9 Photos
Thenardite
Chemical Formula	Na2SO4
Species	Sulfates
Crystal System	Orthorhombic
Mohs Scale	2-3
Specific Gravity	2.67-2.7
Color	White, grayish white, yellowish white, reddish white, brownish white
Streak	White
Luster	Vitreous, Resinous
Refractive Index	n = 1.471 n = 1.477 n = 1.484
Diaphaneity	Transparent, Translucent
Cleavage	PerfectOn perfect; on fair; on incomplete
Fracture	Irregular/Uneven, Hackly
Crystal Habit：Forms crust-like prismatic aggregates on matrix
Geological Setting：Lakes and playas in arid regions; as efflorescences on soils in arid areas, and in caliche. Also occurs around volcanic fumaroles mixed with alkali halides and sulphates and as an incrustation on recent lavas.

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 and was named after the French chemist, Louis Jacques Thénard (1777–1826).

Thenardite crystallizes in the orthorhombic system and often forms yellowish, reddish to grey 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.