Exsolution in the mss-pentlandite system: Textural and genetic implications for Ni-sulfide ores

Experimentation in synthetic systems is essential for a correct interpretation of ore-mineral textures such as the pyrrhotite-pentlandite (po-pn) intergrowths of Ni-sulfide deposits. Within the Fe-Ni-S system, isothermal and cooling exsolution experiments were performed on monosulfide solid solution (mss) of various compositions using sealed, evacuated silica-glass tubes. The charges were annealed between 400° and 200°C for periods ranging from 20 minutes to 5 weeks; also, charges were cooled from 500° to 100°C at 1°C/h. Early-formed isothermal textures form by nucleation and growth, and depend on the initial degree of supersaturation and metal diffusivities; the final textures result from further growth and coarsening of the early forms and are particularly sensitive to the diffusivities, which are a function of temperature. The cooling-rate experiments indicate that massive pn develops by exsolution between 610° and about 250°C during slow cooling from temperatures above the mss-pn solvus; coarsened pn forms along basal planes in the mss matrix between about 250° and 150°C; pn "Flames" result at 150°C or slightly below. Crossing pn lamellae, obtained isothermally at high degrees of supersaturation and virtually unknown in ores, cannot form during slow cooling.