Analysis of the Major Fe Bearing Mineral Phases in Recent Lake Sediments by EXAFS Spectroscopy

Abstract

Abstract. Extended X-ray absorption fine structure (EXAFS) spectroscopy and chemical analyses were combined to determine the Fe bearing minerals in recent lake sediments from Baldeggersee (Switzerland). The upper section of a laminated sediment core, deposited under eutrophic conditions, was compared to the lower part from an oligotrophic period. Qualitative analysis of FeK EXAFS agreed well with chemical data: In the oligotrophic section Fe(II)-O and Fe(III)-O species were present, whereas a significant fraction of Fe(II)-S sulfides was strongly indicated in the eutrophic part. A statistical analysis was performed by least square fitting of normalized reference spectra. The set of reference minerals included Fe(III) oxides and Fe(II) sulfides, carbonates and phosphates. In the oligotrophic regime no satisfying fit was obtained using the set of reference spectra, indicating that siderite (FeCO 3 ) was not present in a significant amount in these carbonate-rich sediments. Simulated EXAFS spectra for a (Ca x , Fe 1−x )CO 3 solid solution allowed reconstructing the specific features of the experimental spectra, suggesting that this phase was the dominant Fe carrier in the oligotrophic section of the core. In the eutrophic part, mackinawite was positively identified and represented the dominant Fe(II) sulfide phase. This finding agreed with chemical extraction, which indicated that 18-40 mol% of Fe was contained in the acid volatile iron sulfide fraction. EXAFS spectra of the eutrophic section were best fitted by considering the admixture of mackinawite and the Fe-Ca carbonate phase inferred to be predominant in the oligotrophic regime.

Aerobic oxidation of mackinawite (FeS) and its environmental implication for arsenic mobilization

“…In our previous study (Jeong et al, 2008), the synthesized FeS was found to be nanoparticulate with a specific surface area of 276-345 m 2 /g. This chemically precipitated nanocrystalline phase is similar to naturally occurring mackinawite (Spadini et al, 2003;Wolthers et al, 2003;Rickard and Morse, 2005).…”

Characterization of synthetic nanocrystalline mackinawite: Crystal structure, particle size, and specific surface area

“…The first approach usually leads to more crystalline mackinawite (Lennie and Vaughan, 1996;Mullet et al, 2002). In contrast, the other methods lead to less crystalline phases that are more similar to those found in natural environments (Berner, 1967b;Spadini et al, 2003;Wolthers et al, 2003).…”

Surface chemistry of disordered mackinawite (FeS)

“…Anoxic marine sediment pore waters are saturated with respect to disordered mackinawite (Berner, 1967;Spadini et al, 2003). It is the first iron sulfide to form in most ambient environments, and with time, it reacts to form more stable iron sulfide phases such as ordered mackinawite, greigite, and ultimately pyrite or pyrrhotite.…”