Thenardite: Difference between revisions

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|photo                = [[File:HJS-Na2SO4-111703-02-10x.jpg|300px]]
|photo                = [[File:HJS-Na2SO4-111703-02-10x.jpg|300px]]
|mineralogical_Name  = Thenardite
|mineralogical_Name  = Thenardite
|chemical_Name        = Sodium sulphate
|chemical_Name        = Sodium sulfate
|Trivial_Name        = Pyrotechnite
|Trivial_Name        = Pyrotechnite
|chemical_Formula      = Na<sub>2</sub>SO<sub>4</sub>
|chemical_Formula      = Na<sub>2</sub>SO<sub>4</sub>
|Hydratforms          = [[Mirabilite]] (Na<sub>2</sub>SO<sub>4</sub>•10H<sub>2</sub>O)<br>Sodiumsulphate heptahydrate (Na<sub>2</sub>SO<sub>4</sub>•7H<sub>2</sub>O)
|Hydratforms          = [[Mirabilite]] (Na<sub>2</sub>SO<sub>4</sub>•10H<sub>2</sub>O)<br>Sodiumsulfate heptahydrate (Na<sub>2</sub>SO<sub>4</sub>•7H<sub>2</sub>O)
|Crystal_System      = orthorhombic
|Crystal_System      = orthorhombic
|Crystal_Structure    =
|Crystal_Structure    =
|Deliqueszenzhumidity = 81,7% (25°C)
|Deliqueszenzhumidity = 81.7% (25°C)
|Solubility          = 162 g/l
|Solubility          = 162 g/l
|Density              = 2,689 g/cm³
|Density              = 2.689 g/cm³
|MolVolume            = 53,11 cm<sup>3</sup>/mol
|MolVolume            = 53.11 cm<sup>3</sup>/mol
|Molweight            = 142,04 g/mol
|Molweight            = 142.04 g/mol
|Transparency        = transparent to translucent
|Transparency        = transparent to translucent
|Cleavage            = perfect
|Cleavage            = perfect
|Crystal_Habit        =
|Crystal_Habit        =
|Twinning            =
|Twinning            =
|Refractive_Indices  = n<sub>x</sub> = 1,468<br> n<sub>y</sub> = 1,473<br> n<sub>z</sub> = 1,483
|Refractive_Indices  = n<sub>x</sub> = 1.468<br> n<sub>y</sub> = 1.473<br> n<sub>z</sub> = 1.483
|Birefringence        = Δ = 0,015
|Birefringence        = Δ = 0.015
|optical_Orientation  = positive
|optical_Orientation  = positive
|Pleochroism          =
|Pleochroism          =
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Authors: [[user:Hschwarz|Hans-Jürgen Schwarz ]], Michael Steiger, [[user:TMueller|Tim Müller]] <br>
Authors: [[user:Hschwarz|Hans-Jürgen Schwarz ]], Michael Steiger, [[user:TMueller|Tim Müller]] <br>
English translation: [[user:MAngeli|Matthieu Angeli]] <br>
English translation: [[user:MAngeli|Matthieu Angeli]] <br>
back to [[Sulphate]]
back to [[Sulfate]]


= Sodium sulphate and thenardite =
= Sodium sulfate and thenardite =


__TOC__
__TOC__
== Abstract  ==
== Abstract  ==


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


== Introduction ==
== Introduction ==
Line 46: Line 46:
== Occurence ==
== Occurence ==


Both thenardite like [[mirabilite]] appear as natural minerals. Sodium sulphate appears in Nature in mineral waters in the form of double salts, as deposits of former salt lakes. Knowledge of the hydrated sodium sulphate 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.
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  ==
== Information on the origin and formation of thenardite / mirabilite in monuments  ==


With the entry of materials that contain soluble sodium compounds, the mineral system of a monument may create sodium sulphate as salt efflorescence when acting with various sources of sulphate 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 Na<sub>2</sub>O can form 520g of [[Mirabilite]] when in contact with air containing sulphuric 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<sup>+</sup>-ions. Road salt consists to a large part of slightly soluble [[Halite|sodium chloride]]. Finally, in the coastal areas, sea water is also a significant source of [[Halite|NaCl]].
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 Na<sub>2</sub>O 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<sup>+</sup>-ions. Road salt consists to a large part of slightly soluble [[Halite|sodium chloride]]. Finally, in the coastal areas, sea water is also a significant source of [[Halite|NaCl]].


