A Possible Source for The Coercivity of Ilmenite-Hematite Minerals

Merrill, Ronald T.

doi:10.5636/jgg.20.181

Cited by 19 publications

(9 citation statements)

References 12 publications

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“…The case of hematite lamellae in ilmenite hosts is interesting because they display a completely different behavior. The remanence‐carrying hematite lamellae interact with the paramagnetic host only in that they are impacted by stress due to elastic and plastic deformations in response to the strain at the ilmenite‐hematite interfaces [ Merrill , ; Shive and Butler , ]. In case of nanoscale lamellae with strain‐limited growth, the elastic contribution will probably be higher than in case of dominant plastic deformation in larger lamellae studied by Shive and Butler [].…”

Section: Discussionmentioning

confidence: 99%

Nonlinear Preisach maps: Detecting and characterizing separate remanent magnetic fractions in complex natural samples

Church¹,

Fabian

McEnroe³

2016

JGR Solid Earth

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Natural remanent magnetization carriers in rocks can contain mixtures of magnetic minerals that interact in complex ways and are challenging to characterize by current measurement techniques. Here a nonlinear mapping scheme is described that efficiently enhances sensitivity and the resolution power of remanent Preisach maps. Using this scheme a large dynamic range of magnetic moments and coercivities can be reliably resolved. The method is applied to synthetic and natural standard samples containing magnetite and hematite, as well as to natural samples from remanent magnetic anomalies where complex microstructures are observed. It is shown that certain offset high‐coercivity patterns in remanent Preisach maps may serve as fingerprints for exsolution structures of ilmenite in hematite or hematite in ilmenite and that in some magnetite‐bearing remanent anomalies the magnetite coercivity is increased beyond its intrinsic coercivity range. Experimental results and theoretical considerations indicate a minimal coercivity of about 10 mT for single‐domain (SD) magnetite, such that observation of lower coercivities implies pseudo‐SD (PSD) or multidomain grain sizes. A diagnostic hematite pattern with a peak downward offset of 17 ± 2% of the intrinsic coercivity is found that is stable over a large range of intrinsic coercivities and may be related to shielding of internal defect or lamellar moments by a spin canting response to the internal field.

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Section: Discussionmentioning

confidence: 99%

Nonlinear Preisach maps: Detecting and characterizing separate remanent magnetic fractions in complex natural samples

Church¹,

Fabian

McEnroe³

2016

JGR Solid Earth

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“…However, because many of the imaged nanoscale haematite lamellae show highly strained coherent interfaces, other controls, such as stress (Merrill 1968) and defects at the interfaces and contact regions, should be considered. All hysteresis loops have a large paramagnetic component carried by the host ilmenite grains.…”

Section: Hysteresis Measurementsmentioning

confidence: 99%

Nanoscale haematite–ilmenite lamellae in massive ilmenite rock: an example of ‘lamellar magnetism’ with implications for planetary magnetic anomalies

McEnroe

Harrison

Robinson

et al. 2002

Geophysical Journal International

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Summary Massive, nearly ‘pure’, haemo‐ilmenite layers from historic ore deposits in Rogaland, Norway contain very few silicates or other oxides and typically produce remanence‐dominated magnetic anomalies. These rocks are ideal for evaluating the magnetic properties of fine exsolution intergrowths and the larger titanohaematite lamellae in the host ilmenite grains. A typical bulk composition, Ilm 84, exsolved at high temperature to produce host ilmenite Ilm 94 and micron‐sized haematite lamellae Ilm 23 as measured by electron microprobe (EMP). Subsequent undercooling of the ilmenite and the micron‐scale haematite lamellae led to metastable nucleation of nanoscale lamellae down to unit‐cell scale, leaving depleted hosts between lamellae with compositions of Ilm 98 and Ilm 15–13 as measured by TEM–EDX. Samples have high coercivities, and average NRM values of 25 A m−1, which typically show ∼2 per cent saturation in the NRM state. The amount of magnetization in these samples is too high to be solely accounted for by a spin‐canted AF moment in the haematite. Based on Monte Carlo simulations of haematite–ilmenite interfaces at the atomic scale and on measured rock‐magnetic properties, we predict that the magnetization is carried by a ferrimagnetic substructure produced at the contacts of the very fine‐scale titanohaematite and ilmenite exsolution lamellae.

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“…It is known that non-reproducible self-reversal has occurred for some titanohematites containing 10 to 15 mole ~ ilmenite in solid solution (Merrill andHeller andEgloff, 1974). Although the mechanism for this self-reversal is not clear, Heller and Egloff (1974) have evidence suggesting that self-reversal may be found in samples that have undergone a late stage of exsolution, a stage which presumably occurs below the Curie temperature of the resulting hematite-rich phase.…”

Section: Self-reversal In Titanohematitesmentioning

confidence: 99%

“…We will not consider this problem in detail here, other than to list the number of factors that change during such division that may have an effect on the magnetic stability: the grain's shape, the grain's volume, the saturation magnetization, the magnetocrystalline anisotropy, the exchange energy, the magnetostrictive anisotropy, and the grain's environment. Grain environment changes includes those due to the internal stress (Merrill, 1968;Shive and Butler, 1969) in addition to changes in the magnetic interactions between grains (Dunlop and West, 1969;Jaep, 1971.) All these factors must be properly evaluated in any viable explanation of change in stability with exsolution.…”

Section: Crm Stabilitymentioning

confidence: 99%

Magnetic effects associated with chemical changes in igneous rocks

Merrill

1975

Geophysical Surveys

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Experimental evidence and theory indicate that chemical changes occur in many igneous rocks at sufficiently low temperatures to significantly affect the remanent magnetization. Some chemical changes lead to self-reversals of magnetization that are not reproducible in laboratory experiments. Such self-reversals appear to be very rare in subaerially-erupted basalts, but they probably are much more common in some other rock types, such as granites and diorites. The stability of the natural remanent magnetization in igneous rocks can be decreased, left unaltered, or increased by chemical changes. In addition, chemical changes will usually affect the intensity of magnetization in a rock; the intensity can increase, decrease, or (rarely) be left unaltered by a chemical change. Such changes are important to consider in the development of improved techniques for obtaining reliable estimates of the intensity of the Earth's magnetic field in the past and in correctly interpreting marine magnetic anomalies. Finally, experiments and theory are presented which suggest that many of the chemical changes in igneous rocks will only occasionally produce significant changes in the direction of the magnetization.

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A Possible Source for The Coercivity of Ilmenite-Hematite Minerals

Abstract: Large coercivities of some ilmenohematite minerals are probably due to magnetostrictive effects associated with exsolution.

Cited by 19 publications

References 12 publications

Nonlinear Preisach maps: Detecting and characterizing separate remanent magnetic fractions in complex natural samples

Nonlinear Preisach maps: Detecting and characterizing separate remanent magnetic fractions in complex natural samples

Nanoscale haematite–ilmenite lamellae in massive ilmenite rock: an example of ‘lamellar magnetism’ with implications for planetary magnetic anomalies

Magnetic effects associated with chemical changes in igneous rocks

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