coast of the Gulf of Mexico, and in evaporites in eastern Europe and western Asia. Native sulfur may be produced by geological processes alone. Fossil-based sulfur deposits from salt domes were once the basis for commercial production in the United States, Russia, Turkmenistan, and Ukraine. Currently, commercial production is still carried out in the Osiek mine in Poland. Such sources are now of secondary commercial importance, and most are no longer worked. Common naturally occurring sulfur compounds include the sulfide minerals, such as pyrite (iron sulfide), cinnabar (mercury sulfide), galena (lead sulfide), sphalerite (zinc sulfide), and stibnite (antimony sulfide); and the sulfate minerals, such as gypsum (calcium sulfate), alunite (potassium aluminium sulfate), and barite (barium sulfate). On Earth, just as upon Jupiter's moon Io, elemental sulfur occurs naturally in volcanic emissions, including emissions from hydrothermal vents. The main industrial source of sulfur is now petroleum and natural gas. Compounds Common oxidation states of sulfur range from −2 to +6. Sulfur forms stable compounds with all elements except the noble gases. Electron transfer reactions Sulfur polycations, S82+, S42+ and S162+ are produced when sulfur is reacted with oxidising agents in a strongly acidic solution. The colored solutions produced by dissolving sulfur in oleum were first reported as early as 1804 by C.F. Bucholz, but the cause of the color and the structure of the polycations involved was only determined in the late 1960s. S82+ is deep blue, S42+ is yellow and S162+ is red. Reduction of sulfur gives various polysulfides with the formula Sx2-, many of which have been obtained crystalline form. Illustrative is the production of sodium tetrasulfide: Some of these dianions dissociate to give radical anions, such as S3− gives the blue color of the rock lapis lazuli. This reaction highlights a distinctive property of sulfur: its ability to catenate (bind to itself by formation of chains). Protonation of these polysulfide anions produces the polysulfanes, H2Sx where x= 2, 3, and 4. Ultimately, reduction of sulfur produces sulfide salts: 16 Na + S8 → 8 Na2S The interconversion of these species is exploited in the sodium–sulfur battery. Hydrogenation Treatment of sulfur with hydrogen gives hydrogen sulfide. When dissolved in water, hydrogen sulfide is mildly acidic: H2S HS− + H+ Hydrogen sulfide gas and the hydrosulfide anion are extremely toxic to mammals, due to their inhibition of the oxygen-carrying capacity of hemoglobin and certain cytochromes in a manner analogous to