MAGNETIC-SUSCEPTIBILITY OF PYRRHOTITE - GRAIN-SIZE, FIELD AND FREQUENCY-DEPENDENCE

The field and frequency dependences of the initial susceptibility of p yrrhotite have been analysed as a function of grain size, motivated by a strong field dependence recently observed for large (mm-sized) pyrr hotite crystals (Worm 1991) and smaller field dependences determined o n smaller grain sizes by Clark (1984). In the present study, the frequ ency ranged from 30 Hz to 27 kHz. At 2 kHz, a field range from 0.05 to 1500 muT was investigated. Separate determinations of in-phase (k') a nd quadrature (k'') susceptibility components allow for the analysis o f eddy current effects. Up to 4 kHz the in-phase susceptibility of a p yrrhotite-ore specimen is practically independent of frequency whereaf ter it decreases while the quadrature component increases linearly wit h frequency to a value on the order of k' at 20 kHz for large grains. k'' is proportional to d2musigmaf where d is grain diameter, mu the in trinsic permeability, sigma the electrical conductivity and f the freq uency. The frequency response of magnetite is essentially flat up to f requencies >20 kHz. Both frequency dependences agree well with calcula tions based on the theory by Wait (1951). The conductivity of the pyrr hotite ore has been determined to be sigma = 1.40 (+/-0.05) . 10(5) OM EGA-1m-1. The susceptibility of pyrrhotite and its field dependence in crease strongly with grain size. While the susceptibility of grains sm aller than 30 mum is field independent (up to 1.5 mT) it may increase as k is-proportional-to H-0.25 for mm-sized crystals in fields >10 muT . For most samples the Rayleigh law is inadequate to characterize indu ced magnetizations in weak alternating fields. When susceptibilities a re measured for geomagnetic anomaly modelling, laboratory fields shoul d be of similar intensity as the Earth's field and of frequency less-t han-or-equal-to kHz.