Iron isotope compositions (expressed as δ56Fe) in sedimentary pyrite have been widely used as tracers of redox and chemical evolution of the ocean through geological time. Previous studies mostly built on the mechanical extraction of sulfides from bulk rock samples, and focused on visible macroscopic pyrites, which may introduce a sampling bias. In situ analyses of micropyrite grains can provide new insights into the processes of pyrite formation and their time evolution. Here, we compile ca. 2000 in situ iron isotope compositions of Archean to Paleoproterozoic sedimentary pyrite, from previous literature as well as new data. Contrasting with bulk analyses, micropyrite displays a large and constant range of δ56Fe values, from -4 to +4‰, through time. Micropyrite δ56Fe values are not significantly influenced by metamorphic grade. A bimodal distribution of positive versus negative δ56Fe values can be attributed to two different processes of pyrite formation, Fe (oxyhydr)oxide sulfidation, versus kinetic and possibly microbially mediated pyrite precipitation. These processes are tightly related to rock lithology and thus to sedimentary conditions, and have existed since 3.8 Ga.