Magnetic Characterization of Synthetic SD-like Pyrrhotite and Its Demagnetization under Hydrostatic Pressure up to 1.8 GPa

Wednesday, 17 December 2014: 3:10 PM
Natalia S. Bezaeva1,2, Dmitriy A. Chareev3, Pierre Rochette4, Myriam Kars5 and Jerome Gattacceca4, (1)Ural Federal University, Yekaterinburg, Russia, (2)Kazan Federal University, Kazan, Russia, (3)Institute of Experimental Mineralogy RAS, Chernogolovka, Russia, (4)CEREGE CNRS/ Aix-Marseille Université, Aix en Provence, France, (5)Center for Advanced Marine Core Research, Kochi University, Nankoku, Japan
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 [1]. 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 [2]. 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 [3]. Superparamagnetic (SP) grains are not present in the sample as no frequency dependence of χ0 is observed. References: [1] Chareev D. A., Voronin M. V., Osadchii E. G. 2014. Amer. Mineral., doi:10.2138/am-2014-4753 (accepted). [2] Sadykov R.A. et al. 2008. Rev. Sci. Instr. 79: 115102. [3] Dekkers M.J. 1988. Phys. Earth Planet. Int. 52, 376-393.