Thenardite is an anhydrous sodium sulfate mineral, Na2SO4 which occurs in arid evaporite environments. It also occurs in dry caves and old mine workings as an efflorescence and as a crusty deposit around fumaroles. It occurs in volcanic caves on Mt. Etna, Italy.
Thenardite crystallizes in the orthorhombic system and often forms yellowish, reddish to grey white prismatic crystals although usually in massive crust deposits. It has a hardness of 2.5 - 3 and a specific gravity of 2.7. Thenardite is fluorescent, white in shortwave and yellow-green in longwave UV radiation.
Thenardite samples will gradually absorb water and convert to the mineral mirabilite, Na2SO4·10H2O.