== Solubility behavior  ==
== Solubility behavior  ==
[[file:Na2SO4_sol.jpg|thumb|350px|right|'''Abbildung 1''': Solubility of Na2SO4 in water, Graph: M. Steiger]]<br>
[[file:Na2SO4_sol.jpg|thumb|350px|right|'''Abbildung 1''': Solubility of Na2SO4 in water, Graph: M. Steiger]]<br>


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 sulphate is highly dependent on temperature. For this reason, a rapid drop of temperature is highly likely to yield very high supersaturation and salt crystallization.<br>
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.<br>


== Hygroscopicity  ==
== Hygroscopicity  ==
Line 83: Line 83:
[[file:Deliqueszenz Mirabilit, Thenardit .JPG|thumb|350px|right|'''Abbildung 3''': Deliquescence points of pure salts thenardite and mirabilite <bib id=Arnold.etal:1991/>]]
[[file:Deliqueszenz Mirabilit, Thenardit .JPG|thumb|350px|right|'''Abbildung 3''': Deliquescence points of pure salts thenardite and mirabilite <bib id=Arnold.etal:1991/>]]


The table below shows additional information for estimating the hygroscopicity of sodium sulphate for the sorption behavior of pure salt and the mixture with [[Halite]] at different relative humidities:
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:


<br>  
<br>  
Line 107: Line 107:
== Crystallization pressure  ==
== Crystallization pressure  ==


When crystallizing from an aqueous solution, the crystallization pressure of thenardite lies in the 29,2-34,5 N/mm<sup>2</sup> range. These values are higher that those calculated for other building-damaging salts <bib id=Winkler:1975/>.
When crystallizing from an aqueous solution, the crystallization pressure of thenardite lies in the 29.2-34.5 N/mm<sup>2</sup> range. These values are higher that those calculated for other building-damaging salts <bib id=Winkler:1975/>.


