Mirabilite

Authors: Hans-Jürgen Schwarz, Nils Mainusch
English version by Christa Gerdwilker
back to Sulfate

Mirabilite
Mineralogical name	Mirabilite
Chemical name	Sodium sulfate decahydrate
Trivial name	Glauber salt, Reussin, Soda Sulfate
Chemical formula	Na2SO4•10H2O
Other forms	Na2SO4•7H2O (sodium sulfate heptahydrate)
Crystal system	monoclinic
Crystal structure
Deliquescence humidity 20°C	95.6 %
Solubility (g/l) at 20°C	1.353 mol/kg
Density (g/cm³)	1.466 g/cm³
Molar volume	219.8 cm3/mol
Molar weight	322.19 g/mol
Transparency	transparent to opaque
Cleavage	perfect
Crystal habit
Twinning
Phase transition
Chemical behavior
Comments	soluble in water and glycerin, insoluble in pure alcohol easily loses some water, converts to thenardite at 32°C abnormal blue or brown interference colors
Crystal Optics
Refractive Indices	nx = 1.394 ny = 1.396 nz = 1.398
Birefringence	Δ = 0.004
Optical Orientation	negative
Pleochroism
Dispersion	76°
Used Literature
[Robie.etal:1978]Title: Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar pressure and higher temperatures Author: Robie R.A., Hemingway B.S.; Fisher J.A. [Dana:1951]Title: Dana's System of Mineralogy Author: Dana J.D.

Abstract

Mirabilite which is the deacahydrate of sodium sulfate and its properties are presented.

Occurrence

Both thenardite and mirabilite appear as natural minerals. Sodium sulfate occurs naturally in mineral waters, as deposits of former salt lakes and in the form of different double salts. Knowledge of sodium sulfate containing crystalline water can be traced back to the 16th century. It was first described by Glauber in 1658, who referred to it as "sal mirable". Based on his name, the trivial name "Glauber's salt" for Mirabilite can also be found in the literature.

Origin and formation of thenardite / mirabilite in monuments

The introduction of materials containing soluble sodium compounds can lead to the formation of sodium sulfate as an efflorescence salt in the mineral system of a monument, if the air is contaminated with sulfurous gases or other sources of sulfate are present. Cements with a high sodium ion content may contain up to 0.5% soluble alkalis according to DIN specifications. In purely mathematical terms, 100 kg of Portland cement with a content of only 0.1% soluble Na2O can form 520 g of mirabilite in air containing sulfuric acid [data according to Arnold/Zehnder 1991]. An abundance of cleaning materials and especially formerly used restoration products (such as water glass) can introduce sodium ions into architectural monuments. Other sources are groundwater and surface water, which may contain Na+ ions. Road salt consists largely of readily soluble sodium chloride. Near the coast, seawater containing NaCl must be considered as a source of sodium.

Solubility properties

Figure1: Solubilities in the system Na₂SO₄-H₂O in dependence of the temperature [according to [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
].

The solubility of mirabilite is 1.35 mol/kg at 20 °C. The solubility shows a high temperature dependence, it decreases with decreasing temperatures and it increases at higher temperatures. Above 32.4 °C mirabilite is unstable, so thenardite is the stable phase at higher temperatures.

see Sodium sulfate

Hygroscopicity

Figure 2: Deliquescence behaviour in the system Na₂SO₄-H₂O and equilibrium humbidities for the transformation thenardite/mirabilite, according to [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

Figure 2 illustrates the influence of temperature on the deliquescence humidities of mirabilite and on the other phases of sodium sulfate. It is notable that for mirabilite and thenardite the graphs of the deliquescence humidities run in opposite directions. Unlike the deliquescence humidity of thenardite those of mirabilite decreases with increasing temperature.
The deliquescence humidity of mirabilite lies always above 87 % in its stable temperature range (table 1). See also sodium sulfate

Table 1: The temperature dependence of the deliquescence humidities of mirabilite [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

0°C	10°C	20°C	30°C
98.8%r.h.	97.8%r.h.	95.6%r.h.	90.1%r.h.

The presence of other ions, as in the case of salt mixtures, can significantly alter the equilibrium humidity parameters, i.e., the temperature and humidity conditions which initiate phase changes. Table 2 lists the experimentally determined equilibrium humidity of different salt mixtures. This shows that the equilibrium humidity of pure mirabilite is higher than that of the other salts.

Table 2: Equilibrium humidity data of saturated salt mixture solutions (mixing ratio: Saturated solution A/saturated solution = 1:1) [Vogt.etal:1993]Title: Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe
Author: Vogt, R.; Goretzki, Lothar
.

