C
CALCITE
Sue J. McLaren
University of Leicester, Leicester, UK
Definition
The minerals calcite, aragonite, and vaterite are naturally
occurring polymorphic forms of calcium carbonate
(CaCO 3 ). At normal Earth surface pressures and temperatures, low magnesian calcites are the only stable CaCO 3
phase. Calcite has a trigonal crystal system, perfect cleavage in three directions, conchoidal fracture, and a hardness
of three on Moh’s scale. Calcite often has a massive habit,
although dog-tooth spar (scalenohedrons) and rhombohedral forms are also common. The mineral’s color is commonly white or colorless, but may be yellow, pink, pale
blue, or even red in hand specimen; in thin section, it is
colorless. Most calcite is relatively pure, but impurities
include ferrous iron, magnesium, manganese, and strontium. Calcites with less than four mole percent MgCO 3
are termed low magnesian calcites, while those containing
4–30 mole percent are known as high magnesian calcites.
Calcite is readily soluble and effervesces vigorously in
cold dilute hydrochloric acid and it is stained red by the
organic dye Alizarin Red S.
Calcite is generally a major constituent of carbonate
rocks and is a commonly found earth mineral. Limestones
must be made of more than 50% carbonate minerals
(Tucker and Wright, 1990). Calcite tends to precipitate
from freshwater where Mg:Ca ratios are generally low
(<1). Carbonate minerals are highly soluble in waters rich
in carbon dioxide, which results in most limestones being
susceptible to diagenesis. Diagenesis leads to the alteration
of a carbonate deposit by various geochemical processes
that result in changes in mineralogy, texture, and fabric.
Over time, skeletal and non-skeletal carbonate grains and
carbonate matrix alter to low magnesian calcite and calcite
cements are precipitated in primary and secondary pore
spaces. In the process, aragonite is often partially or totally
dissolved. During transformation from high to low magnesian calcite, the initial structure is kept but Mg
2þ is leached
from the crystal. The crystal sizes of calcite cements that
form can be grouped into micrite (<4 microns), microspar
(4–10 microns), and spar (>10 microns).
Typical freshwater low magnesian calcite-rich deposits
include calcretes, tufa, travertine, cave speleothems, and
eolianites. High magnesian calcites are common components of beach rocks, shallow marine sediments, and the
skeletons of marine organisms. According to Mackenzie
et al. (1983), there is an increase in the content of magnesium within skeletal high magnesian calcite from polar
regions towards the tropics, which is related to changes
in temperature and seawater saturation of carbonate ions.
Bibliography
Mackenzie, F. T., Bischoff, W. D., Bishop, F. C., Loijens, M.,
Schoonmaker, J., and Wollast, R., 1983. Magnesian calcites:
low temperature occurrence, solubility and solid solution behaviour. In Reeder, R. J. (ed.), Carbonates: Mineralogy and Chemistry. Reviews in Mineralogy. Washington, DC: Mineralogical
Society of America, Vol. 11, pp. 97–144.
Tucker, M. E., and Wright, V. P. 1990. Carbonate Sedimentology.
Oxford: Blackwell, pp. 482.
Cross-references
Aragonite
Beach Rock
Calcrete/Caliche
Diagenesis
Eolianite
Micrite
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011
CALCITE
Sue J. McLaren
University of Leicester, Leicester, UK
Definition
The minerals calcite, aragonite, and vaterite are naturally
occurring polymorphic forms of calcium carbonate
(CaCO 3 ). At normal Earth surface pressures and temperatures, low magnesian calcites are the only stable CaCO 3
phase. Calcite has a trigonal crystal system, perfect cleavage in three directions, conchoidal fracture, and a hardness
of three on Moh’s scale. Calcite often has a massive habit,
although dog-tooth spar (scalenohedrons) and rhombohedral forms are also common. The mineral’s color is commonly white or colorless, but may be yellow, pink, pale
blue, or even red in hand specimen; in thin section, it is
colorless. Most calcite is relatively pure, but impurities
include ferrous iron, magnesium, manganese, and strontium. Calcites with less than four mole percent MgCO 3
are termed low magnesian calcites, while those containing
4–30 mole percent are known as high magnesian calcites.
Calcite is readily soluble and effervesces vigorously in
cold dilute hydrochloric acid and it is stained red by the
organic dye Alizarin Red S.
Calcite is generally a major constituent of carbonate
rocks and is a commonly found earth mineral. Limestones
must be made of more than 50% carbonate minerals
(Tucker and Wright, 1990). Calcite tends to precipitate
from freshwater where Mg:Ca ratios are generally low
(<1). Carbonate minerals are highly soluble in waters rich
in carbon dioxide, which results in most limestones being
susceptible to diagenesis. Diagenesis leads to the alteration
of a carbonate deposit by various geochemical processes
that result in changes in mineralogy, texture, and fabric.
Over time, skeletal and non-skeletal carbonate grains and
carbonate matrix alter to low magnesian calcite and calcite
cements are precipitated in primary and secondary pore
spaces. In the process, aragonite is often partially or totally
dissolved. During transformation from high to low magnesian calcite, the initial structure is kept but Mg
2þ is leached
from the crystal. The crystal sizes of calcite cements that
form can be grouped into micrite (<4 microns), microspar
(4–10 microns), and spar (>10 microns).
Typical freshwater low magnesian calcite-rich deposits
include calcretes, tufa, travertine, cave speleothems, and
eolianites. High magnesian calcites are common components of beach rocks, shallow marine sediments, and the
skeletons of marine organisms. According to Mackenzie
et al. (1983), there is an increase in the content of magnesium within skeletal high magnesian calcite from polar
regions towards the tropics, which is related to changes
in temperature and seawater saturation of carbonate ions.
Bibliography
Mackenzie, F. T., Bischoff, W. D., Bishop, F. C., Loijens, M.,
Schoonmaker, J., and Wollast, R., 1983. Magnesian calcites:
low temperature occurrence, solubility and solid solution behaviour. In Reeder, R. J. (ed.), Carbonates: Mineralogy and Chemistry. Reviews in Mineralogy. Washington, DC: Mineralogical
Society of America, Vol. 11, pp. 97–144.
Tucker, M. E., and Wright, V. P. 1990. Carbonate Sedimentology.
Oxford: Blackwell, pp. 482.
Cross-references
Aragonite
Beach Rock
Calcrete/Caliche
Diagenesis
Eolianite
Micrite
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011