Formation of Pyrite Spherules From Mixtures of Biogenic FeS and Organic Compounds During Experimental Diagenesis

Pyrite ( FeS 2 ) is the most common iron sulfide on the Earth's surface and has widely been used as a paleo‐environmental proxy. Yet the information recorded by pyrite depends on whether it was formed through abiotic or biogenic routes. It is thus of importance to properly identify its origin. Here, we investigate the final morphology of pyrite produced upon a simulated diagenetic history from biogenic and abiotic iron sulfide/phosphate systems. Abiotic starting material obtained by chemical synthesis and biogenic starting material produced from pure culture of Desulfovibrio desulfuricans were submitted to increasing diagenetic conditions (75°C or 150°C from 1 to 10 days). Mineralogical products were characterized by X‐ray diffraction and electron microscopy. For both biogenic and abiotic starting materials, the final state was characterized by the association of pyrite and lipscombite ( Fe 2 +Fe 2 3 +( OH ) 2( PO 4 ) 2 ), the most stable phases in these conditions. Intermediate phases such as greigite for iron sulfides and beraunite/wolfeite for iron phosphates were present in the abiotic residue but were not detected in the biogenic residue. Distinct pyrite morphologies were observed depending on the presence of organic matter. Indeed, while abiotic starting material led to the formation of submicrometric single crystals of pyrite with euhedral shapes similar to the subunits of well crystallized framboids, biogenic starting material produced micrometric spherulitic clusters of pyrite resembling the so‐called pseudo‐framboids. Although further experiments are required to ensure that it can be used as biosignatures, such specific morphologies, likely related to the presence of organic matter, may help recognizing biogenic pyrite in the geological record.