Sulfur, an essential element of life, exists in many forms. Your nose would recognize it easily in the form of hydrogen sulfide (H 2 S), with its distinctive rotten egg smell. Other forms of sulfur are important geochemically, as well. For example, sulfur dioxide (SO 2 ) is one of the most common gasses to be released from volcanoes and causes significant global cooling after eruptions. Sulfur also dissolves in geological fluids and complexes with metals to transport them within the Earth's interior. It makes stable complexes with gold and helps move it to ore deposits at shallow depths.

Many of sulfur’s geochemical activities occur within the Earth’s mantle and crust. In order to understand them, geoscientists must figure out what forms of sulfur exist underneath the Earth’s surface. Currently, scientists have found evidence that sulfate (SO 4 2-) and sulfide (S2-) are the dominant forms of sulfur in the crust and mantle. However, Gleb Pokrovski and Leonid Dubrovinsky argue in a recent of issue of Science that previous studies were hampered by limitations in sample collection and examination procedures.

As you move deeper into the Earth, temperature rises and pressure increases. Some chemical species may only be stable in geological fluids at extreme temperatures and pressures. Thus, when scientists look for chemical components within samples at lower temperatures and pressures, like those in a lab, the unstable chemicals may degrade and be overlooked. But how can researchers study chemicals at extreme conditions?

Pokrovski and Dubrovinsky used a diamond-anvil cell that is equipped with two low-fluorescence diamonds (meaning there’s low background fluorescence intensity from the hardware) and a gold-lined rhenium gasket. Within the cell, the two diamonds are used to compress samples to extreme pressures. Pokrovski and Dubrovinsky studied the chemical profile of the samples using Raman spectroscopy, which gives information related to the vibrational and rotational modes of chemical compounds.

They examined the forms of sulfur in three types of solutions at temperatures of 25 to 450°C, pressures of 0.5 to 3.5 GPa (as a frame of reference, atmospheric pressure is approximately 0.0001 GPa), and pH in the range of 1 to 7. The first solution consisted of sodium thiosulfate (Na 2 S 2 O 3 ) and potassium thiosulfate (K 2 S 2 O 3 ). The second contained elemental sulfur in pure water, and the third contained elemental sulfur in sodium hydroxide (NaOH) solutions.

Contrary to current views, Pokrovski and Dubrovinsky found that the sulfur radical anion S3-—not sulfate and sulfide—is the dominant sulfur species in solutions when the temperature is 250 to 450°C and the pressure is between 0.5 and 3.5 GPa. S3- quickly breaks down to sulfate and sulfide when the temperature and pressure drops. The authors propose that, at elevated pressures and temperatures, the solution becomes dense. The high density causes water molecules to coordinate around the S3-, forming a stabilizing cage.

The sulfur radical anion S3- is responsible for the intense blue color of lapis lazuli (aka, blue ultramarine), a semiprecious stone. The radical anion is stable in the rock at ambient conditions because it is caged in a zeolite framework, and the authors think that the water molecules may form a similar cage.

Considering that the Earth’s crust and mantle have significant amounts of iron, magnesium, and other elements, it is curious that Pokrovski and Dubrovinsky conducted their studies in solutions that are devoid of these components. It would be informative to determine if S3- behaves differently when it is in solution with iron and other elements.

Science, 2010. DOI: 10.1126/science.1199911 (About DOIs)

Listing image by Lawrence Livermore National Labs