Magnetic Properties of Natural Goethite‐I. Grain‐Size Dependence of Some Low‐ and High‐Field Related Rockmagnetic Parameters Measured At Room Temperature

Abstract

Five goethite concentrates, separated from natural samples of different supergene geological origin, are sieved in 12 well‐defined grain‐size fractions ranging from 250 μm down to <5 μm. the goethites show differences in their isomorphous substitution and in their contents of dispersed non‐Fe elements. the crystallite size ranges from 240 to 480 Å which is considerably smaller than the grain size of even the smallest fraction. Nevertheless grain‐size dependent trends are present in most observed rockmagnetic parameters. the behaviour in a grain‐size dependent framework is reported for the initial susceptibility (Xin), the high field susceptibility (Xhf), the saturation magnetization (Js), the saturation remanence (Jrs; acquired in fields of 16 times 106 Am−1 or 20 T), the coercive force (Hc), ferromagnetic coercive force (Hcferr.), remanent coercive force (Hcr) and the remanent acquisition coercive force (Hcr). Xhf, Hc and Hcr decrease with grain size for each sample. A tendency to decrease with grain size is observed for Jrs. Xin shows a weak maximum in the intermediate grain‐size range. Hcferr. is approximately constant in the coarse grain‐size fractions and falls in the same grain‐size range as Xin rises. No grain‐size dependence of Js is observed. Hcr' does not show a consistent grain‐size dependence: in three samples it is grain‐size independent and the other two it decreases with grain size. There is a great variety between the samples in the absolute figures of Js (30−450 times 10−3 Am2 kg−1), Jrs (15−115 times 10−3 Am2 kg−1), Hc (2−20 times 104 Am −1), Hcferr. (2−70 times 104 Am−1), Hcr (40−320 times 104 Am−1) and Hcr' (100−550 times 104 Am−1). Xin shows smaller differences (0.50−1.50 times 10−6 m3 kg−1) and Xhf is approximately equal for all samples (varying from 0.40 times 10−6 m3 kg−1 down to 0.20 times 10−1 m3 kg−1 with decreasing grain size). Sample differences and some ratios of these parameters are discussed in terms of different goethite chemical composition, crystallite size and the presence of intergrowths. the magnetic properties of mirocrystalline goethite are best understood by taking into account crystallite size and magnetic interaction amongst crystallites. Copyright © 1989, Wiley Blackwell. All rights reserved