Chapter 7 Recording materials

Bate, G.

doi:10.1016/s1574-9304(05)80108-7

Cited by 70 publications

(40 citation statements)

References 257 publications

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“…The inversion temperature TI varies, from =250øC [Verwey, 1935;Bernal et al, 1957;Kachi et al, 1971] to >600'C [Wilson, 1961;Sato et al, 1967;Ozdemir and Banerjee, 1984], depending on the method of preparation, impurity content, and particle size and shape (see Bate [1980] for a review). Imaoka [ 1968] found 560øC < TI < 610øC for acicular 7Fe203 but T•250'C for granular material.…”

Section: Inversion Of Maghemitementioning

confidence: 50%

Crystallization remanent magnetization during the transformation of maghemite to hematite

Özdemir¹,

Dunlop²

1988

J. Geophys. Res.

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We have studied crystallization remanent magnetization (CRM) in the γFe2O3 → αFe2O3 transformation, starting from synthetic, equidimensional maghemite crystals of single‐domain size (median diameter of 245 Å). Inversion of maghemite to hematite on heating was indicated by an exothermic peak in differential thermal analysis (DTA) at 555°C. In CRM experiments, samples of dispersed maghemite were heated in zero field to temperatures between 300°C and 604°C, held for 3 hours in a 50‐μT field HCRM, and cooled in zero field to 20°C. The intensity of the resulting CRM MCRM increased with increasing conversion to hematite, reaching a maximum of 784 A m−1 (0.784 emu cm−3) after the 556°C run, then decreased to 1.3 A m−1 (0.0013 emu cm−3) after the 604°C run. Maximum CRM intensity and the most rapid inversion indicated by DTA both occurred around 555°C. MCRM partially inverted samples was always parallel to HCRM. Hematite cannot explain the high intensity and moderate coercivity of the CRM. The bulk of the CRM seems to be carried by maghemite and to be acquired viscously rather than by grain growth. In a second set of experiments, CRM was produced in the presence of a preexisting anhysteretic remanent magnetization (ARM) MARM perpendicular to HCRM. The resulting remanence MT was a composite of surviving anhysteretic remanent magnetization ARM plus CRM directed parallel to HCRM, rather than a single vector of intermediate direction. MT can be synthesized by orthogonally combining CRMs measured in the first set of experiments with ARMs heated in zero field. Phase coupling of maghemite and hematite is not necessary to explain the main features of CRM, with or without an m initial remanence.

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Section: Inversion Of Maghemitementioning

confidence: 50%

Crystallization remanent magnetization during the transformation of maghemite to hematite

Özdemir¹,

Dunlop²

1988

J. Geophys. Res.

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“…Js of the maghemite is -64 Am2/kg at RT which is distinctly lower than the usually quoted value of 74 Am2/kg for pure maghemite [e.g. Bate, 1980]. Reduced values of Js are often reported for natural maghemites and are here possibly caused by the substitution of diamagnetic At [cf.…”

Section: Rock Magnetic Parameters At Room Temperaturementioning

confidence: 63%

Grain‐size dependence of the rock magnetic properties for a natural maghemite

Boer¹,

Dekkers²

1996

Geophysical Research Letters

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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.

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“…A typical room temperature value of 74 Am 2 kg −1 is often found for this maghemite type (e.g. Johnson & Merrill 1974; Bate 1980; Goss 1988; Özdemir & Dunlop 1988; Özdemir 1990; Moskowitz 1993). The presence of part of the vacancies on tetrahedral interstices would result in a higher net magnetic moment.…”

Section: Crystal Structure and Rock‐magnetic Backgroundmentioning

confidence: 89%

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

Boer¹,

Dekkers²

2001

Geophysical Journal International

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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.

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Chapter 7 Recording materials

Cited by 70 publications

References 257 publications

Crystallization remanent magnetization during the transformation of maghemite to hematite

Crystallization remanent magnetization during the transformation of maghemite to hematite

Grain‐size dependence of the rock magnetic properties for a natural maghemite

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

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