The effects of temperature and time on phase transitions in FeS were investigated by X-ray diffraction in the temperature range 296–573 K and by Mössbauer spectroscopy in the range 285–475 K. Natural troilite from the Agpalilik meteorite (Cape York shower), containing ~0.17 wt% Cr, was used in the study. The α transition from troilite (P62c) to MnP structure (Pnma) occurs in an interval of at least 40 K and is complete at 413 K. No change of the NiAs-type (P63/mmc) subcell volume due to the transition could be detected. Magnetic spin flip in the MnP-type phase is time dependent, requiring several weeks at 429 K. The transition from the MnP-type phase to troilite is sluggish. The relative concentrations of the phases simultaneously present below 413 K depend on both the ambient temperature and the thermal history. Below 413 K, the transition is hampered by differing subcell volumes, causing structural incoherence that increases with the degree of undercooling. In addition, the magnetic spins in the MnP-type phase are preferentially oriented perpendicular to the necessary configuration for the transition. The two phases constitute an unstable assemblage at room temperature. The metal-metal bond strength is anisotropic and increases as temperature decreases. This may be an important factor in the crystallographic and magnetic transitions of FeS. The troilite structure probably forms along different reaction paths depending on cooling rate.

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