What is "sodium sesquicarbonate"

Sodium sesquicarbonate (Systematic name trisodium hydrogendicarbonate) NaH(CO) is a double salt of sodium bicarbonate and sodium carbonate, and has a needle-like crystal structure. However, the term is also applied to an equimolar mixture of those two salts, with whatever water of hydration the sodium carbonate includes, supplied as a powder.

The di hydrate, NaH(CO)·2HO, occurs in nature as the evaporite mineral trona.