Magnetic properties of hydrothermally synthesized greigite (Fe3S4) - I. Rock magnetic parameters at room temperature

Abstract

Greigite of variable purity has been synthesized hydrothermally at ∼140°C by injecting a Mohr's salt solution into a sodium sulphide solution containing dissolved zerovalent sulphur, ageing for 30-960 min and rapidly quenching. Oxygen was rigorously excluded during the synthesis. Scanning electron microscope (SEM) observations yield a grain-size range up to 400 nm. The variable purity was deduced from a varying specific saturation magnetization (3-29 Am2 kg-1). The Mr3/M3 or σr3/σ3 ratio varies between 0.3 and 0.4, which is indicative of fine-grained greigite, concurring with SEM observations. Hysteresis measurements at low temperatures indicate the presence of grains that are superparamagnetic at room temperature. Coercivity parameters [(BO)c 13-43 mT; (BO)cr 37-73 mT] are the highest obtained to date for synthetic greigite and compare well with those measured for natural greigite. Ranges for other rock magnetic parameters are discussed. Greigite hysteresis parameters overlap those of magnetite to a large degree. A diagnostic interparametric ratio for greigite is its high σr3/χin ratio; the range of the present greigite is 40-75 kA m-1.