Thénardite
Sodaville, Mineral County, Nevada
General
Category	Sulfate mineral
Formula (repeating unit)	Na2SO4
IMA symbol	Thn[1]
Strunz classification	7.AC.25
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Fddd
Unit cell	a = 5.86 Å, b = 12.3 Å c = 9.82 Å; Z = 8
Identification
Formula mass	142.04 g/mol
Color	White, grayish white, yellowish white, reddish white, brownish white
Crystal habit	Forms crust-like prismatic aggregates on matrix
Twinning	Interpenetration twinning on {001}; also on {100}; common on {110}; {011}
Cleavage	{010} perfect, {101} fair, {100} incomplete
Fracture	Splintery, uneven, hackly
Tenacity	Brittle
Mohs scale hardness	2.5
Luster	Vitreous to resinous
Streak	White
Diaphaneity	Transparent
Specific gravity	2.67–2.7, average = 2.68
Optical properties	Biaxial (+)
Refractive index	nα = 1.471, nβ = 1.477, nγ = 1.484
Birefringence	δ = 0.013
Pleochroism	none
2V angle	83°
Ultraviolet fluorescence	Fluorescent and phosphorescent: short UV=bright white, long UV=yellow-green
Solubility	Soluble in water
Other characteristics	Salty taste
References	[2][3][4]

Thénardite is an anhydrous sodium sulfate mineral, Na₂SO₄ which occurs in arid evaporite environments, specifically lakes and playas. 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 Mount 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).^[3]

Thénardite crystallizes in the orthorhombic system and often forms yellowish, reddish to gray white prismatic crystals although usually in massive crust deposits. Thénardite is fluorescent, white in shortwave and yellow-green in longwave UV radiation.

In humid conditions, thénardite progressively absorbs water and converts to the deca-hydrated mineral mirabilite, Na₂SO₄ · 10 H₂O.

• 
  Crystal structure of thénardite
• 
  Thénardite, cluster of transparent crystals from Soda Lake, California. Width of cluser is 9 cm (3.5 in).
• 
  Tan thénardite pseudomorphing mirabilite crystals from the Boron, California area. Cluster is 6 × 5.5 cm.

• Palache, P.; Berman H.; Frondel, C. (1960). "Dana's System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, Etc. (Seventh Edition)" John Wiley and Sons, Inc., New York, pp. 404-407.