Halite

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

Halite
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
Birefringence
Optical Orientation	isotropic
Pleochroism
Dispersion
Used Literature
[Steiger.etal:2014]Title: Weathering and Deterioration Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja [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. [Dana:1951]Title: Dana's System of Mineralogy Author: Dana J.D.

Occurrence

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
.

Temperature	10°C	20°C	30°C	40°C
Solubility [mol/kg]	6.11	6.13	6.17	6.22

Hygroscopicity

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

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

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

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 []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 will only precipitate out at temperatures below 0.15°C.

Microscopy

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:  n_D = 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 n_D =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

• 

  Halite crust on a brick surface

• 

  Halite whisker

Under the polarizing microscope

• Precipitated from the evaporation of an aqueous solution of the sample on microscope slides
• 

  Sodium chloride, precipitate from an aqueous solution on microscope slide

• 

  Sodium chloride, precipitate from an aqueous solution on microscope slide

• 

  Sodium chloride, precipitate from an aqueous solution on microscope slide

• 

  Sodium chloride, precipitate from an aqueous solution on microscope slide

• 

  Sodium chloride, precipitate from an aqueous solution on microscope slide

• 

  Sodium chloride and calcium sulphate in real sample, precipitate from an aqueous solution on microscopic slide

• 

  Sodium chloride and calcium sulphate in real sample, precipitate from an aqueous solution on microscopic slide

Scanning Electron Microscope photomicrographs

• 

  NaCl whisker growing on brick from a church in Venice. The sample had been kept in a film cartridge so it dried out very slowly. A nice cubic crystal can be seen at the tip of the whisker that had been on the surface of the brick as efflorescence when the sample was taken.

Literature

[Dana:1951]	Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & Sons
[Robie.etal:1978]	Robie R.A., Hemingway B.S.; Fisher J.A. (1978): Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar pressure and higher temperatures. In: U.S. Geol. Surv. Bull, 1452 ()
[Steiger]	The entry doesn't exist yet.
[Steiger.etal:2008c]	Steiger, Michael; Kiekbusch, Jana; Nicolai, Andreas (2008): An improved model incorporating Pitzer’s equations for calculation of thermodynamic properties of pore solutions implemented into an efficient program code. In: Construction and Building Materials, 22 (8), 1841-1850, 10.1016/j.conbuildmat.2007.04.020
[Steiger.etal:2014]	Steiger, Michael; Charola A. Elena; Sterflinger, Katja (2014): Weathering and Deterioration. In: Siegesmund S.; Snethlage R. (eds.): Stone in Architecture, Springer Verlag Berlin Heidelberg, 223-316, 10.1007/978-3-642-45155-3_4.
[Vogt.etal:1993]	Vogt, R.; Goretzki, Lothar (1993): Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe, unveröffentlichter Bericht.
[Winkler:1975]	Winkler, Erhard M. (1975): Stone: Properties, Durability in Man´s Environment, Springer Verlag, Wien

More Literature

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crystallization of a dissolved salt

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concentration given as amount of substance (mol) of the dissolved substance per kilogram solvent; unit: mol•kg-1.

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