	MgSO4	Ca(NO3)2	KNO3
Na2SO4 • 10H2O	87(21°C)	74 (21°C)	81(21°C)

Crystallization pressure

The crystallization of mirabilite from aqueous solution results in a crystallization pressure of 7.2-8.3 N/mm².

Hydration behavior

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Conversion of mirabilite (?) into thenardite

The Na₂SO₄ – H₂O system:

The only stable forms of sodium sulfate are the decahydrate (mirabilite) and the anhydrite (thenardite). The generation of mirabilite by recrystallization of the salt from an aqueous supersaturated solution occurs at 32.4°C. In particular, the transition from thenardite to mirabilite and the incorporation of 10 water molecules in the crystal lattice causes a volume expansion of 320%. This transition happens at a relatively low temperature (32-35°C), the damage caused by this salt is highly dependent on the temperature and thus on the environment. This temperature range is given as a guide, because this transition could happen for example at 25°C at 80% relative humidity, or even at 0°C at 60.7% relative humidity [information from Gmelin]. Due to this strong dependence on the environmental parameters, an estimate of the damage caused on buildings by crystallization and hydration of sodium sulfate are very difficult to obtain.

Analytical evidence

Microscopy

Laboratory analysis:
Observations of the solubility behavior through the microscope allows to verify the high water solubility of sodium sulfate and its insolubility in ethanol. Thenardite and mirabilite do not have morphological characteristics to aid their identification during simple recrystallization experiments. Instead, a broad range of different forms can be observed.

Refractive indices:    n_x = 1,395; n_y =1.396-1.410; n_z =1.398-1.419
Birefringence:      Δ = 0.04-0,023
Crystal classe:            monocline

Under the polarizing microscope:

The crystallization water content of the sample or its re-crystallized form depends on the ambient RH and temperature levels. In dry air (lower than 80% RH at room temperature) mirabilite loses its water of crystallization content and changes to thenardite. This process can be observed during recrystallization under the microscope. Mirabilite has characteristic abnormal interference colors which weaken during water loss and the formation of thenardite.

Differentiation from different salts:

Generally, the differentiation of certain sulfates (listed below and including thenardite) without the microchemical determination of the cations is problematic as their refractive indices are close to each other and all salts display a slight double refraction. The use of an immersion material with a n_D-value of 1.48 is helpful and allows the differentiation of salts within this group. Additionally, the properties listed below can also be taken into consideration. Thenardite can be determined indirectly by the appearance of mirabilite during recrystallization to the hydrated form.

Table 3: Differing characteristics of thenardite and mirabilite

Salt phase	Characteristics
Boussingaultite (NH4)2Mg(SO)4 • 6H20	no abnormal interference colors/oblique extinction
Picromerite K2Mg(SO4)2 • 6H20	no abnormal interference colors/oblique extinction
Bloedite Na2Mg(SO4)2 • 6H20	all indices >1.48 / no abnormal interference colors/oblique extinction / negative optical orientation
Glaserite K3Na(SO4)2	all indices >1.48 / no abnormal interference colors/oblique extinction
Arcanite K2SO4	all indices >1.48 / no abnormal interference colors
Dashkovaite Mg(HCO2)2 • 2H2O	relatively high birefringence/ no abnormal interference colors/oblique extinction

Mixtures:

Mixed systems Na⁺– Ca²⁺– SO₄ ^2-: Due to its lower solubility, gypsum precipitates first during re-crystallization. The characteristic needle shaped gypsum crystals are the first to form while sodium sulfate precipitates later, the crystal growth is noticeably faster but the morphology is non-specific.

Mixed system Na⁺– SO₄ ^2-– Cl^-: The precipitation of both types of particles begins practically at the same time. Halite has a characteristic morphology whereas sodium sulfate occurs in extremely varying forms.

Under the polarizing microscope

• Sodium sulfate crystallization between two glass slides
• 

  Photo taken through crossed polars and red I

• 

  Photo taken through crossed polars

Literatur

[Dana:1951]	Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & Sons
[Robie.etal:1978]	Robie R.A., Hemingway B.S.; Fisher J.A. (1978): Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar pressure and higher temperatures. In: U.S. Geol. Surv. Bull, 1452 ()
[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. In: Geochimica et Cosmochimica Acta, 72 (17), 4291-4306, Url
[Vogt.etal:1993]	Vogt, R.; Goretzki, Lothar (1993): Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe, unveröffentlichter Bericht.

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

Water glass is the common name for a liquid solution of alkaline silicates, also known as Sodium silicate Na2SiO3 or liquid glass.

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

relative humidity

The relationship between a mineral's crystallographic axes and optical indicatrix.

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