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

Abstract

S U M M A R YFive goethite concentrates, separated from natural samples of different supergene geological origin, are sieved in 12 well-defined grain-size fractions ranging from 250 pm down to <5 pm. 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& 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 (Xi"), the high field susceptibility (xhf), the saturation magnetization (J,), the saturation remanence (Jrs; acquired in fields of 16 X lo6 Am-' or 20 T), the coercive force (H,), ferromagnetic coercive force (H, remanent coercive force (Her) and the remanent acquisition coercive force (Her,). X,,, H, and H,, decrease with grain size for each sample. A tendency to decrease with grain size is observed for Jr,. Xi, shows a weak maximum in the intermediate grain-size range. H,,,,,. is approximately constant in the coarse grain-size fractions and falls in the same grain-size range as Xi, rises. No grain-size dependence of J, is observed. Her. does not show a consistent grain-size dependence: in three samples it is grain-size independent and the other two it decreases with grain sue. There is a great variety between the samples in the absolute figures of J, (30-450 X Am2 kg-I), J,, (15-115 X lop3 Am2 kg-I), H, (2-20 x lo4 Am-'), H,,,,,. (2-70 X lo4 Am-'), H,, (40-320 X lo4 Am-') and H,,, (100-550 Xlo4 Am-'). Xi, shows smaller differences (0.50-1.50 X m3 kg-I) and Xh, is approximately equal for all samples (varying from 0.40 x lop6 m3 kg-' down to 0.20 x lop6 m3 kg-' 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.

Low‐temperature magnetic properties of lepidocrocite

“…Goethite has a range of coercive forces (25 -875 mT) and is only saturated in fields above 10 T [Dekkers, 1989a].…”

“…Although there have been numerous studies on the magnetic properties of goethite [Bocquet and Kennedy, 1992;Dekkers, 1989aDekkers, , 1989bDekkers, , 1990Hedley, 1971], less is known about the other common soil iron hydroxides (e.g., ferrihydrite and lepidocrocite). Zergenyi et al [2000] examined the magnetic properties of a highly crystalline ferrihydrite.…”

Magnetic petrology of equatorial Atlantic sediments: Electron microscopy results and their implications for environmental magnetic interpretation

“…[43] Goethite is antiferromagnetic, but can carry a parasitic ferromagnetic moment increasing with the number of lattice defects; the observed crystals should have SD magnetic grain size [e.g., Dekkers, 1989]. The microcrystalline goethite was solely detected at the western SLR site.…”

Non‐saturation of the defect moment of goethite and fine‐grained hematite up to 57 Teslas

“…Room temperature isothermal remanent magnetization (RT IRM) of these two minerals is very hard and magnetic fields available in most rock magnetic laboratories (1 -3 T range) are unable to approach saturation. The use of cryogenic magnets up to 7 T [Maher et al, 2004], pulse fields up to 7 or 9 T [France and Oldfield, 2000;Walden et al, 2000], and Bitter magnets up to 15-20 T [Dekkers, 1989;Rochette and Fillion, 1989], has proved to be insufficient to reach saturation in both minerals. In this study, we have monitored the RT IRM acquisition of selected samples of synthetic and natural goethite and hematite in very high pulse fields up to 57 T and report here that most samples are still far from saturation.…”