Magnetic Anisotropy of Hemo-ilmenite Single Crystals: Testing the Lamellar Magnetism Hypothesis
Abstract
The anisotropy of magnetic susceptibility (AMS) of single crystals of hemo-ilmenite was investigated to examine the relationship between directions of the principal axes of the AMS ellipsoid in minerals considered to contain lamellar magnetism. The crystals, within slabs taken from the Pramsknuten massive hemo-ilmenite "dikes", South Rogaland, Norway, were oriented by electron backscatter diffraction (EBSD) and the c- and a-axes of seventeen individual crystals were identified by comparing both the front and back sides of the slabs. The anisotropy of magnetic susceptibility was measured in low fields with an AGICO KLY-2 susceptibility bridge and in high fields (maximum field: 1.8 T) with a torsion magnetometer. The principal axes of the AMS ellipsoids are k1 > k2 > k3. Low-field AMS shows a triaxial ellipsoid in which the k3 axis lies near to the crystallographic c-axes for 16 of the 17 crystals (13 crystals < 11° , 3 crystals < 17° ). Discordance may be due to misalignment during mounting or contamination by minute amounts of magnetite. Eight crystals show agreement between the alignment of the k2 of the AMS ellipsoid and an a-crystallographic axis (5 crystals < 11° , 3 crystals < 17° ). The significance of this relationship will be discussed. In these crystals the remanent magnetization is also oriented close to the same direction, consistent with the lamellar magnetism hypothesis. In 6 crystals the k1 of AMS ellipsoid is close (< 15° ) to an a-crystallographic axis but there is no correlation with the remanent magnetization. High-field AMS measurements confirm the orientation of the c-crystallographic axes and suggest that hematite is an important contributor to the observed AMS.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFMGP21A0154H
- Keywords:
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- 1518 Magnetic fabrics and anisotropy;
- 1519 Magnetic mineralogy and petrology;
- 1540 Rock and mineral magnetism