Hexahydrite

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Authors: Hans-Jürgen Schwarz, Tim Müller
English version by Christa Gerdwilker
back to Sulphate

Hexahydrite[1][2]
Mineralogical name Hexahydrite, Magnesiumsulfate
Chemical name Magnesiumsulfate Hexahydrite
Trivial name
Chemical formula MgSO4•6H2O
Other forms Kieserite (MgSO4•H2O)
Sanderite (MgSO4•2H2O)
Starkeyite (MgSO4•4H2O)
Pentahydrite (MgSO4•5H2O)
Epsomite (MgSO4•7H2O)
Meridianiite (MgSO4•11H2O)
Magnesium 12-Hydrate
Crystal system monoclinic
Crystal structure
Deliquescence humidity 20°C
Solubility (g/l) at 20°C 3.611 mol/kg
Density (g/cm³) 1.723 g/cm3
Molar volume 132.6 cm3/mol
Molar weight 228.45 g/mol
Transparency transparent to opaque
Cleavage perfect
Crystal habit
Twinning
Phase transition
Chemical behavior
Comments can be produced from an aqueous solution between 48-69 °C
Crystal Optics
Refractive Indices α = 1.426
β = 1.453
γ = 1.456
Birefringence Δ = 0.030
Optical Orientation biaxial negative
Pleochroism
Dispersion 38°
Used Literature
[Lide:1995]Title: CRC Handbook of Chemistry and Physics
Author: Lide D.R.
Link to Google Scholar
[Dana:1951]Title: Dana's System of Mineralogy
Author: Dana J.D.
Link to Google Scholar



Introduction

Hexahydrite is one of the more commonly found salts causing masonry damage. It occurs in many forms on different objects, both externally and internally.

Occurrence of hexahydrite

Hexahydrite is a hydration phase of magnesium sulfate. The presence of magnesium sulfates is particularly damaging to masonry due its different hydrate phases. The hydration and subsequent volume changes result in stresses within the masonry which eventually cause the material to break up during repeated solution crystallization and phase change processes. The properties, damaging effects, occurrence and the determination of hexahydrite are discussed and complemented with illustrations, microscopic images and practical examples. For further information see: epsomite.

Solution behavior

The water solubility of hexahydrite is 3.611 mol/kg at a temperature of 20 °C [Steiger and Asmussen, 2008], and subsequently belongs, like all discussed forms of magnesium sulphate with a solubility of clearly above 100 g/l (at 20 °C) to the group of easily soluble salts. This entails a high risk of salt mobility and frequent re-deposition of salts within the material matrix. Due to the influence of temperature on solubility, a rapid drop in temperature can result in the precipitation of salts [Mainusch:2001]Title: Erstellung einer Materialsammlung zur qualitativen Bestimmung bauschädlicher Salze für Fachleute der Restaurierung
Author: Mainusch, Nils
Link to Google Scholar

Crystallization pressure

Due to the high solubility of the salt, solution and recrystallization processes occur at corresponding humidity levels.

Compared to the hydration pressure, the crystallization pressure is rather low. The hydration of kieserite to hexahydrite at the relevant humidity level can result in an hydration pressure of 57 MPa [Steiger.etal:2008]Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Author: Steiger, Michael; Asmussen, Sönke
Link to Google Scholar
.

Hydration behavior

The six hydrate forms of magnesium sulfate listed above are shown to be stable compounds. With the exception of magnesium sulfate-12-hydrates, all the above crystal water phases of magnesium sulfate have been identified on monuments. These predominantly occur as epsomite, hexahydrite, pentahydrite und kieserite.

Hexahydrite is the magnesium sulfate hexahydrate. It can be formed through the hydration of kieserite or the dehydration of epsomite. During the phase change, water intake causes an increase in volume whereas water loss leads to a reduction in volume. Increased relative humidity also results in increased hydrate-water content within magnesium sulfate. Kieserite is stable at room temperature (25°C) up to a RH of approx. 42 % , above this the change to hexahydrite or epsomite occurs. Hexahydrite is stable below 51 % RH, above this epsomite is formed. The phase changes can happen directly or via solution and re-crystallization. This results in the metastable existence of the lower hydration phase up to its deliquescence humidity. Above this RH the phase dissolves and a supercritical solution is formed, from which the hydrated phase crystallizes [Steiger.etal:2008]Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Author: Steiger, Michael; Asmussen, Sönke
Link to Google Scholar
.


Weblinks


Literatur

[Dana:1951]Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & SonsLink to Google Scholar
[Lide:1995]Lide D.R. (eds.) Lide D.R. (1995): CRC Handbook of Chemistry and Physics, CRC PressLink to Google Scholar
[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.pdfLink to Google ScholarFulltext link
[Steiger.etal:2008]Steiger, Michael; Asmussen, Sönke (2008): Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress. Geochimica et Cosmochimica Acta, 72 (17), 4291-4306, 10.1016/j.gca.2008.05.053Link to Google Scholar