Sodium carbonate crystallizes from water to form three different hydrates:
sodium carbonate decahydrate (natron), Na2CO3·10H2O.
sodium carbonate heptahydrate (not known in mineral form), Na2CO3·7H2O.
sodium carbonate monohydrate (thermonatrite), Na2CO3·H2O.
anhydrous sodium carbonate also known as calcined soda is formed by heating the hydrates. It is also formed when sodium hydrogen carbonate is heated (calcined) e.g. in the final step of the Solvay process. The decahydrate is formed from water solutions crystallizing in the temperature range -2.1 to +32.0 C, the heptahydrate in the narrow range 32.0 to 35.4 C and above this temperature the monohydrate forms.In dry air the decahydrate and heptahydrate will lose water forming causing the crystals to fall apart into a white monohydrate powder. Other hydrates have been reported, e.g. with 2.5 units of water per sodium carbonate unit ("pentahemihydrate").