Listing description
Calcite is a carbonate mineral and the most stable polymorph of calcium
carbonate (CaCO3). The other
polymorphs are the mineralsaragonite and vaterite. Aragonite will change to calcite at
380–470 °C,[5] and
vaterite is even less stable.
Detailed description
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. Calcite exhibits 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 his 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 impurities.
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 Danishscientist Rasmus Bartholin in
1669. At a wavelength of ~590 nm calcite has ordinary and extraordinary refractive indices of 1.658 and 1.486,
respectively.[6] 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.[7]
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
topography.
Use and applications
High-grade optical
calcite was used in World War II for gun sights, specifically in bomb sights
and anti-aircraft weaponry.[8] Also, experiments
have been conducted to use calcite for a cloak of invisibility.[9] Microbiologically
precipitated calcite has a wide range of
applications, such as soil remediation, soil stabilization and concrete repair.
Natural occurrence
The largest
documented single crystals of calcite originated from Iceland, measured 7×7×2 m
and 6×6×3 m and weighed about 250 tons.[10][11]
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 incaverns as stalactites and stalagmites.
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 somebivalves (such as oysters and rudists). Calcite is found in spectacular form in the Snowy River Cave of 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 lenses.[13]
Formation processes
Calcite forms from a
poorly ordered precursor (amorphous calcium carbonate, ACC).[14] The crystallization
process occurs in two stages; firstly, 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.[15] The second stage of
the reaction is approximately 10 times slower than the first. However, the
crystallization of calcite has been observed to be dependent on the starting pH
and presence of Mg in solution.[16] 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.[17] 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 unstabilizes the structure
of vaterite.
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).[18]
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 aragoniteand high-magnesium calcite (hmc)
precipitated today. Calcite seas alternated with aragonite seas over the
Phanerozoic, being most prominent in the Ordovician and 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.[19] Petrographic evidence
for these calcite sea conditions consists of calcitic ooids,
lmc cements, hardgrounds, and rapid early seafloor aragonite
dissolution.[20] The evolution of
marine organisms with calcium carbonate shells may have been affected by the
calcite and aragonite sea cycle.
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