Magnetic Susceptibility of Pyrrhotite: Grain Size, Field and Frequency Dependence

Abstract

The field and frequency dependences of the initial susceptibility of pyrrhotite have been analysed as a function of grain size, motivated by a strong field dependence recently observed for large (mm-sized) pyrrhotite crystals (Worm 1991) and smaller field dependences determined on smaller grain sizes by Clark (1984). In the present study, the frequency ranged from 30 Hz to 27 kHz. At 2 kHz, a field range from 0.05 to 1500 μT was investigated. Separate determinations of in-phase (k′) and quadrature (k″) susceptibility components allow for the analysis of eddy current effects. Up to 4 kHz the in-phase susceptibility of a pyrrhotite-ore specimen is practically independent of frequency whereafter it decreases while the quadrature component increases linearly with frequency to a value on the order of k′ at 20kHz for large grains. k″ is proportional to d2μσf where d is grain diameter, μ the intrinsic permeability, s the electrical conductivity and f the frequency. the frequency response of magnetite is essentially flat up to frequencies >20 kHz. Both frequency dependences agree well with calculations based on the theory by Wait (1951). the conductivity of the pyrrhotite ore has been determined to be σ= 1.40 (±0.05). 105Ω−1m−1. The susceptibility of pyrrhotite and its field dependence increase strongly with grain size. While the susceptibility of grains smaller than 30 μm is field independent (up to 1.5 mT) it may increase as k ∞ H0.25 for mm-sized crystals in fields >10μT. For most samples the Rayleigh law is inadequate to characterize induced magnetizations in weak alternating fields. When susceptibilities are measured for geomagnetic anomaly modelling, laboratory fields should be of similar intensity as the Earth′s field and of frequency ≤1 kHz.