Epsomite

Authors: Hans-Jürgen Schwarz, Tim Müller, Nils Mainusch
English Translation by Sandra Leithäuser
back to Sulfate

Epsomite
Mineralogical name	Epsomite
Chemical name	Magnesium sulfate heptahydrate
Trivial name	Bitter Salts, reichardtite, seelandite
Chemical formula	MgSO4•7H2O
Other forms	Kieserite (MgSO4•H2O) Sanderite (MgSO4•2H2O) Starkeyite (MgSO4•4H2O) Pentahydrite(MgSO4•5H2O) Hexahydrite (MgSO4•6H2O) Meridianiite (MgSO4•11H2O) Magnesium 12-Hydrate
Crystal system	orthorhombic
Crystal structure
Deliquescence humidity 20°C	91.3 %
Solubility (g/l) at 20°C	2.893 mol/kg
Density (g/cm³)	1.68 g/cm3
Molar volume	146.8 cm3/mol
Molar weight	246.48 g/mol
Transparency	transparent to translucent
Cleavage	clear to perfect
Crystal habit	small, acicular, fibre like crystals, granular agregates, crusts
Twinning	rare
Phase transition
Chemical behavior
Comments	can be produced from an aqueous solution under 50°C
Crystal Optics
Refractive Indices	nx = 1.432 ny = 1.453 nz = 1.4609
Birefringence	Δ = 0.0284
Optical Orientation	biaxial negative
Pleochroism	none
Dispersion	51.1°
Used Literature
[Steiger.etal:2011]Title: Weathering and Deterioration Author: Steiger, M.; Charola, A. E.; Sterflinger, K. [Lide:1995]Title: CRC Handbook of Chemistry and Physics Author: Lide D.R. [Dana:1951]Title: Dana's System of Mineralogy Author: Dana J.D.

Occurrence of epsomite

The natural mineral epsomite was first extracted in 1695, from the mineral waters near Epsom (London), where the mineral name was derived from. Just as kieserite (magnesium sulfate mono hydrate), single crystals rarely occur in nature. Both magnesium salts, as well as their hydrate forms are extracted from alpine deposits and occur together with sylvite and rock salt in north Germany, and in the top layer of salt deposits by Staßfurt (near Magdeburg).

Information on the origin and formation of epsomite on monuments

The formation of magnesium sulfate on monuments requires the presence of soluble magnesium compounds, and magnesium ions. Different building materials on historic structures can contain magnesium compounds. Some examples are listed below:

• Utilization of lime with dolomite components: Dolomite (CaMg(CO₃)₂ ) is a double salt. If lime stone containing dolomite components is heated, slaked and used as mortar, CaCO₃ and MgCO₃·xH₂O form through this process (e.g. Nesquehonite MgCO₃·3H₂O including the molecules from the water of crystallization). The water solubility of MgCO₃·3H₂O at 1,76 g/l, is significantly above the solubility of calcite (0,014 g/l) and dolomite (0,078 g/l). If magnesium ions have been dissolved by moisture, they can form several different magnesium salts in the presents of the corresponding anions. The formation of magnesium sulfate can occur if, in connection with dolomite containing plasters, gypsum was used for stucco or plaster, readily supplying sulfates.
• Utilization of magnesia containing binder: Magnesia binder consists essentially of MgO and MgCl₂ or magnesium sulfate. The fully cured and solidified binder can contain highly hygroscopic magnesium salts, which can be converted to magnesium sulfate.
• Utilization of cement: According to the German standard, DIN 1164 [DIN1164:1994]Title: DIN 1164-1: Zement Teil 1, Zusammensetzung und Anforderungen
  Author: DIN
  a maximum content MgO of 5 M.% is tolerated in cements. Apart from the possibility of causing damage by delayed quenching, due to the expansion of the magnesia, magnesium ions can theoretically be released and cause the formation of magnesium sulfate.

Deicing salt can be another source of magnesium sulfate, it often contains a small proportion of readily soluble MgCl₂. Furthermore, the magnesium supply can occur through constant soil leaching in connection with rising damp.

Solubility properties

Figure 1: Solubilities in the system MgSO₄-H₂O and its temperature dependence. The molality m [n(MgSO₄•xH₂O)•kg(H₂O)^-1] is plotted versus the temperature. Equilibria of the different hydrated forms can be distiguished by the different colours of the curves. Dotted lines represent the metastable equilibria. In addition to the solubilities, freezing and boiling points are given. According to [Steiger.etal:2011a]Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars
Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy
.

The solubility of the above magnesium sulfate forms is significantly above 100 g/l (20°C), and they are part of the group of easily soluble salts. In principle, this leads to a great mobility of the salt and frequent shifting of the accumulation zones inside the material fabric. Due to the influence of temperature on solubility, there is a risk of precipitation of the dissolved salts in combination with rapid temperature drops.

Hygroscopicity

The low hygroscopicity of the pure salt epsomite which becomes evident in the high value of the equilibrium moisture content in the range of 88-90% RH, cannot be considered in isolation. In mixed systems, i.e. under the influence of foreign ions the sorption point is lower (see table). The possibility of hygroscopic moisture uptake with the connected difficulties is still possible despite the high Deliquescence point.

