HR: 08:35h
AN: GP11B-03    [Abstracts]
TI: Magnetic Domain Observations on Magnetite Crystals in Biotite and Hornblende Grains
AU: * Dunlop, D J
EM: dunlop@physics.utoronto.ca
AF: Geophysics, Physics Dep't, Univ. of Toronto, 60 St. George St., Toronto, ON M5S 1A7 Canada
AU: Ozdemir, O
EM: ozdemir@physics.utoronto.ca
AF: Geophysics, Physics Dep't, Univ. of Toronto, 60 St. George St., Toronto, ON M5S 1A7 Canada
AB: Magnetic domain structures have been observed on magnetite crystals in 300 micrometer to 1 mm size biotite and hornblende grains separated from drillcore samples of Precambrian granodiorite basement in southern Alberta, Canada. The grains were mounted in epoxy and the surface was polished, first mechanically and then with a suspension of amorphous silica. Domain structures were observed on 6 to 60 micrometer magnetite crystals using the Bitter colloid technique. Observed features include lamellar domains bounded by straight 180 degree walls, closure domains, bent walls, wavy structures, and areas without visible domains. Crystals less than 50 micrometers in size generally have simple domain patterns with domain walls parallel to the long axis of the crystal. We studied domain wall displacements in isolated small magnetite crystals as a function of applied magnetic field up to 24 mT. The number of domains decreases with increasing external field. Some walls are relatively mobile and disappear by 20 mT. Other walls remain immobile. In some cases, crystal defects or inclusions are seen to pin these walls, or segments of walls. An interesting observation was made on a 5 x 25 micrometer elongated magnetite crystal formed in a crack in hornblende. Large quantities of magnetic colloid gathered at either end of the crystal because of the strong flux leakage and pole fields at these locations. Colloid was also concentrated where domain boundaries intersect the sides of the crystal. With increasing applied field, the colloid spread out to form patterns tracing complete external flux closure loops.
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005