Hexahydrite (MgSO4·6H2O, hydrous magnesium sulfate) from Manresa.

Hexahydrite is a fibrous mineral that coats with white, delicate efflorescences the surface of magnesium containing humid rocks. It is so soft (hardness 2-2,5) that it can be scratched with a nail. Hexahidrite tastes salty and bitter because its content in magnesium, while gypsum is tasteless. Hexahydrite is quite similar to epsomite (MgSO4·7H2O) by the appearance and the chemical composition, they only differ by the water of hydration.

Manresa is a well known location of this rare mineral, where it is found round the sanctuary of Sant Ignatius, specially on the rock shelters (photos 2nd, 3rd and 4th).

[photos Joaquim Sanz / Museum of Geology Valentí Masachs (1st) and Jordi Badia (2nd, 3rd and 4th]