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

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

No fakes listed for this mineral species.