APPLICATION OF BONDING MODELS TO SULFIDE MINERALS

Sphalerite (p-ZnS) has a cubic crystal structure in which both Zn and S occur in regular tetrahedral coordination. Pure sphalerite is a diamagnetic semiconductor with a large band gap (—3.6 eV Shuey, 1975). On the basis of the observed structure and properties, the simple MO energy-level diagram shown in Fig. 6.1 can be proposed to describe the bonding in a ZnS4 cluster molecular unit. Overlaps between outermost s and p 

Major subdivision and group Structure type Examples 

Disulfide group (all containing dianion units in the structure) Pyrite-type (cubic) Pyrite (FeS,), cattierite (CoS,), vaesite (NiS,), etc. 

Galena group NaCl-type (cubic) Galena (PbS), clausthalite (PbSe), altaite (PbTe), alabandite (a-MnS) 

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Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

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