Experimental study of formation mechanisms of hydrothermal pyrite

Abstract

Hydrothermal experiments were performed to synthesize pyrite crystals in the system HCl + NaCl + Fe 1− x S + H 2 O ± CaSO 4, in sealed silica capsules at temperatures between 150 and 350°C, for periods of up to eight weeks. The Fe 2+ and H 2S (aq) necessary for the nucleation and growth of the pyrite crystals were generated in situ in the capsules, by the reaction of HCl with pyrrhotite crystals. The solutions were either saturated or undersaturated with respect to pyrrhotite, with anhydrite present or absent from the systems (four experimental series). In the presence of anhydrite in pyrrhotite saturated experiments, considerably larger amounts of pyrite, marcasite, and elemental S formed compared to those without anhydrite present, and has been attributed to substantial sulfate reduction at T > 200° C by H 2(aq) and H 2S (aq) generated in situ in the reaction capsules. Two distinct pathways for pyrite nucleation and growth were recognized in the experiments, and resulted in seven pyrite types differing in crystal habits. The SEM study showed micron-sized pyrite cubes with stepped crystal faces which nucleate oriented on the surfaces of partially dissolved pyrrhotite crystals. Cube-octahedral pyrite crystals overgrow the earlier pyrite cubes, utilizing H 2S (aq) and Fe 2+ from the dissolving pyrrhotite grains. Pyrite crystals also nucleate and grow on the exterior of liquid S droplets, which formed from H 2S (aq) in the experimental capsules. The earliest pyrite crystals recognized in the SEM study on the sulfur surfaces are micron-sized cubes with distinct hopper (i.e., dendritic) morphology. Nucleation appears to be initiated by the reaction between Fe 2+ ions in solution with polysulfides, which occur concentrated on the liquid S droplet surface. Two mechanisms are introduced to account for the polysulfide concentration on sulfur droplet surfaces. The first mechanism suggests that polysulfide liquid, which is solved by liquid sulfur, is expelled to the S droplet surface due to the limited solubility of polysulfides in liquid S. A second mechanism to explain the accumulation of polysulfides in liquid S droplets involves the concept of equilibrium polymerization of S at T > 159° C. This process results in the formation of radical chain S. Polysulfides may form when the sulfur radical reacts with H 2S (aq) from the experimental solution upon which nucleation of pyrite occurs. The early pyrite habits represent transitional habit types which develop into equilibrium pyrite crystals characterized by a pyritohedral habit. This study observed that the type of nucleation surface, either pyrrhotite or S surfaces, controls the habit type of the experimental pyrite crystals. It is suggested that the type of precursor surface, and hence the specific chemical environment, controls the pyrite types that formed in the experiments.