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Villiaumite5 - Poudrette quarry, Mont Saint-Hilaire, Quebec, Canada.jpg
Villiaumite and thermonatrite (powdery coating)
Category Carbonate mineral
(repeating unit)
Strunz classification 5.CB.05
Crystal system Orthorhombic
Crystal class Pyramidal (mm2)
H-M symbol: (mm2)
Space group Pca21
Unit cell a = 10.72 Å, b = 5.24 Å
c = 6.46 Å; Z = 4
Colour Colourless to grey or yellow, white
Crystal habit Acicular crystals rare; typically occurs as powdery crusts
Cleavage Poor to indistinct on {100}
Fracture Sectile
Mohs scale hardness 1 - ​1 12
Lustre Vitreous
Diaphaneity Transparent
Specific gravity 2.255 (measured on synthetic crystal)
Optical properties Biaxial (-)
Refractive index nα = 1.420 nβ = 1.506 nγ = 1.524
Birefringence δ = 0.104
2V angle 48° (measured)
Solubility Soluble in water
Other characteristics Readily dehydrates
References [1][2][3]

Thermonatrite is a naturally occurring evaporite mineral form of sodium carbonate, Na2CO3·H2O.[1][2]

It was first described in 1845.[3] Its name is from the Greek θερμός, "thermos", heat, plus natron, because it may be a dehydration product of natron.[2]

Typical occurrence is in dry saline lake beds and as soil encrustations. It has been reported from volcanic fumaroles and in association with carbonatite related veins. Common associated minerals include trona, natron and halite.[1]


  1. ^ a b c Handbook of Mineralogy
  2. ^ a b c Mindat data
  3. ^ a b Webmineral data
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