Heat Capacities and Thermodynamic Properties of the Pyrrhotites FeS and Fe0.877S from 5 to 350°K

Heat capacities were measured from 5 to 350°K for two pyrrhotites at the iron-rich limit (Fe1.000S) and the sulfur-rich limit (Fe0.877S) of the homogeneity range. The heat-capacity curves follow the normal sigmoid shape with the exception of an upward trend for Fe1.000S above 300°K, and two small, broad humps for Fe0.877S near 8 and 30°K. From these data the entropy and enthalpy functions were calculated. At 298.15°K the values of Cp, S°–S0°, and (H°–H0°)/T are 12.08, 14.415, and 7.496 cal/mole °K, respectively, for Fe1.000S, and 11.92, 14.529, and 7.396 cal/mole °K for Fe0.877S. The values on FeS are lower by approximately 10% over the range 58 to 296°K than those previously reported in the literature.