Thenardite^[1]^[2]

Mineralogical name	Thenardite
Chemical name	Sodium sulfate
Trivial name	Pyrotechnite
Chemical formula	Na₂SO₄
Other forms	Mirabilite (Na₂SO₄•10H₂O) Sodiumsulfate 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, not soluble in pure alcohol
Crystal Optics
Refractive Indices	n_x = 1.468 n_y = 1.473 n_z = 1.483
Birefringence	Δ = 0.015
Optical Orientation	positive
Pleochroism
Dispersion
Used Literature
{{{Literature}}}

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

Sodium sulfate and thenardite[edit]

Abstract[edit]

Sodium sulfate and its phases thenardite, mirabilite and heptahydrate are described. Heptahydrate´s role as a major source of damage is also explained .

Occurrence[edit]

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. Knowledge of the hydrated sodium sulfate dates back to the 16th Century. Its first description has been written by Glauber in 1658 in which he described it as "sal mirable". It is also quite common to read the name "Glauber's salt" for mirabilite in the literature.

Information on the origin and formation of thenardite / mirabilite in monuments[edit]

With the entry of materials that contain soluble sodium compounds, the mineral system of a monument may create sodium sulfate as salt efflorescence, when reacting with various sources of sulfate for example air contaminated with sulphurous gases. In Germany cement exhibits a high content of sodium ions, because the German standardization institute (DIN) allows for 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 520g of Mirabilite when in contact with air containing sulfuric acid [calculation by Arnold/Zehnder 1991]. Sodium ions can also enter into monuments from a plethora of cleaning materials and especially older restoration products (such as water glass). Ground water and surface water are also a possible source of Na⁺-ions. Road salt consists to a large extent of slightly soluble sodium chloride. Finally, in coastal areas, sea water is a significant source of NaCl.

Solubility behavior[edit]

Figure1: Solubility of Na2SO4 in water, Graph: M. Steiger

The structures of both thenardite and mirabilite belong to the group of easily soluble salts and therefore they are easily mobilized (see table hygroscopicity of the salts and ERH). 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[edit]

Figure2:Deliquescence of Na2SO4, Graph: M. Steiger

The temperature effect on the deliquescence points of thenardite and mirabilite is shown below. The striking features here are the opposite curve transitions. 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 1 - 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:

Figure 3: Deliquescence points of pure salts thenardite and mirabilite [Arnold.etal:1991]Title: Monitoring Wall Paintings Affected by soluble Salts
Author: Arnold, Andreas; Zehnder, Konrad

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 2: 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[edit]

When crystallizing from an aqueous solution, the crystallization pressure of thenardite lies in the 29.2-34.5 N/mm² range. These values are higher that those calculated for other building-damaging salts [Winkler:1975]Title: Stone: Properties, Durability in Man´s Environment
Author: Winkler, Erhard M.
.

Hydration behavior[edit]

Error creating thumbnail:

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.

The importance of the heptahydrate in the damage process[edit]

Hydration pressure[edit]

The hydration pressure, which occurs at the transition from thenardite to mirabilite, is highly dependent on the existing relative humidity and temperature conditions. This correlation is shown in table 3 as follows:

Table 3: Hydration pressure thenardite-mirabilite nach [Winkler.etal:1970]**Title:** *Saltburst by Hydration Pressure in Architectural Stone in Urban Atmosphere*
**Author:** *Winkler, Erhard M.; Wilhelm, E.J.*
RH %	20.0 °C	25.0 °C	30.0 °C
100	48.9 N/mm²	40.5 N/mm²	28.9 N/mm²
95,0	41.3 N/mm²	32.7 N/mm²	23.3 N/mm²
90,0	33.5 N/mm²	24.9 N/mm²	13.7 N/mm²
85,0	25.5 N/mm²	16.0 N/mm²	5.1 N/mm²
80,0	16.4 N/mm²	7.8 N/mm²	0.0
75.0	6.7 N/mm²	0.0	-

The change in volume, taking place during phase transition, is to be specified at approx. 320%. [Sperling.etal:1980]Title: Salt Weathering on Arid Environment, I. Theoretical ConsiderationsII. Laboratory Studies
Author: Sperling, C.H.B.and Cooke, R.U.
.

Analytical detection[edit]

Microscopy
[edit]

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 of crystallization 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 become thenardite. This process can be clearly understood and reproduced using a microscopic, when the process of re-crystallization is observed. Mirabillite does not show any characteristic abnormal interference colors. During the moisture loss and the formation of thenardite the abnormal interference phenomena 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/sub>- 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
Pikromerit K₂Mg(SO₄)₂ • 6H₂0	no abnormal interference colors/ inclined extinction
Bloedit Na₂Mg(SO₄)₂ • 6H₂0	all indices >1.48 / no abnormal interference colors/ inclined extinction / optically negative orientation.
Glaserit 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.