Magnetic properties of natural pyrrhotite Part I: Behaviour of initial susceptibility and saturation-magnetization-related rock-magnetic parameters in a grain-size dependent framework

The grain-size dependence of the initial susceptibility ( X in), saturation magnetization ( J s), saturation remanence ( J rs), coercive force ( H c), remanent coercive force ( H cr) and remanent acquisition coercive force ( H cr′), is reported for four natural pyrrhotites in a grain-size range from 250 μm down to < 5 μm. X in decreases with decreasing grain size (range 7 × 10 −5−1 × 10 −5 m 3 kg −1); H c, H cr and H cr′ all increase with decreasing grain size (ranges respectively 7–80, 8–85 and 14–80 kA m −1). The magnitudes of J s (8.9–16.7 A m 2 kg −1) and J rs (2.0–6.0 A m 2 kg −1) correspond with the trend indicated by the microprobe analysis (47.8 down to 46.9 at.% equivalent Fe). J rs shows a maximum in the pseudo-single domain region (range 2.0–6.0 A m 2 kg −1). J s shows a marked decrease for the finest grain-size fractions. Typical for pyrrhotite are a large H c/ H cr ratio and a J rs/ X in ratio which is larger than H cr. When the presence of pyrrhotite has been established in a sample, its grain size is preferable determined by the H cr trend because pyrrhotite compositional differences hardly show up in this trend.