Rockmagnetic properties of fine‐grained natural low‐temperature haematite with reference to remanence acquisition mechanisms in red beds

Abstract

Grain‐size dependence of some remanent magnetic properties of a natural low‐temperature haematite is reported. The haematite was obtained from a gossan in Montarnu (France) and upgraded by magnetic separation. Seven grain‐size fractions were prepared ranging from 10 down to <0.25 μm by means of microprecision sieving and centrifuging. The haematite has an average crystallite size of 60 nm and contains minor contents of Si, Al and H2O. Its unit cell dimensions are very close to those of ideal haematite. The magnetic properties of the haematite are strongly dependent on its thermal history. For the original unheated haematite fractions, the isothermal saturation remanence (Jrs) decreases with grain size. The remanent coercive force (Hcr), remanent acquisition coercive force (Hcr,) and median destructive field of the saturation remanence (H1/2I) are maximal in the 1.0‐0.5 μm fraction. This grain‐size range can be understood as the SD threshold size in analogy with ferrimagnetic minerals. Jrs/(Hcr x Xin) is a useful grain‐size indicator. Lowtemperature cycling of Jrs shows identical behaviour for all fractions; the remanence decreases slightly and shows no sharp Morin transition. After annealing the haematite fractions at 700 °C, Jrs is considerably reduced relative to the original haematite predominantly due to removal of the defect moment. Both Hcr and Hcr, have increased and their maximal values are shifted to the 2.1‐1.0 μm fraction. All fractions show a much sharper Morin transition during low‐temperature cycling of Jrs due to crystallite enlargement. TRM decreases with grain size. The different rock‐magnetic properties before and after annealing might provide a basis for the discrimination in sediments between a haematite DRM with a high‐temperature origin and a haematite CRM which has a low‐temperature origin. Two natural red beds, one with a CRM and one with a DRM associated with high‐temperature haematite, broadly show the expected differences in coercivity and in annealing behaviour, although a full account of all aspects of the observed behaviour can not yet be given. Copyright © 1989, Wiley Blackwell. All rights reserved