MAGNETIC PROPERTIES OF SEDIMENTARY GREIGITE (FE3S4)

Abstract

New data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite (Fe3S4). Several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite. Such discriminants are necessary because greigite displays a similar (or higher) range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals. Sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility (Mrs), which are accompanied by hysteresis ratios of MrsMs ~ 0.5 andBcrBc ~ 1.5 (Ms is the saturation magnetization,Bc is the coercive force and Bcr is the coercivity of remanence). Greigite has no low-temperature phase transition, whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 K, and unoxidized magnetite displays the Verwey transition at 118 K. Low-temperature measurements may therefore prove valuable in distinguishing between magnetite, pyrrhotite and greigite, although the absence of a low-temperature transition cannot be taken as definitive evidence for the presence of greigite. Greigite also displays characteristic high-temperature behaviour, with a major drop in magnetization between 270 and 350°C. Determination of the Mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite. Subsequent low-and high-temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals.