Everything about Calcite totally explained
Calcite is a
carbonate mineral and the most stable
polymorph of
calcium carbonate (
CaCO3). The other polymorphs are the
minerals
aragonite and
vaterite. Aragonite will change to calcite at 470°C, and vaterite is even less stable.
Properties
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
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, 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. It is perhaps best known because of its power to produce strong
double refraction of light, such that objects viewed through a clear piece of calcite appear doubled in all of their parts—a phenomenon first described by
Rasmus Bartholin. A beautifully transparent variety used for optical purposes comes from
Iceland, called
Iceland spar. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar".
Single calcite crystals display an optical property called
birefringence. The birefringent effect (using calcite) was first described by the
Danish scientist
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 1.6 and 1.4, while the extraordinary refractive index varies between 1.9 and 1.5.
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. 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
caverns.
Natural occurrence
Calcite is often the primary constituent of the
shells of
marine organisms, for example,
plankton (such as
coccoliths and planktic
foraminifera), the hard parts of red
algae, some
sponges,
brachiopoda,
echinoderms, most
bryozoa, and parts of the shells of some
bivalves, such as
oysters and
rudists).
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.
Calcite may also be found in
volcanic or
mantle-derived rocks such as
carbonatites,
kimberlites, or rarely in
peridotites.
Calcite 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
Ordovician and
Jurassic.
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 In Literature
A form of calcite,
Iceland spar, plays a critical role in the plot of
Against the Day by
Thomas Pynchon. The same form is referred to in
The Amber Spyglass by
Philip Pullman as it has very similar properties to a mineral found in that story.
Further Information
Get more info on 'Calcite'.
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