Hexahidrite (MgSO4·6H2O, hydrous magnesium sulfate) from Manresa. Hexahidrite is a fibrous mineral that coats with white, delicate efflorescences the surface of magnesium containing humid rocks. Hexahidrite has the typically bitter taste of magnesium and it is so soft (hardness 2-2,5) that can be grated with a nail. Manresa is a well known location of this mineral which is usually rare. Hexahidrite is quite similar to epsomite (MgSO4·H2O) by the appearance and the chemical composition, they only differ from the water of hydration. [photo Joaquim Sanz / Museum of Geology Valentí Masachs]