Figure 2: Deliquescence behaviour in the system MgSO₄-H₂O. The water activity a_w is plotted versus the temperature. Different colours label equilibrium and deliquescence humidities of the different phases of the system. Dotted lines represent metastable equilibria. According to [Steiger.etal:2011a]Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars
Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy
.

Table 1:Deliquescence humidities at different temperatures [according to [Steiger.etal:2011a]Title: Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars
Author: Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy
]

10°C	20°C	25°C	30°C
93.7% r.h.	91.3% r.h.	90.3% r.h.	89.1% r.h.

Moisture Sorption

Tabele 2: Behavior concerning moisture sorption of magnesium sulfate [according to [Vogt.etal:1993]Title: Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe
Author: Vogt, R.; Goretzki, Lothar
]

Moisture sorption at	87%r.F.	81%r.F.	70%r.F.	61%r.F.	50%r.F.
MgSO4	76	75	70	71	27
MgSO4 + NaCl (1:1 molar mixture)	240	146	75	50	20

Crystallization pressure

Due to the high solubility of the salt, dissolving and recrystallization processes take place under the appropriate humidity conditions. In theory the occurring crystallization pressure can be calculated and ranges between 10,5-12,5 N/mm² for epsomite and between 11,8- 14,1 N/mm² for hexahydrite. Compared to other damaging salts these values are thus to be found in the lower third of a calculated scale of values, alltogether ranging from 7,2 bis 65,4 N/mm² [according to [Winkler:1975]Title: Stone: Properties, Durability in Man´s Environment
Author: Winkler, Erhard M.
].

Hydration behavior

The system MgSO₄ – H₂O: the listed hydrate stages of magnesium sulfates are known to be stable compounds. With the exception of magnesium sulfate- 12- hydrate, all the above stages of the chemically combined water of magnesium sulfate on monuments are visible, but essentially only epsomite, hexahydrite, pentahydrite and kieserite occur.

At room temperature and an RH between 50%-90%, epsomite is the most stable hydrate stage. If the RH drops significantly lower than 50% at room temperature, the chemically combined water is released and lower hydrate stages are formed. Hexahydrite (MgSO₄ × 6H₂O) as pure salt is only stable at a temperature range of approx. 48°C to 67,5 °C. Pentahydrite has been thought to be metastable or unstable when exposed to the air, nevertheless the existence of these two salt phases has been detected by X-ray diffraction on building structures. The expulsion of the hydrate water can occur up to the formation of kieserite at elevated temperatures.

Hydration pressure

Possible variations in magnesium sulfate´s content of chemically combined water on constructions are a fact, and it is likely, that changes in the hydrate stages pentahydrite, hexahydrite and epsomite (subject to climatic fluctuations) are taking place in situ. The addition of a water molecule into the crystal lattice of hexahydrite, resulting in the transformation to epsomite, is associated with an increase in volume of around 10%. At temperatures from 0-20°C and 70% RH the resulting hydration pressure can be specified at values between 6,8 –9,7 N/mm². The conversion from kieserite to hexahydrite causes an increase in volume of approx. 140% [according to [Stark.etal:1996]Title: Bauschädliche Salze
Author: Stark, Jochen; Stürmer, Sylvia
].

Conversion reaction

The following case is a good example for the potential damage connected with the formation of epsomite: if magnesium carbonate (as magnesite) is present in the material structure of a monument, epsomite can form through the influence of sulfuric acid. According to [Stark.etal:1996]Title: Bauschädliche Salze
Author: Stark, Jochen; Stürmer, Sylvia
this conversion is linked to an increase in volume of over 400%.

Analytical identification

On objects, crystallized magnesium sulfate can appear in differing morphologies, but specific forms appear more frequently. One examined object displayed a loose crust of an opaque, gray to yellow substance in situ, which was a mixture of epsomite and hexahydrite (part of the thesis by Mainusch [Mainusch:2001]Title: Erstellung einer Materialsammlung zur qualitativen Bestimmung bauschädlicher Salze für Fachleute der Restaurierung
Author: Mainusch, Nils
). At the monastery church St. Johann at Müstair epsomite appeared as “granular crusts”. Magnesium sulfate efflorescence in the shape of salt whiskers was detected at St. Georgs church in Steiermark (Styria).

Microscopy

Laboratory examination:

Breathing onto a (predominantly) magnesium sulfate efflorescence, causes no observable macroscopic changes. However, characteristics are: the good solubility in water, a pH of 7, the formation of a ring-shaped slightly raised seam (visible by the naked eye), or a transparent layer that remains in place after the evaporation of the aqueous solvent. In laboratory tests on objects the results should be double checked with the microscope. Observations of the raw sample material and the recrystallized salt, show that magnesium sulfate crystals appear in unspecific forms in sample material. Experiments to produce well-shaped, single magnesium sulfate crystals, by recrystallization in aqueous solution have not been very successful, because of a strong tendency for intergrowth. Usually the previously mentioned, distinct, ring-shaped seam of intergrown crystals forms, when the magnesium sulfate containing solute is evaporated. Epsomite shows a low solubility in anhydrous ethanol and glycerin under the microscope. For further examinations using polarized light microscopy, a considerable difficulty is the creation of good preparations, due to the tendency of magnesium sulfate to form only few equidimensional or elongated single crystals. The low solubility in ethanol, nevertheless allows to isolate single particles by repeatedly adding recrystallized material. Both on the base material as well as the recrystallized product, examination using polarized light microscopy should be carried out.

