Calcite is a carbonate mineral and the most stable polymorph of
calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based
on scratch hardness comparison, defines value 3 as "calcite".
Other polymorphs of calcium carbonate are the minerals aragonite and
Aragonite will change to calcite over timescales of days or
less at temperatures exceeding 300°C, and vaterite is even less
1.1 "Alabaster", as used by archaeologists
3 Use and applications
4 Natural occurrence
5 Formation processes
6 In Earth history
8 See also
10 Further reading
Calcite is derived from the German Calcit, a term coined in the 19th
century from the Latin word for lime, calx (genitive calcis) with the
suffix -ite used to name minerals. It is thus etymologically related
"Alabaster", as used by archaeologists
When applied by archaeologists and stone trade professionals, the term
alabaster is used not just as in geology and mineralogy, where it is
reserved for a variety of gypsum; but also for a similar-looking,
translucent variety of fine-grained banded deposit of calcite.
Calcite crystals are trigonal-rhombohedral, though actual calcite
rhombohedra are rare as natural crystals. However, they show a
remarkable variety of habits including acute to obtuse rhombohedra,
tabular forms, prisms, or various scalenohedra.
several twinning types adding to the variety of observed forms. It may
occur as fibrous, granular, lamellar, or compact. Cleavage is usually
in three directions parallel to the rhombohedron form. Its fracture is
conchoidal, but difficult to obtain.
It has a defining
Mohs hardness of 3, a specific gravity of 2.71, and
its luster is vitreous in crystallized varieties. Color is white or
none, though shades of gray, red, orange, yellow, green, blue, violet,
brown, or even black can occur when the mineral is charged with
Calcite is transparent to opaque and may occasionally show
phosphorescence or fluorescence. A transparent variety called Iceland
spar is used for optical purposes. Acute scalenohedral crystals are
sometimes referred to as "dogtooth spar" while the rhombohedral form
is sometimes referred to as "nailhead spar".
Single calcite crystals display an optical property called
birefringence (double refraction). This strong birefringence causes
objects viewed through a clear piece of calcite to appear doubled. The
birefringent effect (using calcite) was first described by the Danish
Rasmus Bartholin in 1669. At a wavelength of ~590 nm
calcite has ordinary and extraordinary refractive indices of 1.658 and
1.486, respectively. Between 190 and 1700 nm, the ordinary
refractive index varies roughly between 1.9 and 1.5, while the
extraordinary refractive index varies between 1.6 and 1.4.
Calcite, like most carbonates, will dissolve with most forms of acid.
Calcite can be either dissolved by groundwater or precipitated by
groundwater, depending on several factors including the water
temperature, pH, and dissolved ion concentrations. Although calcite is
fairly insoluble in cold water, acidity can cause dissolution of
calcite and release of carbon dioxide gas. Ambient carbon dioxide, due
to its acidity, has a slight solubilizing effect on calcite. Calcite
exhibits an unusual characteristic called retrograde solubility in
which it becomes less soluble in water as the temperature increases.
When conditions are right for precipitation, calcite forms mineral
coatings that cement the existing rock grains together or it can fill
fractures. When conditions are right for dissolution, the removal of
calcite can dramatically increase the porosity and permeability of the
rock, and if it continues for a long period of time may result in the
formation of caves. On a landscape scale, continued dissolution of
calcium carbonate-rich rocks can lead to the expansion and eventual
collapse of cave systems, resulting in various forms of karst
Use and applications
One of several calcite or alabaster perfume jars from the tomb of
Tutankhamun, d. 1323 BC
Ancient Egyptians carved many items out of calcite, relating it to
their goddess Bast, whose name contributed to the term alabaster
because of the close association. Many other cultures have used the
material for similar carved objects and applications.
High-grade optical calcite was used in World War II for gun sights,
specifically in bomb sights and anti-aircraft weaponry. Also,
experiments have been conducted to use calcite for a cloak of
Microbiologically precipitated calcite has a wide range of
applications, such as soil remediation, soil stabilization and
Calcite, obtained from an 80 kg sample of Carrara marble, is
used as the IAEA-603 isotopic standard in mass spectrometry for the
calibration of δ18O and δ13C.
The largest documented single crystal of calcite originated from
Iceland, measured 7×7×2 m and 6×6×3 m and weighed about 250
Calcite is a common constituent of sedimentary rocks, limestone in
particular, much of which is formed from the shells of dead marine
organisms. Approximately 10% of sedimentary rock is limestone.
Calcite is the primary mineral in metamorphic marble. It also occurs
as a vein mineral in deposits from hot springs, and it occurs in
caverns as stalactites and stalagmites.
Lublinite is a fibrous, efflorescent form of calcite.
Calcite may also be found in volcanic or mantle-derived rocks such as
carbonatites, kimberlites, or rarely in peridotites.
