heating in an applied magnetic field can also induce an irreversible cooling curve. The two processes can be distinguished by stirring the sample between subsequent thermomagnetic runs. Sample redispersion considerably enhances the interpretative value of thermomagnetic analysis and is therefore strongly recommended, in particular when analysing non‐saturated magnetic minerals. Stirring between subsequent runs was extensively used in the analysis of the thermomagnetic behaviour of haematite and goethite as a function of grain size (i.e. coercivity) in various non‐saturating magnetic fields (10–350 mT). The shape of the thermomagnetic heating curves of haematite is shown to be dependent on the competitive interplay between the temperature dependence of the exchange energy and that of the coercive force with respect to the applied field. On heating, pure defect‐poor haematite, which is magnetically dominated by the canted moment, has an initially increasing thermomagnetic heating curve. Further heating causes the magnetization to increase smoothly up to a certain temperature which depends critically on the applied field and the coercivity of the sample. The irreversible block‐shaped thermomagnetic cooling curve lies above the heating curve, and shows hardly any dependence on applied field and grain size. In contrast to the heating curve, the shape of the cooling curve depends only on the temperature variation of the exchange energy. Our data seem to indicate that for defect‐poor haematites the domain configuration acquired at the maximum heating temperature is retained on cooling to room temperature. More defect‐rich haematite has a gently decreasing thermomagnetic heating curve. On heating to increasingly elevated temperatures (800 °C) the defects are annealed out off the lattice, because the thermomagnetic curves approach those of defect‐poor haematite. The defect moment due to lattice defects seems to be additive to, but softer than, the canted moment. The canted and defect moment appear to have the same Néel (or Curie) temperature (≈680 °C), because no change in temperature was observed, whilst the relative contributions did change. The thermomagnetic behaviour of goethite is shown to be dependent on its coercivity and the amount of substituted ions.

The natural maghemite from the Robe River mining district (Australia) is shown to be thermally very stable. The maghemite/hematite inversion occurs at -650øC for sized fractions (250 gm to < 5 gm), enabling determination of T c for maghemite at 610øC. The maghemite is intimately intergrown with hematite and Js of the maghemite (corrected for the hematite content) is 64 Am2/kg ß Jrs, Hcr and H c range between 7.46 -10.18 Am2/kg, 16.90 -23.30 mT, and 6.38 -9.00 mT respectively, indicating PSD maghemite with grain sizes between -1 and -20 gm. The grain-size dependence of the rock magnetic parameters of the maghemite is less prominent than those of magnetite of nominally the same size. It is suggested that the LT-behavior of ARM can be diagnostic of the presence of maghemite: upon cooling to -196øC it shows no change; upon rewarming a gradual decrease in intensity occurs starting at --120øC.

Summary It has been realized previously (e.g. Borradaile 1994) that cycling through the Morin transition (Tm, occurring in ideal α‐Fe2O3 at −10 °C) may have implications for the NRM of some haematite‐bearing rocks. We investigate the behaviour of the low‐field susceptibility (χlf), several magnetizations (in fields of 5, 25, 100 and 1600 mT) and SIRM on cycling through Tm of several well‐characterized haematite types of varying crystallinity and particle shape. Before low‐temperature treatment, χlf of the haematites varied between ∼ 40 and ∼ 235 × 10−8 m3 kg−1. Below Tm, where only haematite's defect moment resides, χlf was much more uniform at ∼ 19 to ∼ 28×10−8 m3 kg−1. After return to room temperature, increases in χlf of up to ∼ 50 per cent were observed (when cycling in the Earth's magnetic field as well as in a field‐free space), inferred to be a function of the domain state of the haematite. This was shown for one of the haematites (LH2 which occurs as small platelets and is particularly well crystalline) where a relation y = (8.60 ± 1.01) ln(x)−2.98 was obtained, where x is the grain size (µm) and y is the percentage susceptibility increase. We suggest that transdomain changes induce the change in χlf. The nucleation of (additional) domain walls in ‘metastable’ single‐domain (SD) to pseudo‐single‐domain (PSD) grains is made possible by the low anisotropy at the Morin transition. In view of this mechanism, small stable SD haematite particles would not be affected and the grain size corresponding to y = 0 (∼ 1.5 µm for LH2) would represent the ‘real’ SD threshold size. Thermal cycling to over the Curie temperature (680 °C) is needed to return to the original domain state before the LT treatment, as expressed by a return to the original χlf values. Measuring χlf between alternating field (AF) demagnetization steps shows that AF demagnetization gradually removes the χlf increase, which appears to be soft; 30 mT is sufficient to erase 90 per cent. Thermal cycling in a 5 mT field between temperatures above Tm showed that irreversible changes in domain structure are noticeable before the isotropic point is passed. After cycling, magnetization is added to PSD and multidomain (MD) grains that intriguingly appears to be remanence, probably induced by the broadening and subsequent irreversible displacement of loosely pinned domain walls. Complete cycling through the isotropic point considerably enhances the new remanence component in ‘metastable’ SD to MD particles by an increase in the number of domains. If this behaviour can be extrapolated to the intensity of the Earth's magnetic field, this would imply that large ‘metastable’ SD to MD specularite crystals with a well‐developed Morin transition are susceptible to acquiring geologically irrelevant remanence components when subjected to low ambient temperatures. Fine‐grained haematite pigment, on the other hand, would not be affected. Thermal demagnetization alone would not be able to separate these two remanences as the new domain structure persists up to close to ...

No 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.