Castellaroite is a rare arsenate mineral with formula Mn<sub>3</sub>(AsO<sub>4</sub>)<sub>2</sub>â¢4H<sub>2</sub>O. It is related to the phosphate mineral metaswitzerite. Castellaroite is monoclinic, with space group P2<sub>1</sub>/n. The other natural manganese arsenate hydrate is manganohörnesite, which is an octahydrate.