Inferred pyrite growth via the particle attachment pathway in the presence of trace metals

Abstract

The morphology of pyrite has been used to infer ancient redox states and biogenicity. However, the influence of trace metals on pyrite morphology is poorly understood. Through batch synthesis experiments, we demonstrate that bioessential trace metals (Co, Cu, Mo, Ni, Zn) accelerate pyrite formation. The first precipitate, FeSam, transformed to an intermediate greigite phase and to pyrite with increasing time and temperature. Trace metals either facilitated polysulphide formation or precipitated as nanoparticles that can serve as nuclei for pyrite growth, depending on the initial metal concentration. Despite varying precipitation rates, the final pyrite morphologies were unaffected. Various morphologies including tabular precipitates (<150 nm), aggregates resembling microframboids (100-250 nm), octahedral (300-1500 nm) and rose-like particles (1000-3000 nm) were observed. This size-shape particle continuum was interpreted as stages of pyrite growth via particle attachment. This process could be important in explaining variations in the mineral's reactivity (e.g., defects), isotopic and trace metal distributions, and morphologies (e.g., framboids) for applications in paleo-proxies, environmental research and biosignatures.