The geologic role of polysutides-Part I

Numerous mcasuremcnts have been made of the pH, Eh, and the potential of a silver-silver sulfide electrode in sodium sulficlo and sodium polysulf?de solutions. By suitably ostimating the second dissociation constant of hydrogen sulfide and various activity coefficients, it has been possiblo to derive a consistent model for the distribution of the polysulfide and sulfide species, Sp2, S,-“, S,-2, Sew2 and HS-l, a t, a t s rengths up to 1111 and for pH values between 7 and 13. This model differs markedly from the models given previously by PESCHANSKI and VALENSI (1949) and by MAZONNY (1989b), who include S-2, S,-‘, S,-2. Sde2, S;-” and HS-1. The free energy of formation values proposed here are 16.36 I_~ 0.10 kcal/mole for SGM2, 16.97 + 0.10 kcal/mole for Ssp2, and 16.29 :& 0.10 kcal/molo for S,-2. Tentative figures for diand tri-sulfide ions are 21.0 +3.0 kcal/mole for S,-” and 62.7 & 3.0 kcal/molc for S2-2. Those are based on a value of 21.42 kcal/mole for Sp2. THIS paper is the first of a series dealing with the possible role of polysulfide solutions in the transport and deposition of chalcophile elements in sulfide vein deposits. The suggested Eh and pH values for the hydrot’hermal solution (BARTON, 1957; NATARAJAN, 1958) lie close to the stability field of elemental sulfur. Since this is exactly the region where polysulfides have their greatest thermodynamic stability, it seems logical that metal-polysulfide complexes may explain hydrothermal transport (BARTON, 1957; BARNES, 1958). Moreover, several sulfides have rather high solubilities in polysulfide solutions. In order to test this idea further and to determine the influence of changing pH on solubilities, experiments on the solubility of acanthite, Ag,S, were begun. It soon became apparent that an understanding of the observed solubilities must be based on an adequate interpretation of the distribution of polysulfide ions in aqueous solutions. It is the purpose of this paper to discuss this interpretation and to present the distribution derived from new experimental work. Subsequent papers will deal with the observed solubilities and the significance for geologic processes. TERMINOLOGY The following definitions and explanations of terms and symbols will be hrlpful to the wader in understanding the text. A, a temperature dependent constant in the &bye-Hiickel equation. <I,,, t,he “mean distance of closest a.pproach” as used in the Debye-Hiickel equation. B, a temperature dependent constant, in the Dcbycl-Hiickel equation. c equals [HS-‘1 + 2 [Szwz]. x=1 E”, a standard potontial; the sign convention of t,he American Electrochemical Society is used. Eh, the oxidation potent,ial in an aqueous solntion. * Published under the auspices of the Committee on Experimental Geology and Geophysics and the Division of Geological Sciences at Harrard University.