== Hydration behavior  ==
== Hydration behavior  ==
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|-
|-
|bgcolor = "#F0F0F0" align=center| '''rel. Feuchte %'''  
|bgcolor = "#F0F0F0" align=center| '''rel. Feuchte %'''  
|bgcolor = "#F0F0F0" align=center|  '''20,0 °C'''  
|bgcolor = "#F0F0F0" align=center|  '''20.0 °C'''  
|bgcolor = "#F0F0F0" align=center|  '''25,0 °C'''  
|bgcolor = "#F0F0F0" align=center|  '''25.0 °C'''  
|bgcolor = "#F0F0F0" align=center|  '''30,0 °C'''
|bgcolor = "#F0F0F0" align=center|  '''30.0 °C'''
|-
|-
|bgcolor = "#F7F7F7" align=center| '''100'''  
|bgcolor = "#F7F7F7" align=center| '''100'''  
|bgcolor = "#FFFFEO" align="center"| 48,9 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 48.9 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 40,5 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 40.5 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 28,9 N/mm<sup>2</sup>
|bgcolor = "#FFFFEO" align="center"| 28.9 N/mm<sup>2</sup>
|-
|-
|bgcolor = "#F7F7F7" align=center| '''95,0'''  
|bgcolor = "#F7F7F7" align=center| '''95,0'''  
|bgcolor = "#FFFFEO" align="center"| 41,3 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 41.3 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 32,7 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 32.7 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 23,3 N/mm<sup>2</sup>
|bgcolor = "#FFFFEO" align="center"| 23.3 N/mm<sup>2</sup>
|-
|-
|bgcolor = "#F7F7F7" align="center"| '''90,0'''  
|bgcolor = "#F7F7F7" align="center"| '''90,0'''  
|bgcolor = "#FFFFEO" align="center"| 33,5 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 33.5 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 24,9 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 24.9 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 13,7 N/mm<sup>2</sup>
|bgcolor = "#FFFFEO" align="center"| 13.7 N/mm<sup>2</sup>
|-
|-
|bgcolor = "#F7F7F7" align="center"| '''85,0'''  
|bgcolor = "#F7F7F7" align="center"| '''85,0'''  
|bgcolor = "#FFFFEO" align="center"| 25,5 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 25.5 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 16,0 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 16.0 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 5,1 N/mm<sup>2</sup>
|bgcolor = "#FFFFEO" align="center"| 5.1 N/mm<sup>2</sup>
|-
|-
|bgcolor = "#F7F7F7" align="center"| '''80,0'''  
|bgcolor = "#F7F7F7" align="center"| '''80,0'''  
|bgcolor = "#FFFFEO" align="center"| 16,4 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 16.4 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 7,8 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 7.8 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 0,0
|bgcolor = "#FFFFEO" align="center"| 0.0
|-
|-
|bgcolor = "#F7F7F7" align="center"| '''75,0'''  
|bgcolor = "#F7F7F7" align="center"| '''75.0'''  
|bgcolor = "#FFFFEO" align="center"| 6,7 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 6.7 N/mm<sup>2</sup>  
|bgcolor = "#FFFFEO" align="center"| 0,0  
|bgcolor = "#FFFFEO" align="center"| 0.0  
|bgcolor = "#FFFFEO" align="center"|  -
|bgcolor = "#FFFFEO" align="center"|  -
|}
|}
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'''Laboruntersuchung:'''<br>Durch mikroskopische Beobachtungen des Lösungsverhaltens sind die gute Wasserlöslichkeit und Ethanolunlöslichkeit zu verifizieren. Thenardit und [[Mirabilit]] besitzen keine morphologische Charakterisitka, die bei einfachen Rekristallisationsversuchen zur Identifizierung beitragen können. Vielmehr ist eine große Bandbreite unterschiedlichster Erscheinungsformen beobachtbar.<br>  
'''Laboruntersuchung:'''<br>Durch mikroskopische Beobachtungen des Lösungsverhaltens sind die gute Wasserlöslichkeit und Ethanolunlöslichkeit zu verifizieren. Thenardit und [[Mirabilit]] besitzen keine morphologische Charakterisitka, die bei einfachen Rekristallisationsversuchen zur Identifizierung beitragen können. Vielmehr ist eine große Bandbreite unterschiedlichster Erscheinungsformen beobachtbar.<br>  


'''Brechungsindizes:''' &nbsp;&nbsp; n<sub>x</sub> = 1,468; n<sub>y</sub> =1,473; n<sub>z</sub> =1,483<br>'''Doppelbrechung''':&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Δ = 0.015<br>'''Kristallklass'''e:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; orthorhombisch<br>  
'''Brechungsindizes:''' &nbsp;&nbsp; n<sub>x</sub> = 1.468; n<sub>y</sub> =1,473; n<sub>z</sub> =1.483<br>'''Doppelbrechung''':&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Δ = 0.015<br>'''Kristallklass'''e:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; orthorhombisch<br>  


