The kinetics of the exsolution of pentlandite from the monosulfide solid solution (mss) have been investigated using a series of anneal/quench and in situ cooling neutron diffraction experiments. Five mss compositions were examined by anneal/quench techniques covering the composition range Fe0.9Ni0.1S to Fe0.65Ni0.35S and using annealing temperatures between 423 and 773 K for periods from 1 h to 5 months. In situ cooling experiments were performed on four mss compositions in the range Fe0.9Ni0.1S to Fe0.7Ni0.3S. The samples of these solid solutions were heated to 973 K, and then cooled to 373 K in steps of 50 K over a 24 h period. The extent of exsolution was monitored by Rietveld phase analysis using powder neutron diffraction data. The anneal/quench experiments established that initial exsolution of pentlandite from mss above 573 K is very rapid and is effectively complete within 1 h of annealing. However, the mss/pyrrhotite compositions remained Ni rich (17 at%Ni) after 5 months annealing, indicating that compositional readjustment at low-temperatures occurs over long periods. Below 573 K, exsolution is less rapid with rate constants in the range 6 × 10−6 to 1 × 10−5 /s and the activation energy for exsolution of pentlandite from mss Fe0.8Ni0.2S between 473 and 423 K is 5 kJ/mol.
The in situ cooling experiments showed that the temperature at which exsolution commences upon cooling decreases from 873 K for Fe0.7Ni0.3S to 823 K for Fe0.9Ni0.1S and that exsolution effectively ceased on the time scale of the experiments at temperatures between 598 and 548 K. The kinetic data were analyzed using the Avrami model where y = 1−exp (-kntn) and the initial rates of exsolution were found to increase with Ni content from 2 × 10−6 /s for Fe0.9Ni0.1S to 4 × 10−5 /s for Fe0.7Ni0.3S. Both high Ni content and high M:S ratio served to facilitate nucleation rate, indicating that nucleation occurs at S vacancies within mss crystals rather than at grain boundaries. Values of the Avrami geometric constant n vary during exsolution upon cooling indicating three possible changes in the growth mechanism during the reaction. The roles of impurities and S fugacity on reaction rates are discussed. The rate constants for exsolution of pentlandite from mss/pyrrhotite in nature are estimated to be 4 or 5 orders of magnitude slower than those reported here, still very rapid on a geological time scale. High metal mobility persists in this system at low temperatures, even at room temperature, and the textures and compositions observed in nature are a consequence of very low-temperature (<100 °C) equilibration of assemblages over geological time scales.