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Authors: Hans-Jürgen Schwarz, Nils Mainusch
English version by Sandra Leithäuser

back to Carbonate

Mineralogical name Thermonatrite
Chemical name Sodium carbonate hydrate
Trivial name Sodium carbonate
Chemical formula Na2CO3•H2O
Other forms Natrite (Na2CO3)
Sodium carbonate heptahydrate (Na2CO3•7H2O)
Sodium carbonate decahydrate (Na2CO3•10H2O)
Crystal system orthorhombic
Crystal structure
Deliquescence humidity 20°C 71% (35°C)
Solubility (g/l) at 20°C 330 g/l
Density (g/cm³) 2.250 g/cm3
Molar volume 55.11 cm3/mol
Molar weight 124.00 g/mol
Transparency transparent
Cleavage poor to indistinct
Crystal habit
Phase transition
Chemical behavior
Comments alkaline in aqueous solution
pH about 12
Crystal Optics
Refractive Indices nx = 1.420
ny= 1.509
nz = 1.525
Birefringence Δ = 0.105
Optical Orientation biaxial negative
Used Literature
[Lide:1995]Title: CRC Handbook of Chemistry and Physics
Author: Lide D.R.
Link to Google Scholar


General information on transition reaction, hydrate pressures and occurrence are available on Sodium carbonate – natrite.


Estimating the hygroscopicity of sodium carbonates in situ is difficult, because the sorption point and saturation values vary greatly as a function of the local environment (i.e., foreign ions, hydrate stages present, temperature conditions). In the following section, values for the deliquescence humidity of thermonatrite and natrite as a function of the ambient air temperature are listed (see also table Gleichgewichtsfeuchte in Abhängigkeit von Temperatur):

Table 1: Deliquescence humidity of thermonatrite
71.0% RH
Table 2: Deliquescence humidity of natrite [Arnold/Zehnder, 1991]
15°C 20°C 25°C 30°C
96.5% RH 97.9% RH 88.2% RH 83.2% RH

Analytical detection[edit]

The identification of thermonatrite and natrite can be carried out in situ by dissolution in water and measuring the resultant pH: According to Bläuer/ Böhm, natrite is the only efflorescent salt known to date, that is alkaline (pH clearly greater than 8) and has the property to dissolve in its chemically bound water at slight warming. The solubility in water is good. Thermonatrite is not soluble in anhydrous ethanol.


Laboratory investigation:

Good solubility in water and low solubility in ethanol can be checked using microscopy. Thermonatrite and natrite
have a marked tendency towards the formation of acicular or dendritic crystal shapes when re-crystallizing. If gypsum is present in the sample material, a rapid precipitation of calcium carbonate takes place, which becomes apparent through a whitish precipitate.

Refractive indices:  nx = 1.420;  ny = 1.509; nz = 1,525;
Birefringence:    Δ  = max. 0,105
Crystal classe:          orthorhombic

Examination with the polarization microscope:

As a function of the prevailing relative humidity and temperature conditions, the crystals of the starting sample material and the re-crystallized specimen change in the amount of chemically bound water. In dry air (< ca. 60% RH) predominantly thermonatrite will be present after a short time.
The assignment of the refractive indices of thermonatrite is carried out according to the immersion method. Single particles that have been embedded in standard immersion oil with a refractive index of nD = 1,518, show strong relief changes on rotation under polarized transmitted light [Blaeuer-Boehm:1994]Title: Salzuntersuchungen an Baudenkmälern
Author: Bläuer-Böhm, Christine
Link to Google Scholar
, a.a.O., S. 86 ff. Due to high birefringence, thermonatrite crystals have lively interference colors. These are a clear differentiating factor from natrite, having a much lower maximum birefringence.
Thermonatrite belongs to the orthorhombic crystal order. This association is connected with the occurrence of parallel and/or symmetrical extinction. Large crystal needles are usually completely extinct.

Please check:

Thermonatrite is clearly assignable, if the following examination criteria have been clarified.

  • high pH-value
  • good solubility in water
  • characteristic habit
  • all refractive indices below nD=1,53
  • high birefringence
  • parallel/symmetrical extinction
Table 3: Salt phases with similar characteristics and the possibility to be confused
Salt phase Distinguishing features to thermonatrite
Nesquehonite MgCO3  3H2O; slightly water soluble/ oblique extinction
Lansfordite MgCO3 • 5H2O slightly water soluble / oblique extinction / low birefringence
Trona Na3H(CO3)2 • 2H2O mainly observed index > 1,53 / oblique extinction
Potash K2CO3 mainly observed index > 1,53 / oblique extinction / strongly hygroscopic



[Blaeuer-Boehm:1994]Bläuer-Böhm, Christine (1994): Salzuntersuchungen an Baudenkmälern. In: Zeitschrift für Kunsttechnologie und Konservierung, 8 (1), 86-103Link to Google Scholar
[Lide:1995]Lide D.R. (eds.) Lide D.R. (1995): CRC Handbook of Chemistry and Physics, CRC PressLink to Google Scholar