Identification and solubility of iron sulfide in anaerobic lake sediment

Abstract

The iron sulfide that darkens the color of some anaerobic sediment has been tentatively identified as mackinawite (tetragonal FeS) in sediment from Linsley Pond, Conn. The mineral was identified by suspending sediment in solutions saturated with H 2 S at one atm pressure and determining the equilibrium ion activity product (equilibrium IAP) of the reaction: FeS=Fe (super ++) + S (super --) . The IAP was 10 (super -17.7) compared to published values of 10 (super -17.6) for synthetic mackinawite. These values can be assumed to approximate the "solubility product' of FeS in environments such as the hypolimnia of Linsley Pond and Lake Mendota. Since griegite has been shown to be stable relative to mackinawite in marine sediment, this mineral may be useful in differentiating Fe (super ++) -rich and H 2 S-rich environments in the fossil record.