Oxidation states and speciation of secondary products on pyrite and arsenopyrite reacted with mine waste waters and air

Three morphologically distinct generations of Fe-oxyhydroxides were identified on pyrite surfaces reacted with unsaturated zone waters of a waste rock pile from the CON Mine (Northwest Territories, Canada). The paragenetic sequence includes an early mottled coating and a late massive (featureless) coating, separated by a generation of Fe(III)-oxyhydroxide of crystalline habit. Gypsum and halite precipitation were the last paragenetic events, and indicate intense wetting and drying in the unsaturated zone of the waste rock pile prior to collection. Fe 2p X-ray photoelectron spectra (XPS) of tarnished pyrite surfaces indicate at least two distinct secondary Fe(III)-oxyhydroxide phases, and combined with O 1s spectra, indicate ferrihydrite, goethite, hematite or maghemite. Minor As(IV) and As(III) are incorporated into these coatings. Fresh arsenopyrite surfaces reacted with air for 14 days, 16 months and 25 years develop exceptionally thin oxidized secondary coatings no more than about 50 Angstrom thick. XPS Fe 2p, O 1s and As 3d spectra indicate that the overlayer is composed of Fe(III)-hydroxides, arsenate (AsO[OH](3) or FeAsO4), and reduced arsenic species, including arsenite (As[OH](3) or FeAsO3). The abundance of reduced arsenic species is explained by diffusion of reduced As (e.g. As degrees) from the unoxidized interior of the mineral to the near-surface where it reacts with oxidants. Continuous supply of reduced As from the bulk, and progressive oxidation of arsenic in the near-surface, result in an effective passivating layer. Whereas these oxidized coatings passivate the surface against air-oxidation, aqueous solutions cause extensive leaching of arsenopyrite surfaces beneath the oxidized coatings. Apparently, the coatings offer little protection against leaching by oxidizing aqueous solutions, perhaps because the oxidized overlayer is compromised by dissolution of acidic and ferric arsenite and arsenate salts.