Efremovite is a rare ammonium sulfate mineral with the chemical formula: (NH4)2Mg2(SO4)3.
It is hygroscopic and when exposed to humid air it slowly converts to the hydrate form, boussingaultite.
[5][6] It was first described in 1989 for an occurrence in the Chelyabinsk coal basin, Southern Urals, Russia.
It was named for Russian geologist Ivan Antonovich Yefremov (1907–1972).
[3] It occurs in association with native sulfur, kladnoite [it], mascagnite, and boussingaultite.