Cutting limestone blocks at a quarry in
Limestone as building material
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as
foraminifera. These organisms secrete shells made of aragonite or calcite, and leave these shells behind when they die. Other carbonate grains comprising limestones are
Limestone often contains variable amounts of
silica in the form of
jasper, etc.) or siliceous skeletal fragment (sponge spicules,
radiolarians), and varying amounts of
detritus) carried in by rivers.
Some limestones do not consist of grains at all, and are formed completely by the chemical
travertine. Secondary calcite may be deposited by
meteoric waters (
precipitates the material in caves). This produces
speleothems, such as
stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular (oolite) appearance.
The primary source of the calcite in limestone is most commonly
marine organisms. Some of these organisms can construct mounds of rock known as reefs, building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone typically does not form in deeper waters (see
lysocline). Limestones may also form in
Calcite can be
precipitated by groundwater, depending on several factors, including the water temperature,
pH, and dissolved
ion concentrations. Calcite exhibits an unusual characteristic called
retrograde solubility, in which it becomes less soluble in water as the temperature increases.
Impurities (such as
clay, sand, organic remains,
iron oxide, and other materials) will cause limestones to exhibit different colors, especially with
Limestone may be crystalline,
clastic, granular, or massive, depending on the method of formation. Crystals of calcite,
barite may line small cavities in the rock. When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures.
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 supersaturated with the 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
metamorphism that occurs during the mountain building process (
orogeny), limestone recrystallizes into
Limestone is a
parent material of
Mollisol soil group.