Thenardite: Difference between revisions

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Revision as of 14:14, 21 February 2012

Thenardite[1][2]
HJS-Na2SO4-111703-02-10x.jpg
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 nx = 1.468
ny = 1.473
nz = 1.483
Birefringence Δ = 0.015
Optical Orientation positive
Pleochroism
Dispersion
Used Literature
{{{Literature}}}


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

Sodium sulfate and thenardite[edit]

Abstract[edit]

Sodium sulfate and its phases thenardite, mirabilite and the heptahydrate, whose importance in the production of damage has been identified, are described.

Occurence[edit]

Both thenardite like mirabilite appear as natural minerals. Sodium sulfate appears in Nature 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 acting with various sources of sulfate such as for example sulphurous gases or contaminated air. Cement exhibits a high content of sodium ions, as they are allowed by the German Standardization Institute to contain up to 0.5 % of 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 from 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 part of slightly soluble sodium chloride. Finally, in the coastal areas, sea water is also 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 easily mobilizable (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 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
Link to Google Scholar
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
Link to Google Scholar

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 humidities:


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
Link to Google Scholar
]
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.
Link to Google Scholar
.

Hydration behavior[edit]

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Conversion of mirabilite (?) into thenardite

The Na2SO4 – H2O system:

The only stable forms 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 happening 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, as 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]. Because of 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.

Pictures of salt and salt damage[edit]

In the field[edit]

Under the polarizing microscope[edit]


Weblinks
[edit]

Literature[edit]

[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 ()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
[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 (), 567-572Link to Google Scholar
[Winkler:1975] Winkler, Erhard M. (1975): Stone: Properties, Durability in Man´s Environment, Springer Verlag, WienLink to Google Scholar