The geologic role of polysulfides—Part II: The solubility of acanthite and covellite in sodium polysulfide solutions

Abstract

The solubility of synthetic low-temperature silver sulfide (acanthite) in solutions of sodium polysulfide of different concentrations, different ratios of sulfur to sulfide and different acidity was determined. The solubility of acanthite increases with rising y ̄ (total S in sulfide and polysulfide species/total sulfide) with rising temperature, and with decreasing pH if y ̄ is constant. For solutions saturated in sulfur, however, addition of acid causes the breakdown of polysulfide to hydrosulfide and sulfur and lowers the y ̄ ; the decrease of y ̄ lessens the solubility more than the lowering of pH raises it. Using the activities of polysulfide ions calculated by Cloke (1963) it is possible to deduce three complexes of silver ion, Ag(S 4) 2 −3, Ags 5S 4 −3 and Ag(HS)S 4 −2, and to calculate the observed solubilities closely. The equilibrium constant for the reaction Ag 2 S + 4 S 4 −2 = 2 Ag( S 4) 2 −3 + S −2 is estimated as 2.14 × 10 −8 and that for the reaction Ag 2 S + 2 S 5 −2 + 2 S 4 −2 = 2 AgS 5 S 4 −3 + S −2 is estimated as 1.74 × 10 −9. The constant for Ag 2 S + H +1 + HS −1 + 2 S 4 −2 = 2 Ag( HS) S 4 −2 is estimated as 2.95 × 10 4. Probably other complexes also exist. The solubility data of Höljte and Beckert (1935) can also be largely explained on the basis of the analogous complexes, Cu(S 4) 2 −3 and CuS 4S 5 −3. The equilibrium constant for the reaction 2 CuS + S 3 −2 + 3 S 4 −2 = 2 Cu( S 4) 2 −3 + S −2 is estimated as 5.00 × 10 −2 and for 2 CuS + 3 S 4 −2 + S 5 −2 = 2 CuS 4 S 5 −3 + S −2 as 3.63 × 10 −7. No data are available to estimate a constant for Cu(HS)S 4 −2. Application of this study to ore deposition depends on somewhat uncertain extrapolations to higher temperatures and pressures. It is tentatively concluded that solubility increases with temperature at constant density and increases with pressure at constant temperature. Thus, drop of temperature and pressure should cause deposition. A critical consideration is the size of the stability field of polysulfides at elevated temperature and pressure. Data relevant to this question are uncertain; to illustrate the possible application of the present study it is assumed that an appreciable stability field exists under magmatic and hydrothermal conditions. A decrease in size of this stability field with changing temperature and pressure conditions could lead to ore deposition, as well as to deposition of barite, alunite and other sulfates. If equilibrium is maintained, either an increase or a decrease of pH would cause disproportionation of polysulfides and deposition of the dissolved sulfides. An increase of pH might be caused by reaction with limestone or by argillization of silicate rocks. A geologically feasible way of adding acid to the polysulfide solution is more difficult to visualize. Perhaps an initial separation of acid gases such as HCl and later recombination with the ore solution is possible. This should give a sulfide body containing native sulfur ; the rarity of this type of ore suggests that this means of deposition is uncommon. Either oxidation or reduction of the polysulfide solution would lead to deposition of sulfides. Oxidation might be brought about by reaction with hematite or magnetite to form pyrite and sulfate ion. The effectiveness of these reactions is limited by the small percentage of these minerals in most rocks. Reduction might be caused by reaction with organic material in black shales, or possibly with reducing gases present in volcanism. Reaction with ferrous minerals, such as biotite, hornblende, or siderite, should also cause ore formation by removing sulfur from the polysulfide solution to form pyrite. Pyritization of wall rocks would thus appear as a cause, not a result, of ore deposition. Until better chemical knowledge exists, it is difficult to know how polysulfide solutions may originate. In this paper it is assumed that an appreciable stability field exists under the conditions in which ore solutions evolve from magmas.