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	Sodiumchloride Dihydrate/Hydrohalite (NaCl•2H2O)
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
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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 salt content of sea water is around 2.7 M.%.

Information on the origins and formation of halite 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 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

The commonly occurring halite found in northern Germany has a solubility of 358 g/l (20°C) and thus belongs to the group of very soluble and, therefore, easily mobilized salts. 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
and [DAns:1933]Title: Die Lösungsgleichgewichte der Systeme der Salze ozeanischer Salzablagerungen
Author: d'Ans, J.

Temperature	10°C	20°C	40°C
Solubility [g/l]	356,5	358,8	364,2

Hygroscopicity

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

Halite’s deliquescence humidity of approx. 75% RH is often crossed in the climate of northern Europe.

Moisture sorption:
Theoretically 1g NaCl can absorb 4,3g moisture. 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

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.
(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

Under normal conditions only the unhydrated form of halite exists. Only at a temperature of below 0.15 °C does a saturated water-based sodium chloride solution result in the precipitation of a deposit of dihydrate hydrohalite.

Microscopy

Laboratory analysis

Sodium chloride crystals can be reliably identified on the basis of 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:

Together with potassium chloride, sodium chloride is one of the few salts belonging to the cubic crystal system which cause damage to masonry. Because of its isotropic internal structure it does not display birefringence.

The classification of the refractive index occurs by immersion method in 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. When viewed between crossed polars, the crystals are not visible but appear (independent of orientation) extinguished.

Differentiation of halite from similar salts:

The group of isotropic salts causing masonry damage consists of halite, sylvite and fluorite. All these phases can be differentiated easily.

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

In situ

• 

  Halite crust on a brick surface

• 

  Halite whisker

Under the polarizing microscope

• 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 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

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

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