The low-temperature heat capacity and thermodynamic properties of greigite (Fe₃S₄)

Heat capacity measurements provide important insights into the energetic, thermodynamic, and magnetic properties of materials. Herein we report the heat capacity of greigite (Fe₃S₄) from 1.8 to 300 K. Greigite is a magnetic spinel mineral and through a ferromagnetic magnon term, C_fsw = B_fswT^3/2, ferrimagnetic ordering is observed in the low-temperature heat capacity. Using a set of theoretical fits of the experimental data, we calculate the thermodynamic functions, including the standard entropy (Δ₀^TS_m°). Greigite is important in iron sulfide formation and reaction pathways in environmental, ore-forming, and technological settings and previous work has measured enthalpies (ΔH_r°) of formation and decomposition to neighboring phases. In this work, the stability of greigite relative to the elements is demonstrated with a negative Gibbs energy (ΔG_r°) of formation and the stability relative to decomposition products of pyrrhotite (FeS_1.092) and pyrite (FeS₂) is demonstrated with a positive Gibbs energy (ΔG_r°) of decomposition. Values of the standard thermodynamic functions C_p,m°, Δ₀^TS_m°, Δ₀^TH_m°, and Φ_m° are tabulated.