Calcite

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

back to Carbonate

Calcite
Mineralogical name	Calcite
Chemical name	Calcium carbonate
Trivial name	Calc spar
Chemical formula	CaCO3
Other forms	CaCO3•H2O CaCO3•6H2O
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
Twinning
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
Pleochroism
Dispersion
Used Literature
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Occurrence of calcite

Calcium carbonate occurs as a natural mineral in three polymorphs, calcite, aragonite, and vaterite. The last two are metastable, vaterite, being the less stable polymorph wever an these two modifications, the unstable modification vaterite can be detected in bone and gallstones, but transforms into calcite at low temperatures and into aragonite at higher temperatures (approx. 60ºC). Aragonite is formed in biological processes, for example egg-shells 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 concerning the damage potential and weathering activity of calcite

Solubility

Calcite is hardly soluble in water. The influence of temperature on solubility is low. However, if water contains CO₂, the solubility increases considerably.

Table 1: Calcite: Solubility in dependance of temperature [Data from H. J. Schwarz 1996 and Stark/Stürmer 1993] [Stark.etal:1996]Title: Bauschädliche Salze
Author: Stark, Jochen; Stürmer, Sylvia

18°C	25°C
0,015g/l	0,014 g/l

The system CaCO₃– H₂O:
Both known hydrate stages of calcium carbonate only exist under special circumstances and dehydrate under normal ambient air-conditions.

Microscopy

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

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

Polarized light microscopic examination:

Calcite crystals originating from lime mortar, plaster or paints are usually very fragmented, and morphological characteristics are rarely clearly identifiable with light microscopy. In samples calcite rather appears as a kind of matrix of fine, rounded particles, in which developed tabular rhombohedra, a characteristic for calcite, rarely appear.

If calcite sample materials stem from another source than calc-spar, e.g. chalk or muscles, then significantly larger equidimensional particles can appear. Strongly elongated particles, laths, needles or crystal aggregates do not appear.

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 n_O – 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 n_D = 1,658.

Possible mistakes:

Calcite is clearly identified, if the following characteristics are recorded:

• calcite is slightly soluble in water
• acid sensitive
• 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 n_o-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)

• 

  Calcite crystals on a sample plate

Literatur

[Stark.etal:1996]

Stark, Jochen; Stürmer, Sylvia (1996): Bauschädliche Salze, Bauhaus-Univ. Weimar

A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

the mathematical difference between the largest and smallest refractive index for an anisotropic mineral

Scanning Electron Microscope