THÉNARDITE

    Class : Sulfates, chromates, molybdates
    Subclass : Anhydrous sulfates
    Crystal system : Orthorhombic
    Chemistry : Na2SO4
    Rarity : Uncommon


Thénardite is a relatively common anhydrous sodium sulfate that forms by evaporation of saline brines above 32.4°C, below this temperature, it is mirabilite that crystallizes. It is a mineral of arid climate, which is mainly found in borate deposits, remains of ancient salt lakes. In some cases, thénardite can develop in volcanic fumaroles. It was named in honor of Louis Jacques Thenard, Professor of Chemistry at the University of Paris. Thénardite forms tabular to flattened crystals on {010}, sometimes orthorhombic bipyramids that can measure several centimeters, powdery encrustations and efflorescences. Thenardite is colorless when pure, but most often takes on a gray, brownish, reddish, or yellowish tint. It is soluble in water and has a slight saline flavor. It is an accessory ore of sodium.

Main photo : Thenardite from Tarapacá, Chile © Rock Currier

Thénardite from Soda Lake, California, USA © Rob Lavinsky
Twinned thénardites from Borax Lake, California, USA © Peter Cristofono
Thénardite pseudomorph after mirabilite from Boron, California, USA © Rob Lavinsky
Thénardite pseudomorph after halite from Searles Lake, California, USA © Rob Lavinsky

Thénardite in the World

Although uncommon, thénardite forms important layers in the deposits of some American salt lakes in Arizona, California and Nevada. The largest known crystals come from the salt deposits of Ténéré (Bilma, Niger).

Thénardite in France

In France, thénardite is known at Avion (Pas-de-Calais), at Cap Garonne (Var) and at the Grotte de Nébélé cave (Pyrénées-Atlantiques).

Twinning

Twins are known on {110}, frequently forming cruciform groups. A twin is also present on {011} resembling that of gypsum.

Fakes and treatments

No fakes listed for this mineral species.



Hardness : 2.5 to 3
Density : 2.664
Fracture : Irregular
Streak : White


TP : Translucent to transparent
RI : 1.471 to 1.484
Birefringence : 0.013
Optical character : Biaxial +
Pleochroism : None
Fluorescence : None


Solubility : Water

Magnetism : NoneRadioactivity : None