ISSN:0094-8276    

DOI:10.1029/GL011i003p00159

年代:1984

数据来源: WILEY

摘要:

The temperature stability of acicular maghemite (γ‐Fe2O3) was studied between 20°C and 600°C by x‐ray diffraction and magnetic measurements. Synthetic monodomain maghemite was found to be stable with respect to hematite (α‐Fe2O3) at temperatures as high as 500°C, inverting to α‐Fe2O3between 510‐660°C. However, a complete inversion of the γ to α form for the sample was not achieved even at 660°C, the final product was a two‐phase mixture, with 65% of the original γ‐Fe2O3unchanged. Two conclusions are made: (a) the absence of maghemite in a natural sample cannot be proven if heating has been restricted to moderate temperatures (300°‐400°C) and (b) the true Curie point of pure

ISSN:0094-8276    

DOI:10.1029/GL011i003p00161

年代:1984

数据来源: WILEY

摘要:

Using neutron diffraction, the cation distribution was determined in compositions of the titanomagnetite spinel series Fe3−xTixO4O ≤ × ≤ 1, with x = 0.6 and 1 at room temperature. The knowledge of the correct cation distribution of x = 0.6 is necessary for predicting the change in magnetization of ozeanic pillow basalts with oxidation. For x = 0.6, Ti ions occupy exclusively B‐sites of the spinel lattice with the possible exception of a minute number occupying A‐sites. In order to explain the diffraction pattern for x = 1, the occupation of additional kinds of sites are discussed. The diffuse scattering in the background for both compounds shows broad peaks which are indicative of magnetic and chemical short r

ISSN:0094-8276    

DOI:10.1029/GL011i003p00165

年代:1984

数据来源: WILEY

摘要:

In an attempt to find conditions suitable for aqueous oxidation of titanomagnetite in submarine basalts, synthetic titanomagnetite powders have been exposed to solutions with different pH's and gas fugacities at different temperatures. Analysis of dissolved Fe and Ti, thermomagnetic measurements, and x‐ray measurements all confirm that aqueous oxidation by Femigration does occur. Furthermore, it is, in the first order, pH dependent, implying that ‘maghemitization’ of ocean floor basalts proceeds most favorably under what geochemists call water‐dominated conditions, where basalt‐seawater interaction leads to an increase i

ISSN:0094-8276    

DOI:10.1029/GL011i003p00169

年代:1984

数据来源: WILEY

摘要:

Monoclinic pyrrhotite (Fe7S8with 4C superstructure) was magnetically separated from a massive pyrrhotitic ore for preparation of nine grain‐size fractions ranging from ∼ 80 µm (large multidomain grains) to<3 µm (single‐domain grains). Grains smaller than 100 µm were found to be magnetically hard with coercive forces ranging from 135 oe for 83 µ grains to 920 oe for<3 µm grains. For grain sizes between 83 µm and 7 µm the coercive force is given by Hc⧜ d−0.79. Variations in hysteresis properties with grain size appear to be gradual, with no evidence of sudden changes associated with domain struc

ISSN:0094-8276    

DOI:10.1029/GL011i003p00173

年代:1984

数据来源: WILEY

摘要:

Rock magnetic investigations of both the natural chromite ores and their purified chromite concentrate (Mg.6Fe.4Cr1.4Al.6O4) were carried out. These include, measurement of natural and saturation remanent magnetism and their stability with respect to alternating field and thermal demagnetisation, IRM acquisition studies, saturation magnetisation versus temperature studies and high field (1000 mT) hysteresis characteristics. From these studies it was observed that the medium/high coercivity component of remanence is associated with the chromite minerals and is carried largely or entirely by them. Therefore, it is concluded that chromites carry permanent magnetisation of palaeomagnetic significance. However, this result is in sharp contrast to the assumption in the literature that the spinels of the above composition are non‐magnetic. The study has also demonstrated that the magnetic properties of artificial Fe‐Cr spinels do not conform to their natural analogues. Reasons for the discrepancy have been postula

ISSN:0094-8276    

DOI:10.1029/GL011i003p00177

年代:1984

数据来源: WILEY

摘要:

It is important to palaeomagnetism to understand the processes of magnetization in submicron magnetite grains. The theoretical contribution of domain wall motion to the anhysteretic susceptibility of two‐domain magnetite spheres has been calculated. Comparison with experimental values shows that domain wall motion can account for the major part of the anhysteretic susceptibility of particles a little above the single‐domain s

ISSN:0094-8276    

DOI:10.1029/GL011i003p00181

年代:1984

数据来源: WILEY

摘要:

Domain state energy calculations have been made for submicron magnetite cubes, based on a model consisting of planar domains of uniform magnetization separated by 180° domain walls (DWs). The magnetization within a DW is assumed to rotate uniformly, resulting in the appearance of magnetic poles of average density ±(2/π)Json the particle faces bounding the DWs. Equilibrium domain structure configurations have been determined by minimization of the sum of the demagnetizing and wall energies. Domain structure transition sizes and particle remanences are reported for two‐domain and three‐domain magnetite

ISSN:0094-8276    

DOI:10.1029/GL011i003p00185

年代:1984

数据来源: WILEY

摘要:

Domain structure observations on synthetic and natural Ti‐rich titanomagnetites (≈TM60), some of which were made under external uniaxial pressure, lead to a new model for the domain state of such materials. Instead of a classical domain configuration, we observed in most cases a more complicated micromagnetic structure with multidomain particles consisting of a conglomerate of quasi‐single domain, multi‐domain and spin cluster zones due to an inhomogeneous anisotropy field caused by internal stress. Pseudo‐single domain properties such as stable TRM can be explained by spin rotation processes as in single domain

ISSN:0094-8276    

DOI:10.1029/GL011i003p00189

年代:1984

数据来源: WILEY

摘要:

Difficulties in domain wall nucleation have been shown to affect the domain state of the fine magnetic particles in rocks. Particles, large enough to contain a domain wall, remain saturated in remanence and only nucleate a wall when a backfield is applied. In this paper, nucleation is described during hysteresis, stress cycles and cycles through the low temperature anisotropy transition in magnetite. It is suggested that much of the grain size dependence of magnetic behaviour in the pseudosingle domain grain size range can be attributed to the increasing difficulty with which walls are nucleated, as particle size decreases.

ISSN:0094-8276    

DOI:10.1029/GL011i003p00193

年代:1984

数据来源: WILEY