Thermonatrite is a naturally occurring evaporite mineral form of sodium carbonate, Na<sub>2</sub>CO<sub>3</sub>÷H<sub>2</sub>O.
It was first described in 1845. Its name is from the Greek øõÃÂüÃÂàthermos, "heat", plus natron, because it may be a dehydration product of natron.
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.