Epsomite

Epsomite is a hydrous magnesium sulfate mineral with formula MgSO4·7H2O. 
Epsomite crystallizes in the orthorhombic system as rarely found acicular or fibrous crystals, the normal form is as massive encrustations. It is colorless to white with tints of yellow, green and pink. The Mohs hardness is 2 to 2.5 and it has a low specific gravity of 1.67. 
Epsomite is the same as the household chemical, Epsom salt, and is readily soluble in water. It absorbs water from the air and converts to hexahydrate with the loss of one water molecule and a switch to monoclinic structure.
Discovery and occurrence.
Epsomite forms as encrustations or efflorescences on limestone cavern walls and mine timbers and walls, rarely as volcanic fumarole deposits, and as rare beds in evaporite layers such as those found in certain bodies of salt water. It was first systematically described in 1806 for an occurrence near Epsom, Surrey, England, after which it was named. It occurs in association with melanterite, gypsum, halotrichite, pickeringite, alunogen, rozenite and mirabilite.
Related minerals.
The epsomite group includes solid solution series with morenosite (NiSO4·7H2O) and goslarite (ZnSO4·7H2O)
Kieserite (MgSO4·H2O) is a less hydrated magnesium sulfate.