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Epsomite formation from a New Mexico cave
CategorySulfate mineral
(repeating unit)
IMA symbolEsm[1]
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
ColorWhite, grey, colorless, or pink, greenish
Crystal habitAcicular to fibrous encrustations
TwinningRarely observed on {110}
Cleavage{010} perfect {101} distinct
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

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.

Discovery and occurrence[edit]

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]

Related minerals[edit]

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

Crystal structure of epsomite


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]

See also[edit]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Webmineral data
  3. ^ a b Mindat.org
  4. ^ a b Handbook of Mineralogy
  5. ^ McGraw-Hill encyclopedia of science & technology (10th ed.). New York: McGraw-Hill. 2007. ISBN 9780071441438. OCLC 84152915.
  6. ^ Rath, Linda. "Why Take an Epsom Salts Bath?". WebMD.
  7. ^ https://www.epsomsaltcouncil.org/wp-content/uploads/2015/10/report_on_absorption_of_magnesium_sulfate.pdf
  8. ^ Browning, Marie (1999). Natural soapmaking (1st paperback ed.). New York: Sterling Pub. Co. ISBN 0-8069-6289-5. OCLC 42598586.