Halite

From Saltwiki
Jump to navigation Jump to search

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


Abstract[edit]

Halite origins[edit]

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[edit]

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[edit]

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
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’s deliquescence humidity of approx. 75% 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 [Vogt.etal:1993]Title: Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe
Author: Vogt, R.; Goretzki, Lothar
Link to Google Scholar
Relative humidity during storge/salt phase NaCl
87% RH 153
81% RH 22
79% RH 7


Crystallization pressure[edit]

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.
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 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[4].


Microscopy[edit]

Laboratory examinations[edit]

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:  nD = 1,544
Crystal category:       cubic

Examinations under the polarizing 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 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. 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[edit]

In situ[edit]

Under the polarizing microscope[edit]


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 gelesen 28.07.2010 Cite error: Invalid <ref> tag; name "hydrohalit" defined multiple times with different content

Literature[edit]

[Filter missing]