Source of stable remanence in chromite ores

Abstract

Rock magnetic investigations of both the natural chromite ores and their purified chromite concentrate (Mg_.6Fe_.4Cr_1.4Al_.6O₄) were carried out. These include, measurement of natural and saturation remanent magnetism and their stability with respect to alternating field and thermal demagnetisation, IRM acquisition studies, saturation magnetisation versus temperature studies and high field (1000 mT) hysteresis characteristics. From these studies it was observed that the medium/high coercivity component of remanence is associated with the chromite minerals and is carried largely or entirely by them. Therefore, it is concluded that chromites carry permanent magnetisation of palaeomagnetic significance. However, this result is in sharp contrast to the assumption in the literature that the spinels of the above composition are non-magnetic. The study has also demonstrated that the magnetic properties of artificial Fe-Cr spinels do not conform to their natural analogues. Reasons for the discrepancy have been postulated.