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Sulfur, the 10th most abundant element in the universe and the 14th most abundant element in the Earth’s crust, is the defining element of sulfide minerals and provides insights into the origins of these minerals through its stable isotopes. The insights come from variations in the isotopic composition of sulfide minerals and related compounds such as sulfate minerals or aqueous sulfur species, caused by preferential partitioning of isotopes among sulfur-bearing phases, known as fractionation. These variations arise from differences in temperature, or more importantly, oxidation and reduction reactions acting upon the sulfur. The oxidation and reduction reactions can occur at high temperature, such as in igneous systems, at intermediate temperatures, such as in hydrothermal systems, and at low temperature during sedimentary diagenesis. At high temperatures, the reactions tend to occur under equilibrium conditions, whereas at low temperatures, disequilibrium is prevalent. In addition, upper atmospheric processes also lead to isotopic fractionations that locally appear in the geologic record.