Mirabilite

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

Mirabilite[1][2]
HJS Na2SO4-slides-110703-10x-1.jpg
Mineralogical name Mirabilite
Chemical name Sodium sulfate decahydrate
Trivial name Glauber salt, Reussin, Sulphate of Soda
Chemical formula Na2SO4•10H2O
Other forms Sodium sulphate heptahydrate Na2SO4•7H2O
Crystal system monoclinic
Crystal structure
Deliquescence humidity 20°C 93.6% (20°C), 90% (23°C), 87% (25°C)
Solubility (g/l) at 20°C 900 g/l
Density (g/cm³) 1.464 g/cm³
Molar volume 219.8 cm3/mol
Molar weight 322.21 g/mol
Transparency transparent to opaque
Cleavage perfect
Crystal habit
Twinning
Phase transition
Chemical behavior
Comments soluble in water and glycerin,
not soluble in pure alcohol
easily loses some water, converts to thenardite at 32°C
abnormal blue or brown interference colors
Crystal Optics
Refractive Indices nx = 1.395
ny = 1.396-1.410
nz = 1.398-1.419
Birefringence Δ = 0.04-0.023
Optical Orientation negative
Pleochroism
Dispersion
Used Literature
{{{Literature}}}


Solubility properties[edit]

Figure1: Solubility of thenardite and mirabilite in comparison to other salts[after [Stark.etal:1996]Title: Bauschädliche Salze
Author: Stark, Jochen; Stürmer, Sylvia
Link to Google Scholar
].


see Sodium sulfate


Hygroscopicity[edit]

Figure 2: Deliquescence points of the pure salts thenardite and mirabilite [Arnold.etal:1991]Title: Monitoring Wall Paintings Affected by soluble Salts
Author: Arnold, Andreas; Zehnder, Konrad
Link to Google Scholar

Figure 2 illustrates the influence of temperature on the deliquescence points of thenardite and mirabilite. Note the opposing curves of the graphs.

The presence of other ions (in salt mixtures) can significantly alter the equilibrium humidity parameters, i.e. the temperature and humidity conditions which initiate phase changes. Table 1 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.


Table1: 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)


Hygroscopicity

To assess the hygroscopicity of sodium sulfates, the table below compares the moisture sorption of pure sodium sulphate and sodium sulphate-halite mixture at different relative humidity (RH) levels.


Table 2: Moisture sorption in M.% after 56 days storage of sodium sulphate [Vogt.etal:1993]Title: Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe
Author: Vogt, R.; Goretzki, Lothar
Link to Google Scholar
storage relative RH 87% RH. 81% RH 79% RH
Na2SO4 79 0 0
Na2SO4+NaCl
(1:1 molar mixture)
157 32 15


Crystallization pressure[edit]

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

Hydration behavior[edit]

see Sodium sulfate

Analytical evidence[edit]

Microscopy
[edit]

Laboratory analysis:
Observations of the solubility behavior through the microscope allow the verification of sodium sulfate’s good water solubility and 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 could 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 water of crystallization content of the sample and re-crystallized material depends on the ambivalent RH and temperature levels. In dry air (with 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 colours 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 anions 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 through the observation of mirabilite during the high hydration stages of salt recrystallization.


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
Magnesium formate Mg(HCO2)2 • 2H2O relatively high birefringence/ no abnormal interference colors/oblique extinction


Mixtures:

Mixed systems Na+– Ca2+– SO4 2-: Due to its poorer solubility, gypsum precipitates first during re-crystallization. The characteristic needle shapes of the individual gypsum crystals as well as those of the aggregates remain. The precipitation of sodium sulfate occurs later, the crystal growth is noticeably faster, the morphology is non-specific.

Mixed system Na+– SO4 2-– Cl-: The precipitation of both types of particles begins approx. simultaneously. Halite has a characteristic morphology whereas sodium sulfate occurs in extremely varying forms.

Under the polarizing microscope[edit]

Weblinks
[edit]

Literatur[edit]

[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. In: 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. In: Geochimica et Cosmochimica Acta, 72 (17), 4291-4306, UrlLink to Google Scholar
[Vogt.etal:1993]Vogt, R.; Goretzki, Lothar (1993): Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe, unveröffentlichter Bericht.Link to Google Scholar