Magnetic Properties of Natural Goethite-Ii. Trm Behaviour During Thermal and Alternating Field Demagnetization and Low-Temperature Treatment

Abstract

Summary The behaviour of a thermoremanent magnetization (TRM), acquired by cooling from 145°C in a field of 16 kAM-1, is reported for five natural goethites in a grain-size range of <5 up to 250nm. the TRM intensity increases with isomorphous substitution in the goethite until a certain maximum, to decrease again at higher levels of isomorphous substitution due to lowering of the goethite Neel temperature. Different grain-size trends are observed for the TRM intensity: in some samples it slightly decreases with grain size, whereas in others a maximum is observed in the intermediate grain-size range. With increasing isomorphous substitution the maximum blocking temperatures during continuous and stepwise thermal demagnetization are decreasing from 90–105°C down to 45–60°C. No grain-size dependence is observed in thermal decay curves. The initial susceptibility measured at room temperature generally decreases some 10 per cent after annealing at 145°C. the decrease after once annealing and its constancy after subsequent annealings at the same temperature is a strong indication that these goethites have never been at that temperature during their geological history. Hence, their NRM is most probably of chemical origin. The goethites are extremely hard with respect to AF demagnetization. Hardly any remanence decrease is observed after application of the maximum alternating fields available (240 kAM-1 or 300 mT), varying from nill to 8 per cent without grain-size dependence. Only in extremely finely crystalline goethite is the decay larger and shows grain-size dependence. For all goethites, the remanence steadily increases with decreasing temperature upon cooling to liquid nitrogen temperature in a field-free space. With increasing isomorphous substitution, the relative remanence increase is larger. Upon rewarming the remanence decreases again, showing virtually reversible behaviour. As goethite is unique in this behaviour, low-temperature treatment can be used for the identification of goethite remanences without the need to heat the sample, avoiding chemical alteration.