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 in the World
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