STUDIES OF PYRITE FORMATION MECHANISMS IN AQUEOUS-SOLUTIONS AT LOW-TEMPERATURES AND PRESSURES

The experimental studies of pyrite formation in aqueous solutions at l ow 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 c onsisted of X-ray amorphous iron sulfide (Fe/S similar to 1/1) and ele mental sulfur. The sulfide fraction of the sediment along with the ads orbed H2O was found to contain the structure protons presumably locali zed within the OH- and HS- groups. Thus the primary sulfide phase was considered as a compound of FeOHHS composition. In the process of sedi ments aging the equilibrium pyrite formation was ascribed in terms of the following reaction: FeOHHS + S-0- > FeS2 + H2O. The synthesized se diments, both the initial and aged ones, were studies by Mossbauer spe ctroscopy on Fe-57 nuclei and by EPR. Iron was found to exist predomin antly in a form of Fe2+. Fe2+ ions in FeOHHS and FeS2 crystalline stru ctures occurred in low spin state. Close vicinity of Fe2+ ions al the initial and final stages of iron sulfide sediment formation was not ma rkedly disturbed. The experimental results are interpreted as an evide nce of two step purite formation mechanism within broad conditional ra nge corresponding to the natural sedimentary, hydrothermal-sedimentary and low temperature hydrothermal processes. The first step involves t he formation of metastable phase-precursors (FeOHHS). The sediment agi ng is resulted in the removal of hydrogen-hearing structurally localiz ed groups as well as formation of pyrite sulfur and dehydration of the sediment. The direct formation of pyrite crystallization nuclei in th e 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.