MECHANISMS OF PYRITE AND MARCASITE FORMATION FROM SOLUTION .3. HYDROTHERMAL PROCESSES

The formation of pyrite and marcasite from solutions between 100 and 300-degrees-C has been examined experimentally, and the solubility product for the initial precipitate upon mixing of Fe2+ and H2S solutions has been determined. Below 300-degrees-C, pyrite and marcasite form via an FeS precursor. The precursor phase is crystalline, nearly stoichiometric FeS with a solubility product of 10(2.9 +/- 0.2) (K = a(Fe2+)a(H2S)/(a(H+)2). It reacts progressively to mackinawite, hexagonal pyrrhotite, and/or greigite before forming pyrite or marcasite. In the presence of elemental sulfur, thiosulfate, or polysulfides, the rate of reaction is extremely fast (minutes at 150-degrees-C). Without these sulfur species and only hydrogen sulfide or bisulfide present, the conversion rate drops significantly (1 h at 300-degrees-C). In the absence of any aqueous sulfur source, the conversion proceeds at an indetectably slow rate. Because the rate of direct FeS2 nucleation is insignificant in (slightly) acidic solutions below 300-degrees-C, most pyrite and marcasite in hydrothermal ores form via the conversion of an FeS precursor.