Measurements of thermal magnetic susceptibility of hematite and goethite

Magnetic susceptibility (MS) of natural specimens of hematite and goethite is studied under continuous heating with various additives: with carbon (sugar), nitrogen (carbamide), and elemental sulfur. It is found that heating of hematite with carbon above 450A degrees C results in the formation of single-domain magnetite, while the magnetic susceptibility rises by a factor of 165. The increase in magnetic susceptibility on heating of hematite with nitrogen above 540A degrees C reflects the generation of a single-domain maghemite with the Curie point of about 650A degrees C, which is stable to heating. After the first heating, the magnetic susceptibility increases by 415 times. The subsequent cycle of thermal treatment results in the transition of maghemite to hematite, a decrease of MS, and an increase of coercivity. Heating with sulfur produces a stable single-domain magnetite at a temperature above the Curie point, which is manifested in the cooling curves. Here, the MS increases by a factor of 400. The heating curves for goethite exhibit a sharp drop in susceptibility to a temperature of 350-360A degrees C, which reflects the transition of hematite to goethite. Heating of hematite with carbon produces stable maghemite at above 530A degrees C, and with sulphur and nitrogen, it produces magnetite. When heated with pyrite, hematite reduces to magnetite under the action of sulfur released from pyrite.