Standard State: solid at 298 K
Discovered by Berzelius in 1817, who found it associated with tellurium, named for the earth. Selenium is found in a few rare minerals, such as crooksite and clausthalite.
In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode mud from electrolytic copper refineries now provide the source of the most of the world's selenium. Selenium is recovered by roasting the mud with soda or sulfuric acid, or by smelting them with soda and niter.
Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is a metallic gray.
These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells.