Аннотация:The experimental studies of pyrite formation in aqueous solutions at low and elevated temperatures by use of X-ray phase analysis, chemical phase analysis, Mossbauer spectroscopy, PMR, EPR were carried out, It was shown that in the conditions of experiments the initial sediment consisted of X-ray amorphous iron sulfide (Fe/S similar to 1/1) and elemental sulfur. The sulfide fraction of the sediment along with the adsorbed H2O was found to contain the structure protons presumably localized within the OH- and HS- groups. Thus the primary sulfide phase was considered as a compound of FeOHHS composition. In the process of sediments aging the equilibrium pyrite formation was ascribed in terms of the following reaction: FeOHHS + S-0- > FeS2 + H2O. The synthesized sediments, both the initial and aged ones, were studies by Mossbauer spectroscopy on Fe-57 nuclei and by EPR. Iron was found to exist predominantly in a form of Fe2+. Fe2+ ions in FeOHHS and FeS2 crystalline structures occurred in low spin state. Close vicinity of Fe2+ ions al the initial and final stages of iron sulfide sediment formation was not markedly disturbed. The experimental results are interpreted as an evidence of two step purite formation mechanism within broad conditional range corresponding to the natural sedimentary, hydrothermal-sedimentary and low temperature hydrothermal processes. The first step involves the formation of metastable phase-precursors (FeOHHS). The sediment aging is resulted in the removal of hydrogen-hearing structurally localized groups as well as formation of pyrite sulfur and dehydration of the sediment. The direct formation of pyrite crystallization nuclei in the initial solution is supposedly favored by acid environment (hP 3,5-4,5). Presented pyrute formation mechanism is valid in redox conditions corresponding to FeS2-S-0 buffer.