Calcite
Authors: Hans-Jürgen Schwarz , Nils Mainusch
English Translation by Sandra Leithäuser
back to Carbonate
Calcite[1][2] | |
![]() | |
Mineralogical name | Calcite |
Chemical name | Calcium carbonate |
Trivial name | Iceland spar, calcite 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 | |
{{{Literature}}} |
Occurrence of calcite
[edit]
Calcium carbonate occurs as a natural mineral in three polymorphs, calcite, aragonite, and vaterite, the last two being metastable. Vaterite, the less 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 concerning the damage potential and weathering activity of calcite[edit]
Solubility[edit]
Calcite is barely soluble in water. The influence of temperature on solubility is low. However, if water contains CO2, the solubility increases considerably.
18°C | 25°C |
0,015g/l | 0,014 g/l |
The system CaCO3– H2O:
The two known hydrate stages of calcium carbonate, mono and hexa-hydrate, only exist under special circumstances and dehydrate under normal ambient air-conditions.
Microscopy
[edit]
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 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 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.
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 no-refractive index is effective
- only parallel and symmetrical extinction is observable
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[edit]
Under the scanning electron microscope (SEM)[edit]
Weblinks
[edit]
- ↑ http://webmineral.com/data/Calcite.shtml viewed on 09/06/2011
- ↑ http://www.mindat.org/min-859.html viewed on 09/06/2011
Literatur[edit]
[Stark.etal:1996] | Stark, Jochen; Stürmer, Sylvia (1996): Bauschädliche Salze, Bauhaus-Univ. Weimar | ![]() |