It can also be formed by the precipitation of calcium carbonate dissolved in the water running in the ground water. A few percentage of other minerals are also present as all limestone contains them. The other minerals can be quartz, siderite, pyrite, clay minerals feldspar and other minerals. It also includes large nodules of chert, pyrite or siderite. The calcium carbonate can easily be identified when treated with acid, it reacts.
The limestone is mainly deposited in calm, warm marine water at shallow depths where organism are capable of generating calcium carbonate shells and skeleton.
These organism when die accumulate in the area to form limestone deposits. The waste product of these organism also contribute to sediment mass which in turn when lithifies produces limestone. The limestone produce from this type are biological sedimentary rocks.
Other limestone is formed by direct precipitation from the marine or fresh water and this type is called chemical sedimentary rock. The limestone formed by evaporation are often formed in the caves where water droplet enters the fracture and when reaching cave ceiling evaporates.
When water evaporates the calcium carbonate present precipitates to form limestone. The main platform is over miles wide, and a great thickness of calcium carbonate sediments have accumulated there.
In this image the dark blue areas are deep ocean waters. The shallow Bahamas Platform appears as light blue. Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts.
These organisms secrete shells made of aragonite or calcite, and leave these shells behind when they die. Limestone often contains variable amounts of silica in the form of chert chalcedony , flint, jasper , etc. Some limestones do not consist of grains at all, and are formed completely by the chemical precipitation of calcite or aragonite, i.
Secondary calcite may 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 oolitic limestone, which can be recognised 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, meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone typically does not form in deeper waters.
Limestone may also form in lacustrine and evaporite depositional environments. Calcite can be dissolved or 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 limestone to exhibit different colours, especially with 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. 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 coral or shells. Strong currents continually shift the oolites around so that new concentric layers of aragonite grow on top of the earlier layers.
The oolites are alternately exposed to pick up a concentric layer and then buried to set the layer. Marble is a hard crystalline rock that takes a high polish and is used for building and sculpture. Chalk is a special form of limestone mainly formed in deeper water from the shell remains of microscopic marine plants and animals such as coccolithophores and foraminifera. Unless deeply buried, most chalks are relatively soft rock with a high calcium carbonate content.
Add to collection. Go to full glossary Add 0 items to collection. Download 0 items. Twitter Pinterest Facebook Instagram. Email Us. See our newsletters here. Would you like to take a short survey? Limestone is a rock with an enormous diversity of uses.
It could be the one rock that is used in more ways than any other. Most limestone is made into crushed stone and used as a construction material, as a crushed stone for road base and railroad ballast, as an aggregate in concrete and is fired in a kiln with crushed shale to make cement. You may be trying to access this site from a secured browser on the server. Please enable scripts and reload this page. Skip to Main Content. Skip to Primary Navigation.
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