Mirabilite

From Saltwiki

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

Mirabilite<ref>http://webmineral.com/data/Mirabilite.shtml accessed 29/07/2010</ref><ref>http://www.mindat.org/min-2725.html accessed 29/07/2010</ref>
HJS Na2SO4-slides-110703-10x-1.jpg
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.
Link to Google Scholar
[Dana:1951]Title: Dana's System of Mineralogy
Author: Dana J.D.
Link to Google Scholar


Abstract

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

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
Link to Google Scholar
].


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
Link to Google Scholar

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
Link to Google Scholar
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
Link to Google Scholar
.
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:    nx = 1,395; ny =1.396-1.410; nz =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 nD-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

Weblinks

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Literatur

[Dana:1951]Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & SonsLink to Google Scholar
[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. U.S. Geol. Surv. Bull, 1452 ()Link to Google Scholar
[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
[Vogt.etal:1993]Vogt, R.; Goretzki, Lothar (1993): Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe, unveröffentlichter BerichtLink to Google Scholar