Calcite is often the primary constituent of the shells of marine
organisms, e.g., plankton (such as coccoliths and planktic
foraminifera), the hard parts of red algae, some sponges, brachiopods,
echinoderms, some serpulids, most bryozoa, and parts of the shells of
some bivalves (such as oysters and rudists).
Calcite is found in
spectacular form in the Snowy River
New Mexico as mentioned
above, where microorganisms are credited with natural formations.
Trilobites, which became extinct a quarter billion years ago, had
unique compound eyes that used clear calcite crystals to form the
Calcite formation can proceed via several pathways, from the classical
Terrace ledge kink model to the crystallisation of poorly ordered
precursor phases (amorphous calcium carbonate, ACC) via an Ostwald
ripening process, or via the agglomeration of nanocrystals.
The crystallization of ACC can occur in two stages: first, the ACC
nanoparticles rapidly dehydrate and crystallize to form individual
particles of vaterite. Secondly, the vaterite transforms to calcite
via a dissolution and reprecipitation mechanism with the reaction rate
controlled by the surface area of calcite. The second stage of the
reaction is approximately 10 times slower. However, the
crystallization of calcite has been observed to be dependent on the
starting pH and presence of Mg in solution. A neutral starting pH
during mixing promotes the direct transformation of ACC into calcite.
Conversely, when ACC forms in a solution that starts with a basic
initial pH, the transformation to calcite occurs via metastable
vaterite, which forms via a spherulitic growth mechanism. In a
second stage this vaterite transforms to calcite via a
surface-controlled dissolution and recrystallization mechanism. Mg has
a noteworthy effect on both the stability of ACC and its
transformation to crystalline CaCO3, resulting in the formation of
calcite directly from ACC, as this ion destabilizes the structure of
Calcite may form in the subsurface in response to activity of
microorganisms, such as during sulfate-dependent anaerobic oxidation
of methane, where methane is oxidized and sulfate is reduced by a
consortium of methane oxidizers and sulfate reducers, leading to
precipitation of calcite and pyrite from the produced bicarbonate and
sulfide. These processes can be traced by the specific carbon isotope
composition of the calcites, which are extremely depleted in the 13C
isotope, by as much as −125 per mil PDB (δ13C).
In Earth history
Calcite seas existed in Earth history when the primary inorganic
precipitate of calcium carbonate in marine waters was low-magnesium
calcite (lmc), as opposed to the aragonite and high-magnesium calcite
(hmc) precipitated today.
Calcite seas alternated with aragonite seas
over the Phanerozoic, being most prominent in the
Jurassic. Lineages evolved to use whichever morph of calcium carbonate
was favourable in the ocean at the time they became mineralised, and
retained this mineralogy for the remainder of their evolutionary
history. Petrographic evidence for these calcite sea conditions
consists of calcitic ooids, lmc cements, hardgrounds, and rapid early
seafloor aragonite dissolution. The evolution of marine organisms
with calcium carbonate shells may have been affected by the calcite
and aragonite sea cycle.
Calcite with mottramite.
Trilobite eyes employed calcite.
Calcite crystals inside a test of the cystoid Echinosphaerites
aurantium (Middle Ordovician, northeastern Estonia).
Rhombohedrons of calcite that appear almost as books of petals, piled
up 3-dimensionally on the matrix.
Calcite crystal canted at an angle, with little balls of hematite and
crystals of chalcopyrite both on its surface and included just inside
the surface of the crystal.
Calcite crystals inside a recrystallized bivalve shell in
Several well formed milky white casts, made up of many small sharp
calcite crystals, from the sulfur mines at Agrigento, Sicily.
Reddish rhombohedral calcite crystals from China. Its red color is due
to the presence of iron.
Calcite (var.: Cobaltoan calcite).
Sand calcites (calcites heavily included with desert sand) in South
Calcite from Ojuela Mine, Mapimí, Mapimí Municipality, Durango,
Wikimedia Commons has media related to Calcite.
Wikisource has the text of the 1911 Encyclopædia Britannica article
List of minerals
Manganoan Calcite, (Ca,Mn)CO3
Ulexite aka "TV rock", another mineral with an optical property often
illustrated in the same way.
^ Dana, James Dwight; Klein, Cornelis and Hurlbut, Cornelius Searle
(1985) Manual of Mineralogy, Wiley, p. 329, ISBN 0-471-80580-7
^ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols,
Monte C., eds. (2003). "Calcite". Handbook of Mineralogy (PDF). V
(Borates, Carbonates, Sulfates). Chantilly, VA, US: Mineralogical
Society of America. ISBN 0962209740.
^ Calcite. Mindat.org
^ Calcite. Webmineral. com
^ Yoshioka S.; Kitano Y. (1985). "Transformation of aragonite to
calcite through heating". Geochemical Journal. 19: 24–249.
