Term: Limestone
**Composition and Characteristics of Limestone:**
– Limestone is primarily composed of calcite and aragonite, forms of calcium carbonate.
– Dolomite is rare in limestone.
– Limestone is chemically pure with less than 5-10% clastic sediments.
– Silica, in the form of chert or siliceous skeletal fragments, is common in limestone.
– Limestone varies in color, density, and hardness.
**Grains in Limestone:**
– Grains in limestone are mainly skeletal fragments of marine organisms like coral and foraminifera.
– Other carbonate grains include ooids, peloids, and limeclasts.
– Ooids are sand-sized grains with calcite or aragonite layers.
– Peloids are structureless grains likely produced by various processes.
**Formation and Uses of Limestone:**
– Limestone forms from the precipitation of minerals like calcite and aragonite from water containing dissolved calcium.
– Most limestone is formed in shallow marine environments.
– Limestone is utilized in cement production, as aggregate for roads, white pigment, soil conditioner, and decorative rock gardens.
– Limestone formations are associated with petroleum reservoirs and cave systems.
**Micrite and Sparry Calcite in Limestone:**
– Carbonate mud in limestone contains micrite, which is fine-grained carbonate mud with microlites.
– Micrite can precipitate from seawater, be secreted by algae, or result from abrasion of carbonate grains.
– Sparry calcite crystals in limestone are typically 0.02-0.1mm in size and have characteristic shapes.
– Sparite cement in limestone indicates high-energy depositional environments.
**Identifying Limestone and Related Characteristics:**
– Limestone outcrops are recognized by their softness and reaction with hydrochloric acid.
– Dolomite shares softness characteristics with limestone.
– Sparite in limestone has a grain size over 20μm and appears white or transparent.
– The White Cliffs of Dover are composed of chalk, a type of limestone with distinctive features.
Limestone (calcium carbonate CaCO3) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.
| Sedimentary rock | |
 Limestone outcrop in the Torcal de Antequera nature reserve of Málaga, Spain | |
| Composition | |
|---|---|
| Calcium carbonate: inorganic crystalline calcite or organic calcareous material |
About 20% to 25% of sedimentary rock is carbonate rock, and most of this is limestone. The remaining carbonate rock is mostly dolomite, a closely related rock, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. Magnesian limestone is an obsolete and poorly-defined term used variously for dolomite, for limestone containing significant dolomite (dolomitic limestone), or for any other limestone containing a significant percentage of magnesium. Most limestone was formed in shallow marine environments, such as continental shelves or platforms, though smaller amounts were formed in many other environments. Much dolomite is secondary dolomite, formed by chemical alteration of limestone. Limestone is exposed over large regions of the Earth's surface, and because limestone is slightly soluble in rainwater, these exposures often are eroded to become karst landscapes. Most cave systems are found in limestone bedrock.
Limestone has numerous uses: as a chemical feedstock for the production of lime used for cement (an essential component of concrete), as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a soil conditioner, and as a popular decorative addition to rock gardens. Limestone formations contain about 30% of the world's petroleum reservoirs.