Petrographic observations of iron, copper and zinc sulfides in freshwater canal sediment

Abstract

Sulfide minerals are known to be important hosts for metals in many freshwater and marine sediments, but little is known about their petrography, particularly in fine-grained recent freshwater sediments. In this paper the results of a Cryogenic SEM investigation of sulfidc textures in two fine-grained anaerobic canal-bed muds are reported. The technique allows direct observation of the undisrupted mineral and organic textures in the canal-bed mud. Iron sulfides occur in two forms: coatings on biofilms and framboids. Iron sulfide-coated biofilms have iron sulfur ratios in the range FeS to Fe3S4. Framboid structures display a continuum of textures from greigite proto-framboids with a poorly developed crystallite texture to pyrite framboids with well developed crystallites. Proto-framboids tend to be smaller than framboids. A positive correlation was observed between crystallite diameter and framboid diameter. By dividing the framboid diameter by the crystallite diameter it was deduced that framboids tend not to have single crystallites at their centers. Copper occurs as a discrete sulfide with a composition similar to chalcopyrite. Zinc occurs as a zinc iron sulfide with metal-to-sulfur ratios in the range 0.59 to 0.87. Copper sulfides tend to nucleate on surfaces whereas the zinc sulfides occur both on surfaces and as floccular precipitates in open pore space.