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

Halite[1][2][3] [4]
NaCl 27.4.2006-10x.JPG
Mineralogical name Halite
Chemical name Sodium chloride
Trivial name Common Salt, Rock Salt
Chemical formula NaCl
Other forms NaCl•2H2O (Hydrohalite)
Crystal system cubic
Crystal structure
Deliquescence humidity 20°C 75.4%
Solubility (g/l) at 20°C 6.135 mol/kg
Density (g/cm³) 2.163 g/cm3
Molar volume 27.02 cm3/mol
Molar weight 58.44 g/mol
Transparency transparent to translucent
Cleavage perfect
Crystal habit cubic crystal, granular, massive aggregates
Twinning none
Phase transition
Chemical behavior
Comments water soluble
Crystal Optics
Refractive Indices nD=1.5443
Optical Orientation isotropic
Used Literature
[Steiger.etal:2014]Title: Weathering and Deterioration
Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja
Link to Google Scholar
[Robie.etal:1978]Title: Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar pressure and higher temperatures
Author: Robie R.A., Hemingway B.S.; Fisher J.A.
Link to Google Scholar
[Dana:1951]Title: Dana's System of Mineralogy
Author: Dana J.D.
Link to Google Scholar



Sodium chloride is obtained through mining or derived from the sea or salt lakes and is commonly used for cooking or as deicing salt for roads.
The sodium chloride content of sea water is around 2.7 M.%.

Origin of the halite found on monuments

Sodium chloride can enter buildings or monuments when these are in contact with materials containing this salt or even other salts containing either sodium or chloride, that might combine to produce NaCl in or efflorescence on them. Contamination with sodium and chloride ions can also occur through contact with salt laden ground or surface water. A range of cleaning materials (e.g., acidic and alkaline cleaners or combination of them), or previously used restoration materials (e.g., water glass) as well as Portland cement, can introduce sodium and chloride ions into monuments. Further common and important sources are deicing salts and maritime environments where the air and fogs may contain a significant amount of sodium chloride in suspension or dissolved in droplets.

Solubility behavior

The commonly occurring halite has a solubility of 6.13 mol/kg (20°C) and can be considered as a very soluble and, therefore, easily mobilized salt. Its solubility changes not significantly within a temperature range of 10 -30°C.

Figure 1: Solubility of sodium chloride in water. The molality m [n(NaCl)•kg(H2O)-1] is plotted versus the temperature.

Table 1: Solubility of halite at different round temperatures [according to [Steiger.etal:2008c]Title: An improved model incorporating Pitzer’s equations for calculation of thermodynamic properties of pore solutions implemented into an efficient program code
Author: Steiger, Michael; Kiekbusch, Jana; Nicolai, Andreas
Link to Google Scholar
Temperature 10°C 20°C 30°C 40°C
Solubility [mol/kg] 6.11 6.13 6.17 6.22


Figure 2: Deliquescence behaviour of sodium chloride. The water activity aw is plotted versus the temperature.

Halite has a deliquescence humidity of about 75% RH, therefore it tends to pick up moisture easily in most temperate climates.

Table 2: Deliquescence humidities of sodium chloride at different round temperatures [Steiger etal: 2014]Title: Weathering and Deterioration
Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja
Link to Google Scholar
0°C 10°C 20°C 30°C 40°C 50°C
75.9%r.h. 75.6%r.h. 75.4%r.h. 75.2%r.h. 75.2%r.h. 74.8%r.h.

Moisture sorption:
Theoretically 1g NaCl can take up 4.3g of moisture, i.e., water vapor. The moisture sorption during varying relative humidity levels is:

Table 3:Moisture sorption in M% after 56 days according to [''Vogt.etal:1993'']The entry doesn't exist yet.
Relative humidity during storge/salt phase NaCl
87% RH 153
81% RH 22
79% RH 7

Hydration behavior

Under normal environmental conditions only halite will crystallize out of a saturated solution. The hydrated form, dihydrate hydrohalite[4] will only precipitate out at temperatures below 0.15°C.


Laboratory analysis

Sodium chloride crystals can be reliably identified on the basis of their morphological features. Individual particles usually form cubic or octahedral shapes and, therefore, clearly display right angles in their crystal construction.

Refractive index:  nD = 1.544
Crystal category:       cubic

Examination by polarized microscopy:

There are few salts belonging to the cubic crystal system which can be found in masonry, i.e., sodium chloride (halite), potassium chloride (sylvite) and calcium fluoride (flourite). Only the two first salts are highly soluble and therefore they are the ones that can cause damage to the masonry. Because of its isotropic internal structure these salts do not display birefringence.

The refractive index can be measured with the immersion method by using a standard oil with a refractive index of nD =1.518. Halite crystals display the same optical density in every direction so that the speed and orientation of linear polarized light waves are not distorted, therefore, when viewed between crossed polars, the crystals are not visible, i.e., they show "extinction".

Differentiation of halite from similar salts:

The three above mentioned isotropic salts can be easily differentiated.

Table 4: Identification features of other chlorides
Salt phase Identification features
Sylvine KCl Refractive index below 1,518.
Fluorite CaF2 Refractive index below 1,518, barely water soluble.

Images of salts and salt damage

In situ

Under the polarizing microscope

Scanning Electron Microscope photomicrographs



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[Steiger]The entry doesn't exist yet.
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