Thenardite is an anhydrous sodium sulfate mineral, Na2SO4 which occurs in arid evaporite environments, specifically lakes and old mine workings as an efflorescence and as a crusty sublimate. It occurs in volcanic. It was first described in 1825 for an occurrence in the Espartinas Saltworks, Ciempozuelos and was named for the French chemist, Louis Jacques Thénard (1777–1826).
Thenardite crystallizes in the orthorhombic system and often forms yellowish, reddish to gray white prismatic crystals although usually in massive crust deposits. Thenardite is fluorescent , white in shortwave and yellow-green in longwave UV radiation.
In humid conditions, thenardite gradually absorbs water and converts to the mineral mirabilite , Na2SO4·10H2O.