<br>  
<br>  
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|-
|-
|bgcolor = "#F7F7F7"|'''[[Astrakanit|Bloedit]]''' Na<sub>2</sub>Mg(SO<sub>4</sub>)<sub>2</sub> • 6H<sub>2</sub>0  
|bgcolor = "#F7F7F7"|'''[[Astrakanit|Bloedit]]''' Na<sub>2</sub>Mg(SO<sub>4</sub>)<sub>2</sub> • 6H<sub>2</sub>0  
|bgcolor = "#FFFFEO"| alle Indizes &gt;1,48 / keine anormalen Interferenzfarben / schiefe Auslöschung / optisch negativ orientiert.
|bgcolor = "#FFFFEO"| alle Indizes &gt;1.48 / keine anormalen Interferenzfarben / schiefe Auslöschung / optisch negativ orientiert.
|-
|-
|bgcolor = "#F7F7F7"| '''[[Aphthitalit|Glaserit]]''' K<sub>3</sub>Na(SO<sub>4</sub>)<sub>2</sub>  
|bgcolor = "#F7F7F7"| '''[[Aphthitalit|Glaserit]]''' K<sub>3</sub>Na(SO<sub>4</sub>)<sub>2</sub>  
|bgcolor = "#FFFFEO"| alle Indizes &gt;1,48 / keine anormalen Interferenzfarben/schiefe Auslöschung
|bgcolor = "#FFFFEO"| alle Indizes &gt;1.48 / keine anormalen Interferenzfarben/schiefe Auslöschung
|-
|-
|bgcolor = "#F7F7F7"| '''[[Arcanit]]''' K<sub>2</sub>SO<sub>4</sub>  
|bgcolor = "#F7F7F7"| '''[[Arcanit]]''' K<sub>2</sub>SO<sub>4</sub>  
|bgcolor = "#FFFFEO"| alle Indizes &gt;1,48 / keine anormalen Interferenzfarben
|bgcolor = "#FFFFEO"| alle Indizes &gt;1.48 / keine anormalen Interferenzfarben
|-
|-
|bgcolor = "#F7F7F7"| '''[[Magnesiumformiat]]''' Mg(HCO<sub>2</sub>)<sub>2</sub> • 2H<sub>2</sub>O  
|bgcolor = "#F7F7F7"| '''[[Magnesiumformiat]]''' Mg(HCO<sub>2</sub>)<sub>2</sub> • 2H<sub>2</sub>O  

Revision as of 16:48, 20 December 2011

<bibimport/>

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.

Introduction[edit]

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]

Abbildung 1: 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]

Abbildung 2: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).


Tabelle 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:

Abbildung 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:


Tabelle 2: Moist sorption of sodium sulphate in M.% after 56 days of storage [after Vogt/Goretzki 1993]
Lagerungsfeuchte 87% r.F. 81% r.F. 79% r.F.
Na2SO4 79 0 0
Na2SO4+NaCl (1:1 molare Mischung) 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]

Error creating thumbnail:
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.

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

Hydration pressure[edit]

Der Hydratationsdruck, der beim Übergang von Thenardit zu Mirabilit aufgebaut wird, ist stark abhängig von den bestehenden Luftfeuchte- und Temperatur-verhältnissen, was in der nachstehenden Tabelle verdeutlicht ist:


Tabelle 3: Hydratationsdruck Thenardit-Mirabilit nach [Winkler.etal:1970]Title: Saltburst by Hydration Pressure in Architectural Stone in Urban Atmosphere
Author: Winkler, Erhard M.; Wilhelm, E.J.
Link to Google Scholar
rel. Feuchte % 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 -


Die Volumenveränderung, die beim Phasenübergang stattfindet, ist mit ca. 320% anzugeben [Sperling.etal:1980]Title: Salt Weathering on Arid Environment, I. Theoretical ConsiderationsII. Laboratory Studies
Author: Sperling, C.H.B.and Cooke, R.U.
Link to Google Scholar
.

Analytical detection[edit]

Microscopy
[edit]

Laboruntersuchung:
Durch mikroskopische Beobachtungen des Lösungsverhaltens sind die gute Wasserlöslichkeit und Ethanolunlöslichkeit zu verifizieren. Thenardit und Mirabilit besitzen keine morphologische Charakterisitka, die bei einfachen Rekristallisationsversuchen zur Identifizierung beitragen können. Vielmehr ist eine große Bandbreite unterschiedlichster Erscheinungsformen beobachtbar.

