Geochimica et Cosmochirnk.a Acla Vol. 53. pp. 229-236 0016-7037/89/$3.00 + .00 Copyright © 1989 Pergamon Press plc. Printed in U.S.A. Solubility product constants of covellite and a poorly crystalline copper sulfide precipitate at 298 K* DAM1AN SHEAt and GEORGE R. HELZ Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, U.S.A. ( Received Apri120, 1988; accepted in revised form August 8, 1988) Abstract--The equilibrium constant at 25"C for the following reaction has been measured in NaC1 media by an indirect method: CuS(COv) + H+(aq) ~ Cu2+(aq) + HS-(aq), K~ = Mcu2.M,s-(10 +p") where CuS(cov) designates synthetic covellite. Values ofpK~ are 21.39, 21.04 and 20.95 at NaCI = 0.2, 0.7 and 1.0 M, respectively; the uncertainty in these K~ values is _+0.15. The free energy of formation of covellite, for which published values are discordant, is calculated to be -11.83 _+ 0.4 kcal/mole at 298 K (-49.50 + 1.7 kJ/mole). This value is obtained by extrapolating the measured pK, p values to infinite dilution with corrections for CI- complexing. Applying similar CI- complexing corrections, based on recent measurements by Seward, to preyiously published solubility data for galena yields a revised pK°w for galena of 12.78. A poorly crystalline precipitate, obtained by mixing Cu 2÷ and HS- solutions, yielded a reversible solubility product 3 orders of magnitude greater than that of covellite but about 3 orders of magnitude less than that of a truly amorphous phase, super-cooled liquid CuS. The poorly crystalline phase has not been studied previously. Its bulk composition was Cu ,.,aS, but microprobe analysis revealed that it was a partially exsolved mixture of roughly Cu L, ,S and Cu t.32S (similar to known blaubleibender covellites). It was kinetically unstable, and converted to coveUite when thermally annealed or when exposed to polysulfide solutions. Because of its instability, a material of this nature is unlikely to account for the amorphous copper sulfide alleged to occur in the Red Sea Brine deposits. However, it is possible that on short time scales dissolved Cu in sulfidic waters is controlled by metastahle, rather than stable phases, as is known to be the case with dissolved Fe. INTRODUCTION M U C H C U R R E N T A T T E N T I O N i s being devoted to understand- ing the geochemical behavior of trace metals in low temper- ature, sultidic environments (BOULEGUE et al., 1982; JACOBS and EMERSON, 1982; EMERSON et al., 1983; KREMLING, 1983; DYRSSEN, 1985; CARIGNAN and NRIAGU, 1985; JA- COBS et aL, 1985; GAmLARD et al., 1986). Among the many problems is the need to establish what phases control trace metal solubilities in these environments. Interfaces between deep-seated sulfidic waters and oxic surface waters are important sites of sulfide mineral deposi- tion. Such interfaces occur in groundwaters around sulfide ore deposits, in pore waters of modern sediments, and in fjords or other types of anoxic marine basins. Mixing of waters across such interfaces can readily generate very high degrees of supersaturation with respect to trace metal sulfide minerals, permitting deposition ofmetastable phases. Indeed, metasta- ble blaubleibender covellite phases, in which the C u / S ratio exceeds1.0, are often found in environments of supergene enrichment. BROCKAMP el al. (1978) even report that an amorphous copper-bearing sulfide phase occurs in deposits from the Red Sea thermal brines. Thus it is conceivable that metastable Cu-S phases may control copper solubilities at temperatures occurring near the Earth's surface. In this regard, copper would be analogous to iron, which is known to be controlled by metastable phases (e.g. DAVISON, 1980). This paper concerns the solubility of phases in the Cu-S * This paper was part ofan invited presentation at the MERI con- ference. ' Present address: Department of Chemistry, Southeastern Mas- sachusetts University, North Dartmouth, MA 02747, U.S.A. system. We have measured the solubility product ofcovellite and of a poorly crystalline, metastable phase formed by rapid mixing of CuCI2 and NariS solutions. Combining these mea- surements with data from the literature, we have estimated the solubility of some additional phases. To determine solubility products, we employed the indirect method of UHLER and HELZ (1984). This involves using a chelating agent, Y, to enhance the solubility of a sulfide min- eral sufficiently to enable convenient measurement. Equilib- rium constants for the following three reactions are deter- mined and then multiplied to get the solubility product con- stant, K,p. CuS(s) + HY 3- ~ CuY 2- + H S - Kdiu (1) CuY 2- + 2H + ~ Cu 2+ + H2 Y2- Kcuv* (2) H2 Y2- ~:~ H + + HY 3- Ka3 (3) CuS(s) + H + ~ Cu 2+ + H S - K~ (4) In measurements on the poorly crystallized precipitate, we used the chelator, ethylenediaminetetraacetic acid (EDTA). For covellite, which is much less soluble, we had to use the stronger chelator, trans 1-2 diaminoeyclohexyltetraacefic acid (DCTA). EXPERIMENTAL Materials Covellite was synthesized by mixing about 10 g of a 1:1 molar ratio of 99.99% pure Cu powder and 99.999% pure S powder in an evacuated pyrex vessel. This mixture was allowed to react for one day at room temperature and was then heated to 2500C for 30 days. 229