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Limestone is a chemical sedimentary rock
composed largely of the mineral calcite (calcium
carbonate: CaCO3). Limestone often
contain variable amounts of
silica in the form
of chert or flint, as well as amounts of clay,
silt and sand as disseminations, nodules, or
layers within the rock unit. The primary source
of this calcite is most commonly marine
organisms. These organisms secrete shells that
settle out of the water column and are deposited
on ocean floors as pelagic ooze (see lysocline
for information on calcite dissolution).
Secondary calcite may also be deposited by
supersaturated meteoric waters (groundwater that
precipitates the material in caves). This
produces speleothems such as stalagmites and
stalactites. Another form is composed of oolites
(oolitic limestone) and can be recognized by its
granular appearance. Limestone makes up about 10
percent of the total volume of all sedimentary
rocks.
Pure limestone are white or almost white.
Because of impurities, such as clay, sand,
organic remains, iron oxide and other materials,
many limestone exhibit different colors,
especially on weathered surfaces. Limestone may
be crystalline, classic, granular, or massive,
depending on the method of formation. Crystals
of calcite, quartz, dolomite or barite may line
small cavities in the rock.
Travertine is a banded, compact variety of
limestone formed along streams, particularly
where there are waterfalls and around hot or
cold springs. Calcium carbonate is deposited
where evaporation of the water leaves a solution
that is supersaturated with chemical
constituents of calcite. Tufa, a porous or
cellular variety of travertine, is found near
waterfalls. Coquina is a poorly consolidated
limestone composed of pieces of coral or shells.
During regional metamorphism limestone
recrystallizes into marble.
Limestone is a parent material of Mollisol soil
group.
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