In situ time-resolved X-ray diffraction of iron sulfides during hydrothermal pyrite growth

Pyrite formation under hydrothermal conditions has been studied using in situ time-resolved X-ray diffraction (XRD). This study employed two different synchrotron X-ray sources (National Synchrotron Light Source (NSLS), Advanced Photon Source (APS); Brookhaven and Argonne National Laboratories, USA, respectively) and two types of reaction cells (capillary and hydrothermal autoclave type) to examine reactions in the Fe–S system under both anoxic and controlled oxic conditions. Starting materials were mackinawite slurries equilibrated in reduced, H2S(aq) solutions and heated to a maximum of 190°C. The results show that under fully anoxic conditions, mackinawite persists to at least 120°C, while aerated (oxic) slurries transform rapidly to greigite, pyrite, magnetite and goethite. Possible reaction mechanisms are discussed in light of these results. These experiments are the first application of previously described in situ reaction cells to anoxic and oxic reactions in the Fe–S system. Finally, the capabilities of each cell type and their applicability to other systems are discussed.