Thermonatrite

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

Thermonatrite
Villiaumite and thermonatrite (powdery coating)
General
Category	Carbonate mineral
Formula (repeating unit)	Na2CO3·H2O
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
Identification
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 1⁄2
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]

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]