Nucleation and growth of FeS

The formation of iron sulfides is of immense importance in our environment, affecting sedimentary or ore deposit formation, heavy metal immobilisation and thus several global bio-geo-chemical cycles.

As such the reactions involving the formation and dissolution of iron sulfide phases in natural settings are key to controlling several global biogeochemical cycles (i.e., S, Fe, C) in both modern and ancient environments. The formation of pyrite (FeS2) – the principal geologically stable iron disulfide – via meta-stable Fe-S precursors (i.e., mackinawite and greigite) is fairly well understood. However, so far little is known about the reactions that control the nucleation and growth of the precursor Fe-S compounds that are crucial in the pyrite pathway. These include a series of highly reactive, often short lived aqueous clusters and also readily oxidised solid Fe-S compounds – among them FeSaq, or the poorly ordered FeSfresh or nano-crystalline mackinawite (nominally FeS) and greigite (Fe3S4). Although key intermediates in the transformation to pyrite, the molecular level understanding of their nucleation and growth kinetics and mechanisms remain currently largely unexplored.

This proposal focuses on the nucleation and growth of meta-stable Fe-S phases. They occur widely in the modern environment (e.g., deep sea vents, marine anoxic sediments), they have been implicated in the origin of life scenarios and are considered potential modern-day catalysts for CO2 conversion for climate change attenuation. This proposal will develop a new and better understanding of the fundamental physical-chemical concepts of mineral formation from solution for the Fe-S system. Through a combination of state-of-the-art experimental (and modelling at UCL) approaches, which now surpass the limitations of conventional techniques, we can follow the earliest stages of nucleation and growth of iron sulfide phases from solution at the scale at which they occur. These early events determine the eventual structures and properties of most Fe-S minerals, but until recent technological advances they have been impossible to investigate. We aim to follow the reactions from the emergence of the first building block in solution, through agglomeration into larger clusters, their aggregation into nano-particles and the eventual transformation into the final crystal. We anticipate that this project, investigating short-lived processes which are only now accessible to study through the development of high temporal and spatial resolution in-situ characterization techniques and High Performance Computing platforms, will lead to in-depth step-by-step quantitative insight into the iron sulfide formation pathways and enhance our fundamental understanding of how a mineral nucleates in solution.

The experimental side of this project In Leeds will follow and quantify the nucleation, growth and aggregation of Fe-S clusters and particles from solution and all the way to anhydrous mackinawite. This will be done via combining in situ and realtime synchrotron-based scattering techniques with high-resolution conventional and cryo-transmission electron microscopy (HR / cryo-TEM). The synchrotron work will be performed both at the ESRF (DUBBLE Beamline) and Diamond Light Source (Beamline 22). 

The ultimate goal of this project is to develop a new and better atomic-level physico-chemical framework that describes the reactions governing and controlling the solution-based nucleation and growth of Fe-S phases from the initial clusters and nuclei, to their subsequent growth and ultimate crystallization to mackinawite.

References:

Ahmed, I.A.M., Benning, L.G., Kakonyi, G.,  Sumoondur, A.D., Terrill, N.J. and Shaw, S. (2010). Formation of Green Rust Sulfate: A Combined in Situ Time-Resolved X-ray Scattering and Electrochemical Study. <cite>Langmuir,</cite> 26 (9), 6593-6603.

Csákberényi-Malasics, D., Rodriguez-Blanco, J.D., Kovács Kis, V., Rečnik, A., Benning, L.G. and Mihály Pósfai, M. (2012). Structural properties and transformations of precipitated FeS. Chemical Geology, 294–295. ://dx.doi.org/10.1016/j.chemgeo.2011.12.009