key words [iron hydroxide sulfur pyrite]

The reactions under study are considered as main reactions of sulphide formation and to govern the oxygen and sulphate contents in the Word ocean at any geological epoch (1-3). Earlier, on the example of pyrite it was demonstrated that the most thermodynamically stable iron sulfides (final phases) cannot be precipitated directly from a solution at low temperatures because of high nucleation barrier. These sulfides more likely are the interactions products of metastable phase-precursors with the initial solutions (4). The aim of this study is to investigate the possible pathways of reactions proceeding namely in the interaction of iron hydroxides with sulphide sulphur at parameters of natural processes.

Solid products of these reactions obtained in laboratory were studied by use of chemical phase analysis (CA), x-ray diffractometry (XRD), M÷ssbauer spectroscopy (MS), electron spin resonance (ESR) and proton magnetic resonance (PMR) techniques.

In the interaction of Fe³⁺ hydroxides with sulphide sulphur the elemental sulphur is formed according to the reaction 2Fe³⁺ + S^2- = 2Fe²⁺ + S⁰. As a result, the redox conditions of the case are determined by the buffer pairs F₂S_2-S^o. In this conditions at the first step of the reaction the x-ray amorphous iron sulphide is formed. According to CA data its content is close to monosulphide (Fe:S=1:1). On the base of PMR data this phase along with absorbed H₂O was found to contain the structure protons presumably localised within OH^- and HS^- atomic groups. Thus, the primary sulphide phase was considered a compound of FeOHHS composition (phase precursors-hydrotroilite). We conclude that the following reaction takes place in this case 2Fe(OH)₃ + 3H₂S =2 FeHSOH +S^o. In the process of precipitate ageing the equilibrium pyrite formation was ascribed in terms of the following reaction: FeOHHS + S^o = FeS₂ + H₂O.

The synthesized phases, both the phase-precursor and final ones, were studied by MS on ⁵⁷Fe nuclei and by ESR. Iron was found to exist predominantly in the form of Fe²⁺. Fe²⁺ ions in FeOHHS and FeS₂ crystalline structures occurred in low spin state. The average values of isomer shift and quadrupole line shift for pyrite and hydrotroilite are in good agreement with corresponding values for the pyrite phase. As a result, we conclude that Fe²⁺ ions in the structure of the phase-precursor presumably as well as that of final phase should be ascribed as having the pyrite-like nearest surrounding, i.e. the cation sublattice of mineral phases in the process of x-ray amorphous to crystalline state transition is not sufficiently transformed. All the compositional changes of sulphide phases are related to changes in anion sublattice. PMR studies of synthesised phases lead to a conclusion that OH^- and HS^- atomic groups in sublattice are substituted for S₂^2-.

In the interaction of Fe²⁺ with sulphide sulphur the redox parameters of corresponding reactions are determined by the series of sulphide-hydroxide sulphide buffer pairs being governed by pH and pS values of the initial solutions. In these conditions phase-precursor is more commonly presented by tochilinite [2 FeS1.5Fe(OH)₂] subsequently transforming to mackinawite (FeS). According to structural studies performed by N.I.Organova the tochilinite structure is interpreted as mixed layered hydroxide-sulphide compound with regular interlayer brycite type layers of Fe(OH)₂ composition with tetrahedral mackinawite like layers of FeS composition. Thus the interaction between Fe²⁺ with sulphide sulphur could be ascribed as follows:

1) 3.5 Fe(OH)₂ + 1.5H₂S = 2Fe(OH)₂1.5FeS + 3H₂O

2) 2 Fe(OH)₂1.5FeS + 2H₂S = 3.5FeS + 4H₂O

The ⁵⁷Fe Mossbauer spectra tochilinite (phase-precursor) and mackinawite (final-phase) have been taken at room temperature. The first showed no magnetic hyperfine structure whereas the second did. Chemical isomer shifts, quadruple interactions, and magnetic hyperfine fields have been determined from these spectra. The spectrum of tochilinite consists of some quadruple doublets which shows that the Fe²⁺ -ions states are the low-spin and high-spin states. For

tochilinite superparamagnetic effect due to the rapid relaxation of atomic spins is observed. In fine particle synthetic tochilinite, magnetic reversals due to the thermal motion take place in a shorter time than the Mossbauer transition. The Fe²⁺-ions in this case are in the low-spin state.

Consequently our study is sufficient to allow the following conclusion: the reactions of iron hydroxide with sulphide sulphur have a steplike character. The first step is the hydroxide-sulphide formation, the latter being subsequently transformed to sulfides (pyrite or mackinawite). The phase-precursor is presented by hydrotroilite on the pyrite stability field whereas the tochilinite is formed in more reducing conditions.

1. Holland H.D. (1978) The Chemistry of the Atmosphere and Ocean. //Willey and Sons, 351p.
2. Garrels R.M. and Lerman A. (1981) // Proc. Nat. Acad. V.78, p.4652.
3. Berner R.A. (1984) // Geochim. et cosmochim. Acta, V.48, p.605.
4. Shoonen M.A.A., Barnes H.L. (1991) // Geochim. et cosmochim. Acta, V.55, p.1495,1505,3491.
5. Organova N.I. (1989) Crystallochemistry of incommensurated and modulated mixed-layer minerals. // Moscow, Nauka, 144p.

key words [pyrrhotite thermodynamic properties]

A double electrochemical cell with Y₂O₃-stabilized ZrO₂ as a solid electrolyte with O^2--conductivity was used to measure the oxygen partial pressures in the equilibrium

3FeSpo + 6Ag + 2O₂ = Fe₃O₄ + 3Ag₂S (I)

Equilibrium (I) in turn results from the combination of the pyrrhotite-magnetite equilibrium

3FeS(po) + 5O₂ = Fe₃O₄ + 3SO₂

and the auxiliary equilibrium

2Ag + SO₂ = Ag₂S + O₂

which fixes the SO₂ partial pressure in the Fe-S-O system (figure). It is noteworthy that the components of the auxiliary system (Ag and Ag₂S) do not yield any solid solutions with pyrrhotite and magnetite.

The oxygen partial pressures in Equilibrium (I) were measured relative to Ni-NiO buffer (pO₂^*) (Pejryd, 1984) in the cell