Epsomite formation in a New Mexico cave
Category Sulfate mineral
(repeating unit)
Strunz classification 7.CB.40
Dana classification
Crystal system Orthorhombic
Crystal class Disphenoidal (222)
H-M symbol: (2 2 2)
Space group P212121
Unit cell a = 11.86, b = 11.99
c = 6.858 [Å]; Z = 4
Color White, grey, colorless, or pink, greenish
Crystal habit Acicular to fibrous encrustations
Twinning Rarely observed on {110}
Cleavage {010} perfect {101} distinct
Fracture Conchoidal
Mohs scale hardness 2
Luster Vitreous, silky when fibrous
Diaphaneity Transparent to translucent
Specific gravity 1.67 - 1.68
Optical properties Biaxial (-)
Refractive index nα = 1.433 nβ = 1.455 nγ = 1.461
Birefringence δ = 0.028
2V angle Measured: 52°
Solubility In water
Alters to Dehydrates in dry air
References [1][2][3]

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. 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.[3]

Related minerals

The epsomite group includes solid solution series with morenosite (NiSO4·7H2O) and goslarite (ZnSO4·7H2O)[2]

Kieserite (MgSO4·H2O) is a less hydrated magnesium sulfate.

Crystal structure of epsomite


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