Grain-size dependence of initial susceptibility and saturation magnetization-related parameters of four natural magnetites in the PSD--MD range

Abstract

Summary. Initial susceptibility, coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain-size fractions of a natural magnetite and titanomagnetite, both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite, both of which show exsolution phenomena. It was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic Fe-rich inclusions. These inclusions are magnetically screened in coarse grains; only for the lower grain sizes studied they have an appreciable influence on the data. The size of the fractions varied from < 5 to 250 μm. The parameters and several parameter ratios are plotted as a function of grain size. Several of these parameters can be used as mineralogical and grain-size indicators. The varying degree of exsolution of the minerals is reflected in the gradients of the plots. From plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size. Variation of the various parameters with grain size is explained in terms of a gradual transition from large PSD grains to MD grains without there being any sharp transition. For titanomagnetite (x= 0.55) the transition takes place at grain sizes that are about 15 μm larger than for magnetite.