Magnetic Characterization of Synthetic SD-like Pyrrhotite and Its Demagnetization under Hydrostatic Pressure up to 1.8 GPa
Abstract:Monoclinic pyrrhotite (Fe7S8) is a common ferrimagnetic mineral in both terrestrial rocks and meteorites (e.g., SNC, Rumuruti chondrites…). It is also recognized as a candidate magnetic mineral for Martian magnetic anomalies. We synthesized SD-like monoclinic pyrrhotite at 245°C using the molten-salt synthesis method . Here we characterize its magnetic properties including its behavior under hydrostatic pressure up to 1.8 GPa. Data were collected in the 10K to 360°C temperature range and include measurements of low-field magnetic susceptibility (χ0), thermomagnetic curves χ0(T), major hysteresis loops, back-field remanence demagnetization curves, first-order reversal curve (FORC), alternating field and pressure demagnetization of saturation isothermal remanent magnetization (SIRM or Mrs), low temperature MPMS datasets (field and frequency dependencies of χ0, field-cooled and zero-field-cooled remanence FC-ZFC and room temperature SIRM heating-cooling cycles) as well as X-ray diffraction (XRD) spectra. χ0(T) indicates a single Curie point at 320°C characteristic of monoclinic pyrrhotite.
For application of hydrostatic pressure up to 1.8 GPa we used a nonmagnetic high-pressure cell of piston-cylinder type entirely made of “Russian alloy” (Ni57Cr40Al3) with 8 mm of inner diameter similar to the cell, described in . Application of 1.8 GPa resulted in demagnetization (decrease in SIRM) of the sample by 38%. Repeated cycling from 1.8 GPa to atmospheric pressure and back resulted in further decrease in remanence by 44% (for 3 cycles). The characteristic Besnus transition of pyrrhotite is observed at ~34 K. The observed hysteresis parameters (Mrs/Ms=0.53, Bcr/Bc=1.17, Bcr=41 mT, where Ms is saturation magnetization, Bc and Bcr are coercivity and remanent coercivity, respectively) are consistent with pseudo-single-domain range, previously established in literature . Superparamagnetic (SP) grains are not present in the sample as no frequency dependence of χ0 is observed. References:  Chareev D. A., Voronin M. V., Osadchii E. G. 2014. Amer. Mineral., doi:10.2138/am-2014-4753 (accepted).  Sadykov R.A. et al. 2008. Rev. Sci. Instr. 79: 115102.  Dekkers M.J. 1988. Phys. Earth Planet. Int. 52, 376-393.