^ Staudigel P.T.; Swart P.K. (2016). "Isotopic behavior during the
aragonite-calcite transition: Implications for sample preparation and
proxy interpretation". Chemical Geology. 442: 130–138.
^ Online Etymology Dictionary, "calcite"
^ More about alabaster and travertine, brief guide explaining the
different use of the same terms by geologists, archaeologists and the
stone trade. Oxford University Museum of Natural History, 2012 
^ Elert, Glenn. "Refraction". The Physics Hypertextbook.
^ Thompson, D. W.; Devries, M. J.; Tiwald, T. E.; Woollam, J. A.
(1998). "Determination of optical anisotropy in calcite from
ultraviolet to mid-infrared by generalized ellipsometry". Thin Solid
Films. 313–314: 341–346. Bibcode:1998TSF...313..341T.
^ "Borrego's calcite mine trail holds desert wonders". Retrieved
^ Chen, Xianzhong; Luo, Yu; Zhang, Jingjing; Jiang, Kyle; Pendry, John
B.; Zhang, Shuang (2011). "Macroscopic invisibility cloaking of
visible light". Nature Communications. 2 (2): 176. arXiv:1012.2783 .
PMC 3105339 . PMID 21285954.
^ Department of Nuclear Sciences and Applications,
Laboratories (16 July 2016). "Reference Sheet: Certified Reference
Material : IAEA-603 (calcite) – Stable Isotope Reference
Material for δ13C and δ18O" (PDF). IAEA. p. 2. Retrieved 28
^ "IAEA-603 , Calcite". Reference Products for Environment and Trade.
International Atomic Energy Agency. Retrieved 27 February 2017.
^ Rickwood, P. C. (1981). "The largest crystals" (PDF). American
Mineralogist. 66: 885–907.
^ Lublinite. Mindat.org
^ Angier, Natalie (3 March 2014). "When Trilobites Ruled the World".
The New York Times. Retrieved 10 March 2014.
^ De Yoreo, J J; Vekilov, P G (2003). "Principles of crystal
nucleation and growth". Reviews in Mineralogy and Geochemistry. 54:
^ De Yoreo, J; Gilbert, PUPA; Sommerdijk, N A J M; Penn, R L;
Whitelam, S; Joester, D; Zhang, H; Rimer, J D; Navrotsky, A; Banfield,
J F; Wallace, A F; Michel, F M; Meldrum, F C; Cölfen, H; Dove, P M
(2015). "Crystallization by particle attachment in synthetic,
biogenic, and geologic environments". Science. 349 (6247): aaa6760.
doi:10.1126/science.aaa6760. PMID 26228157.
^ Rodriguez-Blanco, J. D.; Shaw, S.; Benning, L. G. (2011). "The
kinetics and mechanisms of amorphous calcium carbonate (ACC)
crystallization to calcite, via vaterite". Nanoscale. 3 (1): 265–71.
^ Rodriguez-Blanco, J. D.; Shaw, S.; Bots, P.; Roncal-Herrero, T.;
Benning, L. G. (2012). "The role of pH and Mg on the stability and
crystallization of amorphous calcium carbonate". Journal of Alloys and
Compounds. 536: S477. doi:10.1016/j.jallcom.2011.11.057.
^ Bots, P.; Benning, L. G.; Rodriguez-Blanco, J. D.; Roncal-Herrero,
T.; Shaw, S. (2012). "Mechanistic Insights into the Crystallization of
Amorphous Calcium Carbonate (ACC)".
Crystal Growth & Design. 12
(7): 3806–3814. doi:10.1021/cg300676b.
^ Drake, H.; Astrom, M.E.; Heim, C.; Broman, C.; Astrom, J.;
Whitehouse, M.; Ivarsson, M.; Siljestrom, S.; Sjovall, P. (2015).
"Extreme 13C depletion of carbonates formed during oxidation of
biogenic methane in fractured granite" (PDF). Nature Communications.
6: 7020. Bibcode:2015NatCo...6E7020D. doi:10.1038/ncomms8020.
PMC 4432592 . PMID 25948095.
^ Porter, S. M. (2007). "Seawater Chemistry and Early Carbonate
Biomineralization". Science. 316 (5829): 1302.
^ Palmer, Timothy; Wilson, Mark (2004). "
Calcite precipitation and
dissolution of biogenic aragonite in shallow
Ordovician calcite seas".
Lethaia. 37 (4): 417–427. doi:10.1080/00241160410002135.
^ Harper, E.M.; Palmer, T.J.; Alphey, J.R. (1997). "Evolutionary
response by bivalves to changing Phanerozoic sea-water chemistry".
Geological Magazine. 134 (3): 403–407.
Schmittner, Karl-Erich; and Giresse, Pierre; 1999.
"Micro-environmental controls on biomineralization: superficial
processes of apatite and calcite precipitation in Quaternary soils",
Roussillon, France. Sedimentology 46/3: 463–476.