Brassite is a rare arsenate mineral with the chemical formula Mg(AsO3OH)·4(H2O). It was named brassite, in 1973, to honor French chemist R`ejane Brasse, who first synthesized the compound. The type locality for brassite is Jáchymov of the Czech Republic. It occurs as an alteration of magnesium carbonate minerals by arsenic bearing solutions. It occurs associated with pharmacolite, picropharmacolite, weilite, haidingerite, , native arsenic, realgar and dolomite.