Mirabilite

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

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

see Sodium sulfate

Hygroscopicity

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

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
.

	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

storage relative RH	87% RH.	81% RH	79% RH
Na2SO4	79	0	0
Na2SO4+NaCl (1:1 molar mixture)	157	32	15

Crystallization pressure

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

Hydration behavior

see Sodium sulfate

Analytical evidence

Microscopy

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:    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 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 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 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
Dashkovaite 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

• 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.

relative humidity

In physics, interference is the addition of two or more waves that results in a new wave pattern. The colours are produced by the difference in speed in the fast and slow rays. Minerals have specific and predictable colours in cross-polarized light, and this can help identify them.

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