Hexahydrite

Hexahydrite
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) Meridianite (MgSO4•11H2O) Magnesium 12-Hydrate
Crystal system	monoclinic
Crystal structure
Deliquescence humidity 20°C
Solubility (g/l) at 20°C	660 g/l
Density (g/cm³)	1.76 g/cm3
Molar volume	132.6 cm3/mol
Molar weight	228.46 g/mol
Transparency	transparent to opaque
Cleavage	perfect
Crystal habit
Twinning
Phase transition
Chemical behavior
Comments	can be produced from an aqueous solution at 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
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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 660 g/l 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

Crystallization pressure

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

Compared to the hydration pressure, the crystallization pressure is rather low. During the hydration of kieserite to hexahydrite at the relevant humidity level, a hydration pressure of 57 MPa can be occur [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
.

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 found 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
.

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

relative humidity

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