thénardite

{{Short description|Anhydrous sodium sulfate mineral}}

{{Infobox mineral

| name = Thénardite

| category = Sulfate mineral

| boxwidth =

| boxbgcolor =

| image = Thenardite Sodium sulfate near Sodaville Mineral County Nevada.jpg

| caption = Thénardite from Sodaville, Nevada

| formula = Na₂SO₄

| IMAsymbol = Thn{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| molweight = 142.04 g/mol

| strunz = 7.AC.25

| dana =

| system = Orthorhombic

| class = Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)

| symmetry = Fddd

| unit cell = a = 5.86 Å, b = 12.3 Å
c = 9.82 Å; Z = 8

| color = White, grayish white, yellowish white, reddish white, brownish white

| 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 = 2.5

| luster = Vitreous to resinous

| refractive = n_α = 1.471, n_β = 1.477, n_γ = 1.484

| opticalprop = Biaxial (+)

| 2V = 83°

| birefringence = δ = 0.013

| pleochroism = none

| streak = White

| gravity = 2.67–2.7, average = 2.68

| density =

| melt =

| fusibility =

| diagnostic =

| solubility = Soluble in water

| diaphaneity = Transparent

| other = Salty taste

| fluorescence = Fluorescent and phosphorescent: short UV=bright white, long UV=yellow-green

| references = [http://webmineral.com/data/Thenardite.shtml Thénardite at Webmineral][http://www.mindat.org/min-3935.html Thénardite at Mindat][http://rruff.info/doclib/hom/thenardite.pdf Handbook of Mineralogy]

}}

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 in Ciempozuelos, Spain, by the Spanish chemist José Luis Casaseca (1800 - 1869). Casaseca named the mineral after his master, the French chemist Louis Jacques Thénard (1777–1857).

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.