Fontarnauite is a relatively recently described, rare sulfate, borate mineral with the chemical formula . It is found in an evaporite boron deposit. It coexists with other evaporite boron minerals, especially probertite. It is monoclinic, crystallizing in the space group P2<sub>1</sub>/c.
It was named for Ramon Fontarnau i Griera, a materials scientist of the University of Barcelona.