Brechungsindizes:    nx = 1.468; ny =1,473; nz =1.483
Doppelbrechung:      Δ = 0.015
Kristallklasse:            orthorhombisch


Polarisationsmikroskopische Investigation:

In Abhängigkeit von den vorliegenden Luftfeuchte- und Temperaturbedingungen verändern Kristalle des Rohprobematerials und des rekristallisierten Präparates ihren Kristallwassergehalt. An trockner Luft (mit r.F. < 80% und Raumtemperatur) verliert Mirabilit sein Kristallwasser und geht in Thenardit über. Dieser Vorgang kann mikroskopisch klar nachvollzogen werden, wenn der Prozess der Rekristallisation beobachtet wird. Mirabilit weist charakteristische anormale Interferenzfarbe auf, im Zuge des Wasserverlustes und Entstehen von Thenardit schwächen sich die anormalen Interferenzphänomene zunehmend ab.

Die Zuweisung der Brechungsindizes von Thenardit erfolgt entsprechend der Immersionsmethode. Aufgrund der niedrigen maximalen Doppelbrechung zeigt Thenardit zumeist graue Interferenzfarben. Die Auslöschung ist parallel oder symmetrisch.


Verwechslungsmöglichkeiten:

Generell ist die Unterscheidung einer bestimmten Anzahl von Sulfaten (die unten aufgelistet sind und wozu Thenardit zählt) ohne mikrochemische Bestimmung der Anionen problematisch, da die Brechungsindizes der Salze dicht beieinander liegen und alle Salze eine niedrige Doppelbrechung aufweisen. Hilfreich ist die Verwendung eines Immersionsmittels mit einem nD-Wert von 1,48. Eine Differenzierung innerhalb dieser Gruppe wird damit möglich. Außerdem können die unten genannten Eigenschaften als Abgrenzungskriterien hinzugezogen werden.

Eindeutig bestimmbar wird Thenardit durch die Möglichkeit, nach Auflösung des Probematerials im Zuge der Rekristallisation das Phänomen anormaler Interferezfarben beobachten zu können, sprich Mirabilit in der hohen Hydratstufe zu identifizieren und somit indirekt Thenardit nachzuweisen.

Tabelle 3: Unterscheidungsmerkmale zu anderen Sulfaten
Salzphase Unterscheidungsmerkmal
Boussingaultit (NH4)2Mg(SO)4 • 6H20 keine anormalen Interferenzfarben / schiefe Auslöschung
Pikromerit K2Mg(SO4)2 • 6H20 keine anormalen Interferenzfarben / schiefe Auslöschung
Bloedit Na2Mg(SO4)2 • 6H20 alle Indizes >1.48 / keine anormalen Interferenzfarben / schiefe Auslöschung / optisch negativ orientiert.
Glaserit K3Na(SO4)2 alle Indizes >1.48 / keine anormalen Interferenzfarben/schiefe Auslöschung
Arcanit K2SO4 alle Indizes >1.48 / keine anormalen Interferenzfarben
Magnesiumformiat Mg(HCO2)2 • 2H2O vergleichsweise hohe Doppelbrechung / keine anormalen Interfernzfarben / schiefe Auslöschung


Betrachtung von Mischsystemen:

Mischsystem Na+– Ca2+– SO4 2-: Der Ausfall von Gips erfolgt im Zuge der Rekristallisation entsprechend der geringeren Löslichkeit desselben zuerst. Der charakteristische nadelige Habitus von einzelnen Gipskristallen wie auch von Aggregaten bleibt bestehen. Der Ausfall von Natriumsulfat erfolgt später, das eigentliche Kristallwachstum vollzieht sich merklich schneller. Die Morphologie ist unspezifisch.

Mischsystem Na+– SO4 2-– Cl-: Der Ausfall der beiden Partikelsorten beginnt etwa zeitgleich. Halit mit charakteristischer Morphologie, Natriumsulfat in extrem variierender Gestalt.


Pictures of salt and salt damage[edit]

Am Objekt[edit]

Under the polarized microscope[edit]


Unter dem Rasterelektronenmikroskop[edit]

Weblinks
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Literatur[edit]

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