Identification of Mackinawite and Constraints on Its Electronic Configuration Using Mössbauer Spectroscopy

Christian Schröder,Alastair Tait,Catherine A Mccammon,Stefan Peiffer,Ian B Butler,Moli Wan

doi:10.3390/min10121090

Christian Schröder, Alastair Tait + Show 4 more

Open Access

https://doi.org/10.3390/min10121090

Copy DOI

Abstract

The Fe(II) monosulfide mineral mackinawite (FeS) is an important phase in low-temperature iron and sulfur cycles, yet it is challenging to characterize since it often occurs in X-ray amorphous or nanoparticulate forms and is extremely sensitive to oxidation. Moreover, the electronic configuration of iron in mackinawite is still under debate. Mössbauer spectroscopy has the potential to distinguish mackinawite from other FeS phases and provide clarity on the electronic configuration, but conflicting results have been reported. We therefore conducted a Mössbauer study at 5 K of five samples of mackinawite synthesized through different pathways. Samples show two different Mössbauer patterns: a singlet that remains unsplit at all temperatures studied, and a sextet with a hyperfine magnetic field of 27(1) T at 5 K, or both. Our results suggest that the singlet corresponds to stoichiometric mackinawite (FeS), while the sextet corresponds to mackinawite with excess S (FeS1+x). Both phases show center shifts near 0.5 mm/s at 5 K. Coupled with observations from the literature, our data support non-zero magnetic moments on iron atoms in both phases, with strong itinerant spin fluctuations in stoichiometric FeS. Our results provide a clear approach for the identification of mackinawite in both laboratory and natural environments.

Highlights

Iron(II) monosulfide, Fe(II) monosulfide mineral mackinawite (FeS), known as the mineral mackinawite, is widespread in low-temperature aqueous environments
Our results suggest that both the singlet and the sextet (27 T phase) represent FeS identified as mackinawite by other methods
The wet-filtered material was synthesized following a protocol commonly used for FeS synthesis, e.g., [25], and Hellige and coauthors [26] and Peiffer and

Summary

Introduction

Iron(II) monosulfide, FeS, known as the mineral mackinawite, is widespread in low-temperature aqueous environments. Mackinawite has likely been present on Earth since the Hadean eon [2] and might have played a role in the origin of life [3,4]. It has potential for microbial fuel cells [5] and exhibits the magnetic characteristics of isostructural high-temperature. Despite its significance and it has been studied for several decades, identification and characterization of mackinawite using standard mineralogical tools remain challenging. Mackinawite crystallizes in a tetragonal structure with space group P4/nmm where iron atoms occupy edge-shared

Methods

Results

Conclusion