Structural properties and transformations of precipitated FeS

Abstract

Nanocrystalline iron sulfides form in diverse anoxic environments. The initial precipitate is commonly referred to as nanocrystalline mackinawite (FeS) or amorphous FeS. In order to better understand the structure of the initial precipitate and its conversion to mackinawite and greigite (Fe3S4), we studied synthetic iron sulfide samples that were precipitated from hydrous solutions near room temperature. The transformation of precipitated FeS was followed in both aqueous and dry aging experiments using X-ray powder diffraction (XRD) and scanning and transmission electron microscopy (SEM/TEM) and selected-area electron diffraction (SAED). Under tightly controlled anoxic conditions the first precipitate was nanocrystalline mackinawite. In contrast, when anaerobic conditions during synthesis were not completely ensured, freshly precipitated iron sulfide was typically X-ray amorphous (FeSam), and showed only one broad Bragg-peak at 2Θ = 16.5° (5.4 Å). A distribution of interatomic distances calculated from pair-distribution function analysis of SAED patterns of FeSam showed that only short-range (< 7 Å) order was present in the bulk of the material, with Fe mainly present in tetrahedral coordination. SEM and TEM images confirmed the poorly ordered structure and showed that FeSam formed aggregates of curved, amorphous sheets that contained 3–8 structurally ordered layers at their cores. Such layers are generally assumed to be structurally similar to the tetrahedral iron sulfide layers in mackinawite. However, both inter- and intralayer spacings measured in high-resolution TEM images (~ 5.3 to 6.3 and ~ 3.0 to 3.1 Å, respectively) were significantly larger than the corresponding spacings in crystalline mackinawite (5.03 and 2.6 Å, respectively), suggesting that short-range structural order within the semi-ordered layers of FeSam was not mackinawite-like. In aqueous aging experiments at room temperature, FeSam transformed into a mixture of mackinawite and greigite in ~ 2 months, and completely converted to platy greigite crystals after ~ 10 months. These aqueous transformations were likely driven by excess sulfur in the reacting solutions. We also studied the conversions of nanocrystalline mackinawite. In order to accelerate phase transitions, the initial FeS precipitate was heated to 120 °C, resulting in the formation of crystalline mackinawite within 2 h; at 150 °C, the material converted directly to pyrrhotite. Finally, when stored in a dry state at room temperature, crystalline mackinawite converted to greigite in 3 months, much faster than in the equivalent experiments in the aqueous solution, probably as a result of a more oxidative environment. The distinction between FeSam and nanocrystalline mackinawite is significant, since conditions for the formation of both phases are present in natural settings. Our experiments in a well-sealed anaerobic chamber simulate iron sulfide formation under anoxic conditions, whereas the samples that were prepared under less tightly controlled conditions can be regarded as representative of the oxic–anoxic transition zone in sediments. Our observations of the structural and morphological features of precipitated FeSam and the details of its aqueous conversion to greigite at ambient conditions are relevant to problems related to the biogeochemical cycling of elements in anoxic and suboxic marine sediments. An additional important finding is that even at moderately high temperatures (up to 170 °C), the conversions of iron monosulfides follow different pathways than at ambient conditions.