Identifying goethite and hematite from rock magnetic measurements of soils and sediments

Abstract

We describe a sequence of experiments involving the acquisition of isothermal remanent magnetization (IRM) in high (>2 T) fields, followed by orthogonal demagnetization and the cooling and heating of remanent magnetization (Mr) over the temperature range −196°C to +680°C. The main aim of these experiments is to improve the basis for identifying the imperfect antiferromagnetic minerals contributing to the magnetic properties of natural soil and sediment samples, which often contain complex mineral mixtures. The procedures developed are first applied to a series of ore and synthetic samples of hematite and goethite, each characterized by X‐ray diffraction (XRD) spectra, and second to controlled mixtures of the two minerals. The results confirm the mineral‐diagnostic nature of the experiments used. They are subsequently used to identify goethite and hematite contributions to the rock magnetic properties of a variety of soils and recent sediments. Goethite can be unambiguously identified, often accompanied by hematite, in the majority of the samples studied, despite indications from the synthetic mixtures that the ratio of goethite to hematite may have to exceed an order of magnitude before the goethite signatures are readily recognizable. The rock magnetic tests are a much more sensitive basis for recognizing the presence of goethite in natural samples than are XRD analyses. The results suggest that goethite's contribution to the rock magnetic properties of soil and sediment samples may often have been underestimated.