|Artists impression of an S3- molecule in a diamond cell.|
The rich blue color is that of the mineral lapis lazuli, and is due
the S3- molecule.
Credit: Copyright Pokrovski and Dubrovinsky as shown in
Science Daily here
Sulfur is a key element in the transportation of gold in hot water geothermal systems. Although there are a number of forms, the most common are sulfide (sulfur in a -2 oxidation state, hydrogen sulfide, H2S) and sulfate (SO4, in which a sulfur ion in a +6 oxidation state is surrounded by four oxygens in a tetrahedron). It has been difficult to characterize the chemistry of the hot aqueous fluids because rates of reaction are too fast to allow the hot fluid to preserve its chemistry upon cooling. Attempts to deduce the chemistry by looking at fluid inclusions trapped in minerals have been frustrated by these reactions, and reveal almost exclusively sulfate and sulfide. In a paper in the February 25, 2011, issue of Science, (vol. 331, p. 1052), Gleb Pokrovski and Leonid Dubrovinsky used Raman spectroscopy to examine the species in situ in a diamond anvil cell at temperatures between 25 and 450 C, and pressures of 0.5-3.5 GPa, temperatures and pressures characteristic of hydrothermal systems. They found that sulfate and sulfide were the dominant species at temperatures less than 250 C, as expected, but that at higher temperatures the dominant form was trisulfur, S3-. It was identified by S-S bending and stretching modes at characteristic wavelengths in the Raman spectrum.
Combined with known thermodynamics, the data allowed them to predict the S3- concentrations in natural fluids, they calculate that in a 1 wt% S concentration, the S3- accounts for a major part of the dissolved sulfur over a wide range of conditions of pressure, temperature and pH. S3- forms at the expense of sulfides and sulfates, and thus reduces the amount of sulfur retained in deposited minerals such as pyrite, pyrrhotite, anhydrite and barite. It increases sulfur mobility by preserving it in the aqueous solution. It binds well with gold, copper and platinum, and so competes with sulfide and sulfate as a transporting agent for these elements at depth. Upon rising into lower pressure and temperature environments, it will decompose into sulfate and sulfide, resulting in precipitation of the gold. It may rise high enough to form a low-density vapor phase. Finally, Pokrovski and Dubrovinsky conclude that if S3- is as abundant as they predict, it will influence the thermodynamic properties, kinetic models of reactions, and sulfur isotope-fractionation models which ignore its formation at this time.
In an accompanying perspective (p. 1018), Craig Manning discusses the role of S3- in mineralogy, specifically the role of it in giving lapis lazuli it's deep blue color by charge transfer between groups of S3-. Because sulfur has such different oxidation states (6+ and 2-) changes between these states leads to the transfer of many electrons. These transfers can lead to oxidation or reduction reactions in host rocks. He notes that in the 1991 eruption of Mount Pinatubo more sulfur was degassed than could be accounted for by the chemistry of the parent magma. The simplest explanation is that there was a sulfur-rich vapor phase deep in the volcano that was carried upward in the eruption, and that much of the SO2 released could have formed from precursory aqueous S3-. Manning concludes with the intriguing thought that deep waters may be ultramarine blue, an old idea dating back to 1856, revived by by Walter Giggenbach in 1971, but attributed to S2-, not S3-.