Epsomite

Sulfate mineral

MgSO₄·7H₂OIMA symbolEsm^[1]Strunz classification7.CB.40Dana classification29.6.11.1Crystal systemOrthorhombicCrystal classDisphenoidal (222)
H-M symbol: (2 2 2)Space groupP2₁2₁2₁Unit cella = 11.86, b = 11.99
c = 6.858 [Å]; Z = 4IdentificationColorWhite, grey, colorless, or pink, greenishCrystal habitAcicular to fibrous encrustationsTwinningRarely observed on {110}Cleavage{010} perfect {101} distinctFractureConchoidalMohs scale hardness2LusterVitreous, silky when fibrousDiaphaneityTransparent to translucentSpecific gravity1.67–1.68 g/mlOptical propertiesBiaxial (−)Refractive indexn_α = 1.433 n_β = 1.455 n_γ = 1.461Birefringenceδ = 0.0282V angleMeasured: 52°SolubilityIn waterAlters toDehydrates in dry airReferences^[2]^[3]^[4]

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. It is a soft mineral with variable Mohs hardness around 2.0~2.5, and it has a low specific gravity of 1.67 g/ml.

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.

Etymology

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

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

Related minerals

The epsomite group includes solid solution series with morenosite (NiSO
₄·7 H
₂O) and goslarite (ZnSO
₄·7 H
₂O).^[3]

Research

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

Uses

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]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. Vol. 85, no. 3. pp. 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ "Epsomite". Web Mineral (webmineral.com).
^ ^a ^b "Epsomite". Mindat.org.
^ ^a ^b Epsomite (PDF) (Report). Handbook of Mineralogy – via Dept. Geology, University of Arizona.
^ McGraw-Hill Encyclopedia of Science & Technology (10th ed.). New York, NY: McGraw-Hill. 2007. ISBN 9780071441438. OCLC 84152915.
^ Rath, Linda. "Why take an epsom salts bath?". WebMD (webmd.com). A-to-Z guides.
^ Report on absorption of magnesium sulfate (PDF) (commercial / promotional). Epsom Salt Council. October 2015 – via epsomsaltcouncil.org.
^ Browning, Marie (1999). Natural Soapmaking (1st ppb ed.). New York, NY: Sterling. ISBN 0-8069-6289-5. OCLC 42598586.