Epsomite, Epsom salt, or magnesium sulfate heptahydrate, 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.

It 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.

It was first systematically described in 1806 for an occurrence near Epsom, Surrey, England, after which it was named.

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.^[5] It occurs in association with melanterite, gypsum, halotrichite, pickeringite, alunogen, rozenite and mirabilite.^[4]

The epsomite group includes solid solution series with morenosite (NiSO₄·7H₂O) and goslarite (ZnSO₄·7H₂O).^[3]

Research on topical magnesium (for example Epsom salt baths) is very limited.^[6] The Epsom Salt Council recommends bathing 2 or 3 times/week, using 500–600 g (18–21 oz) Epsom salts each time.^[7]

Epsom salt is commonly sold as the main ingredient in bath salt, with additives such as glycerin (used as a humectant) and fragrances. The purpose of bath salts is mostly to make the bathing experience more enjoyable and serve as a vehicle for cosmetics, though they are said to improve cleaning and aid in exfoliation.^[8]

• Magnesium sulfate