HR: 0800h
AN: GP41A-0865    [Abstracts]
TI: Intergrowths in Multidomain Hematite: Source of High NRM Intensities and Coercivity?
AU: * Schmidt, P W
EM: phil.schmidit@csiro.au
AF: CSIRO, Industrial Physics, Lindfield, NSW 2070 Australia
AU: McEnroe, S
EM: suzanne.mcenroe@ngu.no
AF: NGU, N-7491, Trondheim, 7491 Norway
AU: Harrison, R
EM:
AF: Dept. Earth Sci., Univ. Cambridge, Cambridge, CB23EQ United Kingdom
AU: Clark, D
EM:
AF: CSIRO, Industrial Physics, Lindfield, NSW 2070 Australia
AU: Robinson, P
EM:
AF: NGU, N-7491, Trondheim, 7491 Norway
AB: Magnetic properties of samples from Mid Proterozoic massive hematite deposits in South Australia were examined in detail. The Peculiar Knob and Manxman B massive hematite deposits have large magnetic anomalies that can only be modeled with a significant remanent component. Samples show very high natural remanent magnetization (NRM) and high Q values. Based on susceptibilities, high- and low-field thermomagnetic curves, hysteresis loops, and IRM acquisition/demagnetization curves, samples can be separated into hematite ores and hematite-magnetite ores. The NRMs for the hematite-magnetite ores are at most 10% of their SIRMs. NRM values of the nearly pure massive hematite ore samples are in the hundreds of A/m, but well below saturating values predicted by theory for a TRM in MD-size hematite. AF demagnetization of the hematite ores reveals MDFs of ~50mT. These nearly pure hematite samples have N‚el temperatures near 670C. Mrs/Ms values range from 0.5 to 0.9, and Hcr/Hc is close to 1. Samples are composed of well equilibrated MD-size massive hematite grains or massive hematite with magnetite. SEM and TEM work has shown that some samples contain trace intergrowths of Al spinel and other Fe oxides. A very fine texture is observed in reflected light, but no chemical difference was found with microprobe analyses. Transmission electron microscopy has shown significant twinning of the hematite that likely increases the coercivity of the MD grains. FORC measurements show that the microcoercivity is nearly identical to the bulk coercivity, that is higher than expected for MD hematite. Further TEM imaging and analyses will focus on the presence of maghemite-hematite nanoscale intergrowths that are postulated, based on some of the rock-magnetic properties. These intergrowths could account for the higher than expected coercivities and NRM intensities
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1541 Satellite magnetics: main field, crustal field, external field
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005