Picropharmacolite, Ca₄Mg(AsO₃OH)₂(AsO₄)₂·11H₂O, is a rare arsenate mineral. It was named in 1819 from the Greek for bitter, in allusion to its magnesium content, and its chemical similarity to pharmacolite. The mineral irhtemite, Ca₄Mg(AsO₃OH)2(AsO₄)₂·4H₂O, has the same composition as picropharmacolite, except that it has only four water molecules per formula unit, instead of eleven. It may be formed by the dehydration of picropharmacolite.