The Collaborative International Dictionary
Thenardite \The*nard"ite\, n. [Named after the French chemist, L. J. Th['e]nard.] (Min.) Anhydrous sodium sulphate, a mineral of a white or brown color and vitreous luster.
Wiktionary
n. (context mineralogy English) An orthorhombic-dipyramidal mineral containing oxygen, sodium, and sulfur.
Wikipedia
Thenardite is an anhydrous sodium sulfate mineral, NaSO which occurs in arid evaporite environments. It also occurs in dry caves and old mine workings as an efflorescence and as a crusty sublimate deposit around fumaroles. It occurs in volcanic caves on Mt. Etna, Italy. It was first described in 1825 for an occurrence in the Espartinas Saltworks, Ciempozuelos, Madrid, Spain 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, NaSO·10HO.