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Authors: Hans-Jürgen Schwarz , Nils Mainusch, A. Elena Charola
English Translation by Sandra Leithäuser

back to Carbonate

Calcit Kristalle Musterplatte.jpg
Mineralogical name Calcite
Chemical name Calcium carbonate
Trivial name Iceland spar, calcite spar
Chemical formula CaCO3
Other forms CaCO3•H2O
Crystal system trigonal
Crystal structure
Deliquescence humidity 20°C
Solubility (g/l) at 20°C 0.014 g/l (25°C)
Density (g/cm³) 2.72 g/cm3
Molar volume
Molar weight 100.09 g/mol
Transparency transparent to opaque
Cleavage perfect
Crystal habit
Phase transition
Chemical behavior
Comments well soluble in 2M HCl
Crystal Optics
Refractive Indices no = 1.658
ne = 1.486
Birefringence Δ = 0.172
Optical Orientation negative
Used Literature

Occurrence of calcite

Calcium carbonate occurs as a natural mineral in three polymorphs, calcite, aragonite, and vaterite, the last two being metastable. Vaterite, the least stable one transforms into calcite at low temperatures and into aragonite at higher temperatures (approx. 60ºC). Vaterite can be detected in bone and gallstones, while aragonite is formed in biological processes, for example, egg- or mollusk-shells. Iceland spar, calcite spar or calc-spar is a very pure, transparent variety of calcium carbonate which shows a strong birefringence, was discovered and scientifically explained by C. Huygens in the 17th century.

Information on calcite relevant to its formation during weathering


The system CaCO3– H2O:
The two known hydrated forms of calcium carbonate, mono and hexa-hydrate, only exist under special circumstances and dehydrate under normal ambient air-conditions to calcite.

Calcite is barely soluble in water. The influence of temperature on solubility is low.

Table 1: Calcite: Solubility in dependance of temperature [Data from H. J. Schwarz 1996 and Stark/Stürmer 1993] [Stark.etal:1996]Author: Stark, Jochen; Stürmer, Sylvia
Note: 51 S.
Publisher: Bauhaus-Univ. Weimar
Series: Schriften der Bauhaus-Universität Weimar, Bauhaus-Universität (Weimar).
Title: Bauschädliche Salze
Volume: 103
Year: 1996
Link to Google Scholar
18°C 25°C
0,015g/l 0,014 g/l

However, if water contains CO2, the solubility of calcite increases considerably because of the formation of carbonic acid that will react forming soluble calcium bicarbonate, Ca(HCO3)2. This is a slow reaction but over centuries of water flowing through limestone, some of it is slowly carried away and deposited when the water evaporates, as for example in the limestone caves where stalactites and stalactites form. When a drop of saturated solution of calcite is exposed to air, the water evaporates and a film or calcite is formed around the drop. As more of the saturated solutions filters out, it will cover the film of calcite allow the formation of a thicker film, while a new water surface is exposed and a new film created resulting in concentric tubes as shown in the SEM figures below. Larger crystals will develop towards the evaporating surface linking the concentric films to each other. Eventually, the whole stalactite may fill with a crystalline growth of calcite.

A similar phenomenon can be observed in the formation of small stalactites, the most common ones being those that form on concrete bridges. In this case, the mechanism is different: water percolating through concrete will dissolve the unreacted calcium hydroxide, Ca(OH)2, in it. This is a fast reaction compared to the slow dissolution of calcite. When the alkaline solution reaches the exterior, it will react with the CO2 from the air, another very fast reaction. This explains why the stalactite from concrete grow so fast, and why, except in special cases, they are small.

Deterioration Potential

The low solubility of CaCO3 precludes it from being a "deteriorating" salt. However, because of its slow dissolution as mentioned above, in some cases it may result in the formation of hard calcite concretions, for example, in marble fountains with water sprays that enhance water evaporation so that concretions form over the edge of the basin or any sculptures washed over by the water. Similar disfigurement can be seen in concrete structures either in stains or the formation of stalatites at the edge of cracks or fissures where more water can percolate. Even along the edges of joints filled with Portland cement, particularly if these got wet before they set completely.


Laboratory examination:
Calcite is only slightly soluble in water, therefore recrystallization cannot be initiated.

Refractive indices:  n0 = 1.658,  nE = 1.486
Birefringence:    Δ  = max. 0,172
Crystal class:          trigonal

Polarized light microscopic examination:

Calcite crystals growing during the setting of lime mortars, renders or in lime-paints are very poorly formed and seldom can be identified by their morphology using light microscopy. In these samples, calcite appears as a matrix of fine, rounded particles, in which developed tabular rhombohedra, the characteristic shape for calcite, rarely appear.

If calcite sample materials stem from another source than calcite-spar, e.g., chalk or mussels, then significantly larger equidimensional particles can appear. Strongly elongated particles, laths, needles or crystal aggregates are rarely found.

The very high birefringence leads to interference phenomena, even with very small particles. In addition the strong relief changes are very noticeable, especially when using an immersion medium with a refractive index near nO – value of 1,658. Since calcite is optically uni-axial, independent of the orientation and the viewing direction, in one of the two normal positions it always displays a fixed refractive index value, being effective at nD = 1,658.

Positive identification:

Calcite can clearly be identified, if the following characteristics are observed:

  • Low solubility in water
  • Sensitivity to acids
  • Very strong birefringence
  • High relief in most immersion media
  • When using an embedding medium with a refractive index of 1,662 it reveals a strong relief change, wherein a crystal nearly disappears in the normal position, when no-refractive index is effective
  • Only parallel and symmetrical extinction is observable

Table 2: Salt phases, with chemical and optical properties similar to calcite
Salt phase Distinguishing features to calcite
Dolomite MgCa(CO3)2  Both indices above 1.5
Magnesite MgCO3 Both indices above 1.5, no- Index with 1.7-1.719 significantly higher than calcite.

Calcite in Pictures

Under the scanning electron microscope (SEM)



[Stark.etal:1996]%editor% Stark, Jochen; Stürmer, Sylvia (1996): Bauschädliche Salze, %edition%, Bauhaus-Univ. Weimar, %address%, %pages%, %url%, %doi%Link to Google Scholar

More Literature in the bibliography:

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