Physics
Scientific paper
Oct 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988pepi...52..150w&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 52, Issue 1-2, p. 150-158.
Physics
18
Scientific paper
Taenite with 48-57% Ni is magnetically soft, but when transformed via atomic ordering below 320°C it becomes the new magnetic mineral tetrataenite with considerable magnetic hardness and other distinctive magnetic properties. Tetrataenite is ubiquitous in chondrite meteorites, occurring as discrete grains, as a clear taenite rim, and in the etched cloudy zone. Measured remanent coercivities range from ~ 30 mT in Appley Bridge (LL4) to ~ 600 mT in some Bjurbole (L4) chondrules. This range suggests a considerable variation in tetrataenite properties that might be related to degree of order, size and properties of the ordered regions, possible shock induced effects, and other causes, including the initial grain size, particularly in the cloudy zone.
Tetrataenite is characterized during heating to the apparent Curie point near 550°C, and taenite is characterized during subsequent cooling to room temperature. This record is possible because of the large magnetic anisotropy contrasts between tetrataenite and taenite and the fact that heating to the apparent Curie point destroys the magnetic anisotropy of tetrataenite, thereby allowing a taenite cooling record. Failure to recognize the presence of tetrataenite can result in considerable misinterpretation of thermomagnetic analysis since the curve shapes for tetrataenite and taenite can be different. Magnetic hysteresis loops, thermomagnetic curves, coercivity-temperature curves, and saturation remanence thermal demagnetization curves provide unquestionable records of two vastly different magnetic materials. Classical magnetic techniques can be used to monitor the progress of disordering of the tetrataenite during isothermal annealing. Magnetic hysteresis loop measurements of a Bjurbole chondrule at 488°C clearly record the magnetic parameter changes associated with disordering. Because of the large coercivity and other distinctive properties, tetrataenite exerts significant control on the bulk magnetic properties of meteorites regardless of its volumetric significance. Paleofield estimates to date have not considered the role of tetrataenite and should therefore be accepted with caution.
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