Magnesium sulfate

Author: Amelie Stahlbuhk
back to Sulfate

This article will be released soon.

Abstract

The different hydrated forms of magnesium sulfate and the behaviour concerning solubility and hygroscopicity will be presented.

Hydrated forms

Kieserite MgSO₄•H₂O
Sanderite MgSO₄•2H₂O
Starkeyite MgSO₄•4H₂O
Pentahydrite MgSO₄•5H₂O
Hexahydrite MgSO₄•6H₂O
Epsomite MgSO₄•7H₂O
Meridianiite MgSO₄•11H₂O

Solubility

As it is shown in table 1, the different hydrated forms of magnesium sulfate are easily soluble salts, which leads to a high mobility of the salts in porous materials.

Table 1: Solubilities in mol/kg of different hydrated forms of magnesium sulfate at 20°C [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
].

Hydrated form	Solubility [mol/kg] at 20°C
Kieserite	5.60
Starkeyite	5.04
Pentahydrite	4.40
Hexahydrite	3.61
Epsomite	2.84

Due to the different hydrated forms of magnesium sulfate with stable and meta stable equilibria, the solubility diagram of the system MgSO₄-H₂O contains more information than diagrams of salts with less or also without any hydrated forms. With the temperature dependence of the solubility it is possible that temperature changes are accompanied by a hydration or a dehydration of a considered phase.

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
.

Hygroscopicity

In the system MgSO₄-H₂O changes in temperature or relative humidity may lead to hydration/dehydration or deliquescence/crystallization processes. At 20 °C epsomite is the present crystalline phase when the relative humidity is below its deliquescence humidity of 91.3 %. When the relative humidity reaches values below 47 % the dehydration to lower hydrated levels sets in, as it is represented by the curves of the equilibrium humidities in figure 2.

Table 2: Deliquescence and equilibrium humidities at 20 °C [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
].

Phase transition	Deliquescence or equilibrium humidity at 20°C
Epsomite-solution	91.3 %
Epsomite-Hexahydrite	46.6 %
Epsomite-Kieserite	46.7 %
Hexahydrite-Starkeyite	39.1 %

In the temperature range of -10 to 100 °C the deliquescence humidities of the present hydrated forms (depending on the temperature) lie always above 80 % r.h., so the salts do not belong to the hygroscopic salts.

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 from a water solution on a glass slide
• 

  Sample of a salt crust of magnesium sulfate whiskers

• 

  Real sample with magnesium sulfate salts, crystallized from a water solution on a glass slide

• 

  Real sample with magnesium sulfate salts, crystallized from a water solution on a glass slide

• 

  Real sample with magnesium sulfate salts, crystallized from a water solution on a glass slide

• 

  Real sample with magnesium sulfate salts, crystallized from a water solution on a glass slide

• 

  Real sample with magnesium sulfate salts, 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

• 

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

Literature

[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: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,

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

the relative humidity, above which a material, e.g. a salt, absorbes humidity from the air and dissolves.

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

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