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, FeS am , 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.

Microbially mediated iron and sulfur cycling have impacted redox transitions and the bioavailability of nutrients throughout Earth's history. Here, we incubated Geobacter sulfurreducens in the co-presence of ferrihydrite and S 0 at pH 6.5, 7.2 or 8.0. Microbial reduction of Fe(III) and S 0 resulted in a shift from ferruginous (Fe 2þ -rich) to sulfidic (sulfide-rich) conditions and the precipitation of mackinawite, greigite and vivianite. The initial pH controlled the timing of the ferruginous-sulfidic transition and the relative abundance and crystallinity of the formed minerals. Vivianite formation was attributed to phosphate initially added to the medium. Phosphate showed a dynamic cycle, with low dissolved concentrations initially due to sorption to ferrihydrite, followed by vivianite precipitation under ferruginous conditions, and a significant release under sulfidic conditions. Co-reduction of Fe(III) and S 0 therefore regulates Fe-S biomineral formation and P bioavailability, which could be particularly important to consider in microbial mats and the sulfate-poor Archean ocean.