Abstract

This study investigates relationships between microstructures revealed by transmission electron microscopy (TEM) and the magnetic properties of strongly magnetized magnetite iron ore previously described as type I Iodestone. The sample consists mostly of magnetite with some maghemite (~30%) and minor hematite and goethite oxidation products (<5%). The maghemite exhibits a weakly developed superstructure that in some areas is consistent with the enantiomorphous space groups P4132 and P4332. High-resolution imaging indicates that magnetite and maghemite contain closely spaced planar faults parallel to {101}. Displacement vector analyses and high-resolution imaging indicate 〈¼¼0〉 translations across most faults. Faults cannot arise merely by oxidation and vacancy ordering because they offset the magnetite substructure. Boundaries that show no offset of the magnetite substructure probably represent junctions between enantiomorphous maghemite domains. Although superlattices are not sufficiently well developed to allow detailed characterization of the vacancy ordering patterns, we can establish that the 〈¼¼0〉 boundaries generally separate regions of the same phase and not regions of magnetite from maghemite. Needle-shaped areas defined by planar faults are elongate along the magnetically soft [111] direction, and defect orientations correspond to the usual orientations of magnetic domain walls in magnetite. As noted previously, Fe-O-Fe bond angles across {101} faults allow direct reversal of the magnetic moments. Modification of magnetic vector orientations would require either nucleation of new domain walls or the movement of faults.

Three specific thermal regimes (20–280, 280–425, and 425–600 °C) are derived from a series of thermomagnetic experiments; each regime is associated with the movement of the natural remanent magrretization (NRM) vector. The ratio of NRM to saturation remanent magnetization is large, consistent with lightning-discharge induced magnetization. In the first regime, 80% of the NRM is destroyed, the coercivity is substantially reduced, and the magnetic susceptibility increases. These changes correlate with the elimination of most stacking faults from the sample. We suggest that the initial high coercivity is directly associated with the presence of stacking faults and that pinning of domain walls by these features primarily explains the large and stable natural remanence. The shape anisotropy of the magrretite and maghemite regions may also be important. We tentatively suggest that the stacking faults may also have been induced in the lightning event. The second thermal regime, characterized by rapid reduction in susceptibility and saturation magnetization, corresponds to the conversion of maghemite to hematite. The third regime is dominated by magrretite and hematite. Results demonstrate that microstructural detail revealed by TEM contributes substantially to the understanding of mineral magnetic properties.

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