Sphalerite

Abstract: Abundant, micrometer-scale, spherical aggregates of 2- to 5-nanometer-diameter sphalerite (ZnS) particles formed within natural biofilms dominated by relatively aerotolerant sulfate-reducing bacteria of the family Desulfobacteriaceae. The biofilm zinc concentration is about 106 times that of associated groundwater (0.09 to 1.1 parts per million zinc). Sphalerite also concentrates arsenic (0.01 weight %) and selenium (0.004 weight %). The almost monomineralic product results from buffering of sulfide concentrations at low values by sphalerite precipitation. These results show how microbes control metal concentrations in groundwater- and wetland-based remediation systems and suggest biological routes for formation of some low-temperature ZnS deposits.

Abstract: The authors studies of aqueous, abiotic pyrite oxidation in oxygen-saturated and anaerobic Fe(III)-saturated systems span pH values from 2 to 9 and include analyses of sulfite, thiosulfate, polythionates, and sulfate. In addition, they evaluated procedures for cleaning oxidation products from pyrite surfaces. As in silicate experiments, the preparation of the pyrite surface is critical to a valid interpretation of the onset of pyrite oxidation. The rates in oxygen-saturated systems (1) were relatively independent of pH, (2) gave linear sulfoxy anion production, (3) produced thiosulfate and polythionates at pH >3.9, and (4) produced intermediate sulfoxy anions only at high stirring rates. In anaerobic Fe(III)-saturated systems no intermediates were observed. From these results, along with the generally faster rate of oxidation in Fe(III)-saturated systems, we conclude that Fe(III) is the more important, direct oxidant of pyrite. This conclusion is supported by theoretical consideration regarding the low probability of a direct reaction between paramagnetic molecular oxygen and diamagnetic disulfide. The authors findings extend earlier models for the pyrite oxidation mechanism because of the larger range of experimental conditions that they have studied. Preliminary results from studies of sphalerite oxidation, in which they have found thiosulfate, support the hypothesis that thiosulfate is a keymore » intermediate related to the reaction mechanisms, independent of the bounding structure of the sulfide mineral.« less