Epsomite

Epsomite is a hydrous magnesium sulfate mineral with formula MgSO4·7H2O.

Epsomite
Epsomite formation in a New Mexico cave
General
CategorySulfate mineral
Formula
(repeating unit)		MgSO4·7H2O
Strunz classification7.CB.40
Dana classification29.6.11.1
Crystal systemOrthorhombic
Crystal classDisphenoidal (222)
H-M symbol: (2 2 2)
Space groupP212121
Unit cella = 11.86, b = 11.99
c = 6.858 [Å]; Z = 4
Identification
ColorWhite, grey, colorless, or pink, greenish
Crystal habitAcicular to fibrous encrustations
TwinningRarely observed on {110}
Cleavage{010} perfect {101} distinct
FractureConchoidal
Mohs scale hardness2
LusterVitreous, silky when fibrous
DiaphaneityTransparent to translucent
Specific gravity1.67 - 1.68
Optical propertiesBiaxial (-)
Refractive indexnα = 1.433 nβ = 1.455 nγ = 1.461
Birefringenceδ = 0.028
2V angleMeasured: 52°
SolubilityIn water
Alters toDehydrates in dry air
References[1][2][3]

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.[4] 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]

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

References
  1. Webmineral data
  2. Mindat.org
  3. Handbook of Mineralogy
  4. McGraw-Hill encyclopedia of science & technology (10th ed.). New York: McGraw-Hill. 2007. ISBN 9780071441438. OCLC 84152915.
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