Iron sulfides were synthesized via a co-precipitation method. In addition, synchrotron-radiation experiments were performed under a range of pH and temperature conditions (up to 100 °C) to compare the results of in situ and ex situ crystal growth investigation of iron sulfides. In acidic environments, H2S acts as an oxidant, oxidizing Fe2+ to Fe3+ and allowing formation of greigite from mackinawite. However, under neutral conditions, due to very low H2S concentrations, the oxidant may be S (instead of H2S), allowing mackinawite to transform into greigite. Both mackinawite and magnetite were present under alkaline conditions, with possible transitions of Fe2+ + 2OH− → Fe(OH)2, followed by 3Fe(OH)2 → Fe3O4 + 2H2O + H2. In situ X-ray diffraction results indicate that the mineral transformation rate under acidic conditions is faster than under neutral and alkaline conditions. This means that acid environments can enhance rapid phase transformation of iron sulfides. The results under different experimental conditions suggest that there is a variety of formation pathways for iron-sulfide minerals owing to the presence of different oxidants in different geochemical environments.