The Catalytic Role of Pyrrhotite in Hydrothermal Alteration of Pentlandite to Violarite
The catalytic role of pyrrhotite in the hydrothermal alteration of pentlandite to violarite was investigated at 125°C under vapor saturated pressure. The starting minerals used were pure pentlandite, pure pyrrhotite and pyrrhotite-bearing pentlandite assemblage. The hydrothermal fluid used was a 200 mM acetate buffer solution (pH 3.90). The textures of the minerals before and after reactions were monitored using a scanning electron microscopy (SEM), indicating a dissolution-reprecipitation mechanism. The progress of the reaction was monitored by Rietveld quantitative phase analysis based on X-ray diffraction (XRD) experiments. The reaction kinetics of the pyrrhotite-bearing pentlandite assemblage is 2.5 times faster than that of pure pentlandite. However, the kinetics of pentlandite physically mixed with pyrrhotite was even slower than pure pentlandite, and the kinetics of both pure pentlandite and pyrrhotite-bearing pentlandite slowed down when physically mixed the two materials. These experiments indicate that pyrrhotite may kinetically favor the reaction when it exists in the assemblage but kinetically impede the reaction when it is only physically mixed with pentlandite. The results are discussed in terms of pyrrhotite dissolution. The dissolution of pyrrhotite reverses the dissolution of pentlandite and consequently impedes the kinetics. On the other hand, pyrrhotite dissolution in the boundary of the two phases in pyrrhotite-bearing pentlandite assemblage produces more surface area of pentlandite and favors its dissolution kinetics, and thus accelerates the reaction.