World of Earth Science | Limestone

Limestone is a sedimentary rock composed almost entirely of the mineral calcite (calcium carbonate, CaCO₃). The precursor calcium-carbonate sediment that existed prior to lithification of limestone can be of several types. These sediment types include carbonate mud, carbonate fossil fragments, carbonate pellets and rip-up clasts, and ooids. Carbonate mud is made of microcrystalline calcite crystals (crystals of a few microns in size) that form directly from seawater and from the disintegration of some calcareous marine algae. Carbonate fossil fragments include all shelly organic debris originally composed of CaCO₃ (in the form of calcite or a denser phase, aragonite). Carbonate pellets and rip-up clasts are small lumps of carbonate mud (a few millimeters in size) that have been consolidated either by being eaten and excreted (pellets), or by settling and then being ripped-up by wave energy (rip-up clasts). Ooids are sand-sized, concentrically layered grains that form by inorganic precipitation of calcium carbonate during agitation of seawater (usually by rolling on a shallow shoal area of the ocean).

The sediment precursor to limestone forms in the shallow marine realm or, less commonly, within carbonate-rich lakes. A special kind of marine limestone, composed entirely of the remains of marine micro-plankton, is called chalk. There is also a special kind of limestone, travertine, which forms from fresh-water deposition of dissolved carbonate within cave and cavern systems.

The original sediment determines limestone texture (i.e., the size and nature of grains in the rock). Micritic limestone is made of microcrystalline calcite like the original carbonate mud. Fossiliferous limestone is made of a large proportion of fossil fragments and the balance of the rock is either carbonate

mud, carbonate cement (sparry calcite crystals between grains), or a mixture of both. Pelletal limestone and limestone with ripup clasts is much like fossiliferous limestone regarding the balance of the rock (mud, spar, or both). Ooid limestone has a high proportion of ooid grains in it. Limestone that is made of sediment formed within or near organic reefs is sometimes referred to as reef rock or boundstone.

Limestones contain extensive fossil records spanning much geologic time, including microfossils, megafossils, and trace fossils (or ichnofossils). Limestones are particularly common sedimentary rocks, representing times in Earth history when globally warm conditions prevailed along with particularly high sea levels. High sea levels during these times promoted development of extensive, shallow seas (i.e., epicontinental seas) across much of Earth's continental space. Epicontinental seas with extensive limestone deposits characterize the geological time intervals such as Late Cambrian-Ordovician, Mississippian, and Cretaceous.

Limestones are particularly susceptible to dissolution by acidic groundwater, and for this reason, extensive subterranean cave and cavern systems are known from within many limestone formations. The calcite in limestones is also susceptible to replacement by dolomite where the limestone meets magnesium-rich, briny groundwaters. Where this replacement is extensive, limestone formations are changed to dolostones by this process. Considerable porosity (approximately seven percent by volume) is created within the former limestone (now dolostone), and this pore space can host important hydrocarbon and mineral deposits. Limestone is also commonly replaced by chert in places where silica has been transported into the limestone by groundwaters.

Limestone is economically important as a source of quicklime (CaO), which can be produced by heating calcite to drive off carbon dioxide (CO₂). In addition to agricultural uses, limestone is used in cement making and as a flux in the smelting of iron. Limestone can be used also for road construction and as ornamental building stone.