SUMMARY

Grain-size dependence of some remanent magnetic properties of a natural low-temperature haematite is reported. The haematite was obtained from a gossan in Montarnu (France) and upgraded by magnetic separation. Seven grain-size fractions were prepared ranging from 10 down to <0.25 μm by means of microprecision sieving and centrifuging. The haematite has an average crystallite size of 60 nm and contains minor contents of Si, Al and H2O. Its unit cell dimensions are very close to those of ideal haematite.



The magnetic properties of the haematite are strongly dependent on its thermal history. For the original unheated haematite fractions, the isothermal saturation remanence (Jrs) decreases with grain size. The remanent coercive force (Hcr), remanent acquisition coercive force (Hcr,) and median destructive field of the saturation remanence (H1/2I) are maximal in the 1.0-0.5 μm fraction. This grain-size range can be understood as the SD threshold size in analogy with ferrimagnetic minerals. Jrs/(Hcr x Xin) is a useful grain-size indicator. Lowtemperature cycling of Jrs shows identical behaviour for all fractions; the remanence decreases slightly and shows no sharp Morin transition.



After annealing the haematite fractions at 700 °C, Jrs is considerably reduced relative to the original haematite predominantly due to removal of the defect moment. Both Hcr and Hcr, have increased and their maximal values are shifted to the 2.1-1.0 μm fraction. All fractions show a much sharper Morin transition during low-temperature cycling of Jrs due to crystallite enlargement. TRM decreases with grain size.



The different rock-magnetic properties before and after annealing might provide a basis for the discrimination in sediments between a haematite DRM with a high-temperature origin and a haematite CRM which has a low-temperature origin. Two natural red beds, one with a CRM and one with a DRM associated with high-temperature haematite, broadly show the expected differences in coercivity and in annealing behaviour, although a full account of all aspects of the observed behaviour can not yet be given.

/pdf/rockmagnetic-properties-of-fine-grained-natural-low-1ab5e5bxmk.pdf

Rockmagnetic properties of fine-grained natural low-temperature haematite with reference to remanence acquisition mechanisms in red beds

SUMMARY 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 defectpoor 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 oV 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

Thermomagnetic behaviour of haematite and goethite as a function of grain size in various non‐saturating magnetic fields

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.

/pdf/unusual-thermomagnetic-behaviour-of-haematites-neoformation-55qaqtqdsc.pdf

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

Received 22 June 2011; revised 20 December 2011; accepted 24 December 2011; published 23 February 2012. [1] Hematite, a ubiquitous mineral in aerobic sediments and soils of temperate and warm areas, is weakly magnetic. However, it carries a stable natural remanent magnetization, and thus can reflect paleoenvironment changes. To quantify the influence of Al content in hematite on its magnetic properties, two series of hematite particles were prepared by hydrothermal transformation of ferrihydrite in aqueous suspension (HFh* series) and by thermal dehydration of goethite (HG* series). Crystal morphological and mineral magnetic properties of these two types of hematites differ distinctively. More specifically, the HFh* series samples display oblate (plate-like) morphologies, while the HG* series samples are prolate (highly acicular). HFh* series samples display higher saturation magnetization but lower magnetic coercivity than that of the HG* series. It is tenable that a better lattice ordering of Al substitution occurs during the process of dehydration of goethite than after transformation from ferrihydrite, resulting in weaker saturation magnetization for HG* series samples. The origin of single domain (SD) hematite in nature can be diagnosed by the correlation of unblocking temperature and magnetic coercivity: a positive correlation indicates the presence of pure (Al-free) SD hematite, while a negative correlation indicates a chemical origin of SD Al-substituted hematite. These results bear new information on decoding the complex magnetic properties of SD Al-hematite in nature environments, and thus deepen our understanding of the mechanism of variations in both paleomagnetic and paleoenvironmental signals carried by Al-hematite.

/pdf/magnetic-discrimination-between-al-substituted-hematites-2xfbklsk1g.pdf

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

The magnetic mineralogy, palaeomagnetism, and magnetic fabric of a Carboniferous mudstone-dominated red bed sequence and associated sandstones are described. Natural remanent magnetization (NRM) is carried by hematite both as relatively coarse particles and as pigmentary material; magnetite and goethite also contribute to the remanence of some mudstones. The magnetization of red sandstones is carried exclusively by hematite. Magnetic carriers have been identified using a combination of rock magnetic measurements, X-ray diffraction and 57 Fe Mossbauer spectroscopy. Both sandstones and mudstones carry a primary style magnetic fabric apparently related to the deposition of hematite particles. The reddening is associated with areas which developed better drainage and is genetically related to complex palaeosols in the mudstones, a feature indicating that it (and the magnetization) must have been approximately contemporaneous with sediment deposition.

Remanence acquisition in red beds from the Coal Measures of Central England

Summary. The properties of the ‘defect moment’ of two fine-particle synthetic haematite powders were studied at low and room temperatures, in weak and moderate fields with respect to the saturation field of this mineral, and as a function of annealing. Two components were separated, (a) a component of low coercivity due to lattice defects, which disappears at the Morin transition, (b) a component with higher coercivities, due to nonmagnetic impurities, which persists below the Morin transition and is unaffected by annealing.

/pdf/experimental-study-of-the-origin-and-properties-of-the-38fadvvx61.pdf

Ileana Bucur

Papers

Experimental study of the origin and properties of the defect moment in single domain haematite