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.
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.
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. 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 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, 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 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 are now extinct, had unique compound eyes. They used clear calcite
crystals to form the lenses of their eyes.
Lublinite is
a fibrous, efflorescent form of calcite.
