Thermonatrite
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Thermonatrite is a naturally occurring evaporite mineral form of sodium carbonate, Na₂CO₃·H₂O.
It was first described in 1845. Its name is from the Greek θερμός thermos, "heat", plus natron, because it may be a dehydration product of natron.... Wikipedia
Colour: Colourless to grey or yellow, white
Crystal class: Pyramidal (mm2); H-M symbol: (mm2)
Crystal habit: Acicular crystals rare; typically occurs as powdery crusts
Crystal system: Orthorhombic
Formula (repeating unit): Na2CO3·H2O
Optical properties: Biaxial (−)
Strunz classification: 5.CB.05