A common trigonal rock-forming mineral, typically
forming white, pink, colorless, yellow, or gray crystals with a
variety of shapes, including rhombohedrons and clusters of
small crystals known as nailhead spar and dogtooth spar.
Distinct crystal faces and striations, or cleavage traces, on the
crystal faces make many varieties of calcite easily distinguished
from quartz. Some of the clear crystals of calcite
known as Iceland spar form perfect rhombohedrons that
exhibit strong double refraction. When an object is viewed
through the clear crystals of Iceland spar, a double image
appears through the crystal. Calcite is the main constituent of
many carbonate rocks such as limestone, and also occurs as
crystals in marble, in gangue mineral in many mineral
deposits, as stalactites and stalagmites in cave deposits, and
as loose earthy material in chalk deposits.
Calcite has the chemical formula CaCO3, and it is trimorphous
with aragonite and vaterite. Aragonite has
orthorhombic symmetry, is denser and harder than calcite,
has a less distinct cleavage, and typically forms as fibrous
aggregates with gypsum in hot springs and in shallow
marine muds and coral reefs. Vaterite is a rare hexagonal
form of CaCO3.
Most fossil shells are made of calcite, either as an original
precipitate from seawater or from aragonite that reverted
to the more stable calcite. Calcite also forms a common
cement in sedimentary rocks and in many vein and fault systems.
See also MINERALOGY.
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