Refractive indices:  n_X = 1,433, n_y = 1,455, n_z
Birefringence:    Δ = max.  0,028
Crystal class:          orthorhombic

Polarized light microscopic examination:

The allocation of epsomite to the refractive indices is carried out according to the immersion method (successively by means of embedding media n_D=1,518; n_D=1,47; n_D=1,46). When using an immersion medium with a refractive index of n_D=1,45 on many single particles a slight but distinct variation in relief is visible at rotation. Because epsomite belongs to the class of orthorhombic crystals, it never appears in an oblique but always in a parallel symmetrical extinction. Beside epsomite, hexahydrite often forms, which is monoclinic and has nearly identical refractive indices. A distinction between these two hydrate stages of magnesium sulfate is only possible through the allocation of the crystal class. Due to the low path difference epsomite crystals usually only show low inference colors in the range of the first order.

Possibility for mistakes:

Epsomite/hexahydrite are to be allocated, once the examination criteria below have been clarified:

• good water solubility
• characteristic appearance at recrystallization 
• low birefringence

Salts and deterioration pattern

Under the polarising microscope

• Crystallized on monuments
• 

  Sample of a salt crust of magnesium sulfate whiskers (image width 440 um)

• Crystallized from a water solution on a glass slide
• 

  Sample of magnesium sulfate re-crystallized from water solution on a glass slide

• 

  Sample of magnesium sulfate re-crystallized from water solution on a glass slide

• 

  Sample of magnesium sulfate re-crystallized from water solution on a glass slide

• 

  Sample of magnesium sulfatere-crystallized from water solution on a glass slide

• 

  Sample of magnesium sulfate re-crystallized from water solution on a glass slide

• 

  Magnesium- sulfate, crystallized from a water solution on a glass slide

• 

  Magnesium sulfate, crystallized from a water solution on a glass slide

• 

  Magnesium sulfate, crystallized from a water solution on a glass slide

• 

  Magnesium sulfate, crystallized from a water solution on a glass slide

• 

  Magnesium sulfate, crystallized from a water solution on a glass slide

• 

  Magnesium sulfate, crystallized from a water solution on a glass slide

• 

  Magnesium sulfate, crystallized from a water solution on a glass slide

• 

  Magnesium sulfate, crystallized from a water solution on a glass slide

Literature

[DIN1164:1994]	DIN (1994): DIN 1164-1: Zement Teil 1, Zusammensetzung und Anforderungen.
[Dana:1951]	Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & Sons
[Lide:1995]	Lide D.R. (eds.) Lide D.R. (1995): CRC Handbook of Chemistry and Physics, CRC Press
[Mainusch:2001]	Mainusch, Nils (2001): Erstellung einer Materialsammlung zur qualitativen Bestimmung bauschädlicher Salze für Fachleute der Restaurierung, Diplomarbeit, HAWK Hochschule für angewandte Wissenschaft und Kunst Hildesheim/Holzminden/Göttingen, file:Diplomarbeit Nils Mainusch.pdf
[Stark.etal:1996]	Stark, Jochen; Stürmer, Sylvia (1996): Bauschädliche Salze, Bauhaus-Univ. Weimar
[Steiger.etal:2011]	Steiger, M.; Charola, A. E.; Sterflinger, K. (2011): Weathering and Deterioration. In: Siegesmund, Siegfried; Snethlage, Rolf (eds.): Stone in Architecture - Properties, Durability, Springer, 227-316.
[Steiger.etal:2011a]	Steiger, Michael; Linnow, Kirsten; Ehrhardt, Dorothee; Rohde, Mandy (2011): Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na+-Mg2+-Cl--SO42--H2O systems with implications for Mars. In: Geochimica et Cosmochimica Act, 75 (12), 3600-3626, 10.1016/j.gca.2011.03.038,
[Vogt.etal:1993]	Vogt, R.; Goretzki, Lothar (1993): Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe, unveröffentlichter Bericht.
[Winkler:1975]	Winkler, Erhard M. (1975): Stone: Properties, Durability in Man´s Environment, Springer Verlag, Wien

A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

concentration given as amount of substance (mol) of the dissolved substance per kilogram solvent; unit: mol•kg-1.

relative humidity

A molecule is an electrically neutral group of at least two atoms.

crystallization of a dissolved salt

the mathematical difference between the largest and smallest refractive index for an anisotropic mineral

Hochschule für angewandte Wissenschaft und Kunst Hildesheim/Holzminden/Göttingen, University of Applied Sciences and Arts Hildesheim/Holzminden/Göttingen