Halite

<bibimport /> 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	Natriumchlorid Dihydrat/Hydrohalit (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	slightly watersoluble
Crystal Optics
Refractive Indices	n=1.544
Birefringence
Optical Orientation	isotropic
Pleochroism
Dispersion
Used Literature
{{{Literature}}}

Halite origins

Sodium chloride is obtained through mining or derived from the sea or salt lakes and is commonly used for cooking or as road gritting salt.
The salt content of sea water is approx. 2,7 M.%.

Causes of halite formation on monuments

Contact with materials containing soluble sodium-based ingredients can result in the efflorescence of sodium chloride on monuments. A primary example is the high sodium content of cements. 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 caustic cleaners) and especially previously used restoration materials (e.g. water glass) can introduce sodium and chloride ions into monuments. Further common sources of halite are rock salt used in road gritting which predominantly consists of sodium chloride, as well as salt laden seawater in coastal areas.

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.

Image 1: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

Image 3: The system NaCl/H₂O within temperature range of -20°C to 80°C. Graphic: Michael Steiger

Halite’s deliquescence humidity of approx. 75% is often crossed in the climate of northern Europe. Temperature variations hardly affect the deliquescence point of halite which is illustrated below in comparison with potassium nitrate and natrite.

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 [Vogt.etal:1993]Title: Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe
Author: Vogt, R.; Goretzki, Lothar

Relative humidity during storge/salt phase	NaCl
87% RH	153
81% RH	22
79% RH	7

Crystallization pressure

The crystallization of halite from a water-based 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 examinations

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

Examinations under the polarising microscope:

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 polarised light waves are not distorted. When viewed between crossed polars, the crystals are not visible but appear (independent of orientation) extinguished.

Differentiating of halite from similar salts:

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

Table 3: Identification features of other chlorides

Salt phase	Identification features
Sylvine KCl	Refractive index under 1,518.
Fluorite CaF2	Refractive index under 1,518, barely water soluble.

Images of salts and salt damage

In situ

• 

  Halite crust on a brick surface

• 

  Halite whisker

Under the polarising microscope

• Precipitate from watery sample on microscope slide
• 

  Sodium chloride, precipitate from watery sample on microscope slide

• 

  Sodium chloride, precipitate from watery sample on microscope slide

• 

  Sodium chloride, precipitate from watery sample on microscope slide

• 

  Sodium chloride, precipitate from watery sample on microscope slide

• 

  Sodium chloride, precipitate from watery sample on microscope slide

• 

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

• 

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

Literature

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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

Light is called polarised, when the oscillation of the light waves has a certain orientation. The electric field may be oriented in a single direction (linear polarization), or it may rotate as the wave travels (circular or elliptical polarization) This can be achieved by using polarising filters. Natural daylight is not polarized.