Heat capacities of iron disulfides Thermodynamics of marcasite from 5 to 700 K, pyrite from 300 to 780 K, and the transformation of marcasite to pyrite

Abstract

The heat capacity of purified natural marcasite has been determined by adiabatic-shield calorimetry in the region 5 to 700 K where it transforms to pyrite exothermically. Values of thermodynamic functions at 298.15 K are Cp, {So(T) - So(0)}, and {Ho(T) - Ho(0)} are 14.92 calth K-1 mol-1, 12.88 calth K-1 mol-1, and 2328 calth mol-1, respectively, for marcasite (FeS2). Our earlier measurements on pyrite have been extended to 770 K, and show that the heat capacity of marcasite is slightly higher than that of pyrite over the entire range of mutual existence. The transformation to pyrite is significantly exothermic at 700 K, [Delta]Ht = - (1.05 +/- 0.05) kcalth mol-1, and correspondingly, Ho(T = 0, marcasite) - Ho(T = 0, pyrite) = (0.99 +/- 0.05) kcalth mol-1. Marcasite is thus metastable with regard to pyrite over the whole temperature region and owes its formation and persistence to kinetic factors. At 298.15 K the standard enthalpies, entropies, and Gibbs energies of formation for FeS2 phases are: