Thenardite

Authors: Hans-Jürgen Schwarz , Michael Steiger, Tim Müller
English translation by Matthieu Angeli
back to Sulfate

Thenardite
Mineralogical name	Thenardite
Chemical name	Sodium sulfate
Trivial name	Pyrotechnite
Chemical formula	Na2SO4
Other forms	Mirabilite (Na2SO4•10H2O) Sodium sulfate heptahydrate (Na2SO4•7H2O)
Crystal system	orthorhombic
Crystal structure
Deliquescence humidity 20°C	81.7% (25°C)
Solubility (g/l) at 20°C	162 g/l
Density (g/cm³)	2.689 g/cm³
Molar volume	53.11 cm3/mol
Molar weight	142.04 g/mol
Transparency	transparent to translucent
Cleavage	perfect
Crystal habit
Twinning
Phase transition
Chemical behavior
Comments	soluble in water and glycerin, insoluble in pure alcohol
Crystal Optics
Refractive Indices	nx = 1.468 ny = 1.473 nz = 1.483
Birefringence	Δ = 0.015
Optical Orientation	positive
Pleochroism
Dispersion
Used Literature

Sodium sulfate and thenardite

Abstract

Thenardite as the anhydrous and stable phase of sodium sulfate and its properties will be presented.

Occurrence

Both thenardite and mirabilite occur as natural minerals. In nature sodium sulfate occurs in mineral waters in the form of double salts, as deposits of former salt lakes. The hydrated sodium sulfate was first described by Glauber in 1658 where he called it "sal mirabilis". Mirabilite is also known as "Glauber's salt" in honor of its discoverer.

Origin and formation of thenardite / mirabilite in monuments

When sodium ions in conjunction with other anions enter porous inorganic building materials, sodium sulfate may be formed by reaction with sulfate contributed by other sources, for example air contaminated with sulfur oxide gases. Portland cement contains a certain amount of sodium or potassium sulfate. In Germany, the standardization institute (DIN) allows a content of up to 0.5 % soluble alkalis. This means that 100 kg of Portland cement containing only 0.1% soluble Na₂O can form 520 g of Mirabilite when reacting with sulfate [calculation by Arnold/Zehnder 1991]. Sodium ions can also enter into monuments from various cleaning materials and, in older restoration products, such as water glass. Ground water, and even surface water, are also a possible source of Na⁺-ions as well as sulfate ions. De-icing road salt may contain a large amount of soluble sodium chloride. Finally, in coastal areas, sea water is a significant source of NaCl.

Solubility behavior

Figure 1: Solubility of Na₂SO₄ in water, 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 structures of both thenardite and mirabilite belong to the group of easily soluble salts (solubility of thenardite at 20 °C: 3.7 mol/kg) and therefore they are easily mobilized (see table hygroscopicity of salts and their equilibrium moisture content). The solubility of sodium sulfate is highly dependent on temperature. For this reason, a rapid drop of temperature is highly likely to yield very high supersaturation and salt crystallization.

Hygroscopicity

Figure 2:Deliquescence of Na₂SO₄, 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 temperature effect on the deliquescence points of thenardite and mirabilite is shown below. The striking features here are the opposite curve transitions for.
For thenardite the deliquescence humidity reaches higher values with increasing temperature (table 1).

Tabelle 1: Deliquescence humidity of thenardite and its temperature dependence,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

0°C	10°C	20°C	30°C	40°C	50°C
84.4%r.h.	85.6%r.h.	86.6%r.h.	87.3%r.h.	87.9%r.h.	88.4%r.h.

In the presence of other ions (in salt mixtures), the parameters of the equilibrium moisture content, as well as the necessary temperature and humidity conditions for recrystallization, change significantly. The following table shows experimental data of equilibrium moisture for different salt mixtures at different temperatures.It turns out that all the values of equilibrium moisture content are lower than those of the pure salt mirabilite (see table equilibrium moisture content as a function of temperature).

Table 2 - Information about the equilibrium moisture of saturated solid solutions (mixing ratio: saturated sol.A / saturated sol.B = 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)

Water vapor sorption:

The table below shows additional information for estimating the hygroscopicity of sodium sulfate for the sorption behavior of pure salt and the mixture with halite at different relative humidity levels:

Table 3: Moist sorption of sodium sulphate in M.% after 56 days of storage [after [Vogt.etal:1993]Title: Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe
Author: Vogt, R.; Goretzki, Lothar
]

Air humidity	87% r.F.	81% r.F.	79% r.F.
Na2SO4	79	0	0
Na2SO4+NaCl (1:1 molar mixture)	157	32	15

Crystallization pressure

For the crystallization from an aqueous solution a crystallization pressure of 29.2 - 34.5 N/mm² for thenardite can be expected. In comparism to other calculated pressures of other salts that might damage building materials, thenardite is able to exert high crystallization pressure [Winkler:1975]Title: Stone: Properties, Durability in Man´s Environment
Author: Winkler, Erhard M.
.

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 detection

Microscopy

Laboratory investigation:
Through microscopic observations regarding the solubility behavior, good solubility in water and no solubility in ethanol can be confirmed. Thenardite and mirabilite do not have morphological characteristics that could help identification with the use of simple re-crystallization experiments.

Refractive indices:    n_x = 1.468; n_y =1,473; n_z =1.483
Birefringence:      Δ = 0.015
Crystal class:            orthorhombic

Polarized light microscopy examination:

The raw sampling material and the re-crystallized preparation change their water content, depending on the conditions of relative humidity and temperature. In dry air conditions (RH < 80% and room temperature) mirabilite looses its chemically bound water and changes to thenardite. This process can be clearly understood and reproduced using a microscope, when the process of re-crystallization is observed. Mirabillite does show the characteristic abnormal interference colors. During the moisture loss and the formation of thenardite these abnormal interference colors become weaker.

The refractive index assignment of thenardite is carried out using the immersion method. Due to the low maximum birefringence, thenardite mostly displays gray interference colors. The extinction is parallel or symmetrical.

Possible mistakes:

Generally, the differentiation between certain kinds of sulfates (they are listed below, thenardite included) is a problem without micro-chemical determination of the anions, because the refractive indices of the salts lie very closely together and all salts show a low birefringence. It is best to use an immersion medium with a n_D- value of 1,48, thus making the differentiation within this group possible. Moreover, the properties mentioned below can be consulted as criteria for determination. Thenardite is unambiguously, but indirectly determined by re-crystallizing a sample and observing the abnormal interference colors, which occur when mirabiltite is identified in its high hydrate form.

Table 3: Characteristics for differentiating thenardite from other sulfates

Salt phase	Characteristic
Boussingaultite (NH4)2Mg(SO)4 • 6H20	no abnormal interference colors/ inclined extinction
Pikromerite K2Mg(SO4)2 • 6H20	no abnormal interference colors/ inclined extinction
Bloedite Na2Mg(SO4)2 • 6H20	all indices >1.48 / no abnormal interference colors/ inclined extinction / optically negative orientation.
Glaserite K3Na(SO4)2	all indizes >1.48 / no abnormal interference colors/ inclined extinction
Arcanite K2SO4	all indices >1.48 / no abnormal interference colors
Magnesium formiate Mg(HCO2)2 • 2H2O	comparatively high birefringence / no abnormal interference colors/ inclined extinction

Observation of mixed systems:

The mixed system Na⁺– Ca²⁺– SO₄ ^2-: The precipitation of gypsum takes place first during re-crystallization, which is due to its low solubility. The distinct needle like habit of single gypsum crystals and aggregates remains. The precipitation of sodium sulfate takes place later. The actual crystal growth takes place much faster. The morphology is non-specific.

Mixed system Na⁺– SO₄ ^2-– Cl^-: The precipitation of both kinds of particle starts approximately at the same time, halite with its characteristic morphology, sodium sulfate in extremely varying forms.

Pictures of salt and salt damage

In the field

• Thenardite efflorescences
• 

  Thenardite crystals on a wall in the old church in Idensen, Germany

• 

  Thenardite efforescences in the Ev. Ref. church in Eilsum, Germany

Literature

[Sperling.etal:1980]	Sperling, C.H.B.and Cooke, R.U. (1980): Salt Weathering on Arid Environment, I. Theoretical ConsiderationsII. Laboratory Studies. In: Papers in Geography, 8 ()
[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.
[Winkler.etal:1970]	Winkler, Erhard M.; Wilhelm, E.J. (1970): Saltburst by Hydration Pressure in Architectural Stone in Urban Atmosphere. In: Geological Society of America, Bulletin, 81 (), 567-572
[Winkler:1975]	Winkler, Erhard M. (1975): Stone: Properties, Durability in Man´s Environment, Springer Verlag, Wien

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

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