|
|
|
|
Limestone |
|
|
|
|
|
|
|
Limestone is a sedimentary
rock composed largely of the mineral calcite (calcium carbonate:
CaCO3). Limestones often contain variable amounts of silica in the
form of chert or flint, as well as varying amounts of clay, silt and
sand as disseminations, nodules, or layers within the rock. The
primary source of the calcite in limestone 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 or
alternatively is conglomerated in a coral reef (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 taken by calcite is that of
oolites (oolitic limestone) which can be recognised by its granular
appearance. Limestone makes up about 10 percent of the total volume of
all sedimentary rocks.
Pure limestones are white or almost white. Because of impurities, such
as clay, sand, organic remains, iron oxide and other materials, many
limestones exhibit different colors, especially on weathered surfaces.
Limestone may be crystalline, clastic, granular, or massive, depending
on the method of formation. Crystals of calcite, quartz, dolomite or
barite may line small cavities in the rock. Folk and Dunham
classifications are used to describe limestones more precisely.
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 that occurs during the mountain building
process (orogeny) limestone recrystallizes into marble. |
|
|
