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Transformation of synthetic mackinawite to hexagonal pyrrhotite: A kinetic study

Abstract The kinetics of the transformation of synthetically prepared mackinawite to hexagonal pyrrhotite have been studied to clarify the relationship between these two phases. Kinetic analysis using the Johnson-Mehl equation is based on measurements of the fraction of hexagonal pyrrhotite obtained by heating mackinawite samples in the temperature range of 530-545 K. This gives isothermal rate constants from which an activation energy of 493 kllmol and a frequency factor of 3.7 × 1045min-1 have been established. The mechanism proposed for this transformation is that of a solid-state diffusion process. Kinetic data derived from this experiment may be used to show that persistence of mackinawite in nature can be explained by kinetic factors alone. Reactions at low temperatures, in which mackinawite is oxidized or further sulfidized, are therefore more important than the kinetically inhibited transformation to hexagonal pyrrhotite. The irreversible mackinawite to hexagonal pyrrhotite relation is compared with the reversible tetragonal FeSe to NiAs-type FeSe phase transformation. This contrast in stability behavior is examined in terms of the thermal stability of the respective tetragonal phases relative to the upper magnetic ordering temperatures of their corresponding hexagonal phases.