Halite

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


Halite[1][2][3]
NaCl 27.4.2006-10x.JPG
Mineralogical name Halite
Chemical name Sodium chloride
Trivial name Common Salt, Rock Salt
Chemical formula NaCl
Other forms Sodiumchloride Dihydrate/Hydrohalite (NaCl•2H2O)[4]
Crystal system cubic
Crystal structure
Deliquescence humidity 20°C 75.7% (10°C), 75.3% (25°C)
Solubility (g/l) at 20°C 358 g/l
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 n=1.544
Birefringence
Optical Orientation isotropic
Pleochroism
Dispersion
Used Literature
{{{Literature}}}


Abstract[edit]

Occurrence[edit]

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 salt content of sea water is around 2.7 M.%.

Information on the origins and formation of halite on monuments[edit]

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 react to produce NaCl 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) and especially previously used restoration materials (e.g., water glass) or even 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[edit]

The commonly occurring halite has a solubility of 358 g/l (20°C) and can be considered a very soluble and, therefore, easily mobilized salt. Its solubility changes comparatively little within a temperature range of 10 -30°C.

Figure1:Phase diagram of halite. Graphic: Michael Steiger


Table 1: Solubility of halite in relation to temperature [according to [Stark.etal:1996]Title: Bauschädliche Salze
Author: Stark, Jochen; Stürmer, Sylvia
Link to Google Scholar and [DAns:1933]Title: Die Lösungsgleichgewichte der Systeme der Salze ozeanischer Salzablagerungen
Author: d'Ans, J.
Link to Google Scholar
Temperature 10°C 20°C 40°C
Solubility [g/l] 356,5 358,8 364,2


Hygroscopicity[edit]

Figure 2: The system NaCl/H2O within temperature range of -20°C to 80°C. Graphic: Michael Steiger

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



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:


Tabelle 2: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


Crystallization pressure[edit]

The crystallization of halite from an aqueous solution results in a crystallization pressure of 55,4-65,4 N/mm2 [Winkler:1975]Title: Stone: Properties, Durability in Man´s Environment
Author: Winkler, Erhard M.
Link to Google Scholar (for comparison, the crystallization pressure of different salts can range from 7,2-65,4 N/mm2). These values need to be considered in conjunction with temperature and concentration and can, therefore, only act as indicators of damage potential in relation to salt crystallization pressure. In comparison to other salts, the crystallization pressure of halite is extremely high.

Hydration behavior[edit]

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.


Microscopy[edit]

Laboratory analysis[edit]

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 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 are "estinguished".


Differentiation of halite from similar salts:

The three above mentioned isotropic salts can be easily differentiated.


Table 3: Identification features of other chlorides
Salt phase Identification features
Sylvine KCl Refractive index below1,518.
Fluorite CaF2 Refractive index below 1,518, barely water soluble.


Images of salts and salt damage[edit]

In situ[edit]

  • Halite crust on a brick surface

    Halite crust on a brick surface

  • Halite whisker

    Halite whisker

Under the polarizing microscope[edit]

  • Precipitate from watery sample 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, 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, 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

  • 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


Weblinks[edit]

  1. ↑ http://webmineral.com/data/Halite.shtml accessed 28.07.2010
  2. ↑ http://www.mindat.org/min-1804.html accessed 28.07.2010
  3. ↑ http://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Halit accessed 28.07.2010
  4. ↑ 4.0 4.1 http://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Hydrohalit accessed 28.07.2010

Literature[edit]

[Dana:1951]Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & SonsLink to Google Scholar
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[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.2007.04.020Link to Google Scholar
[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.Link 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:1975] Winkler, Erhard M. (1975): Stone: Properties, Durability in Man´s Environment, Springer Verlag, WienLink to Google Scholar

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A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

crystallization of a dissolved salt

Water glass is the common name for a liquid solution of alkaline silicates, also known as Sodium silicate Na2SiO3 or liquid glass.

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

X-ray diffraction

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